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United States Government Accountability Office: 
GAO: 

Report to Congressional Requesters: 

September 2011: 

Center for Science, Technology, and Engineering Natural Resources and 
Environment: 

Technology Assessment: 

Neutron detectors: 
Alternatives to using helium-3: 

GAO-11-753: 

Cover image from GAO represents the three neutron detection reactions 
that are suitable for conversion materials in neutron detectors 
discussed in the report. The reactions are: 

n + He-3 - H-1 + H-3; 

n + Li-6 - H-3 + He-4; 

n +B-10 - Li-7* + He-4 + y (94 percent of the time) or n + B-10 - He-4 
+ Li-7 (6 percent of the time). 

Where:
* n is a neutron, depicted as a light circle in the cover image; 

* H-1 is a proton, depicted as a dark circle in the cover image; 

* H-3 is the hydrogen isotope hydrogen-3 (tritium) which has a nucleus 
containing one proton and two neutrons, depicted as a collection of 
three circles in the cover image, one dark circle for the proton and 
two light circles for the neutrons; 

* He-3 is the helium isotope helium-3 which has a nucleus containing 
two protons and one neutron, depicted as a collection of three circles 
in the cover image, two dark circles for the protons and one light 
circle for the neutron; 

* He-4 is the helium isotope helium-3 (an alpha particle) which has a 
nucleus containing two protons and two neutrons, depicted as a 
collection of four circles in the cover image, two dark circles for 
the protons and two light circles for the neutrons. 

* Li-6 is the lithium isotope lithium-6 which has a nucleus containing 
three protons and three neutrons, depicted as a collection of six 
circles in the cover image, three dark circles for the protons and 
three light circles for the neutrons; 

* Li-7 is the lithium isotope lithium-7 which has a nucleus containing 
three protons and four neutrons, depicted as a collection of seven 
circles in the cover image, three dark circles for the protons and 
four light circles for the neutrons; 

* Li-7* is the lithium isotope lithium-7 in a high energy state.
* B-10 is the isotope boron-10 which has a nucleus containing five 
protons and five neutrons, depicted as a collection of 10 circles in 
the cover image, five dark circles for the protons and five light 
circles for the neutrons; 

* y is a gamma-ray which is not depicted in the cover image. 

GAO Highlights: 

Highlights of GAO-11-753, a report to congressional requesters. 

Why GAO Did This Study: 

Neutron detectors are used to detect neutron radiation in science, 
security, and other applications. For example, large-area detectors 
detect neutrons at science facilities across the world and radiation 
portal monitors screen vehicles and cargo at ports and border 
crossings for nuclear material that terrorists could use in a nuclear 
weapon. Helium-3 is a critical component of such neutron detectors, 
and in 2008 the U.S. government became aware of a severe shortage of 
helium-3 gas. While demand for it has increased, helium-3 is currently 
produced as a byproduct of the radioactive decay of tritium, and the 
United States ceased tritium production in 1988. The shortage has led 
science facilities and federal agencies such as the DOD and DHS to 
identify or develop alternative detector technologies. 

GAO was asked to review the effectiveness of alternative neutron 
detector technologies that do not use helium-3. GAO assessed (1) what 
alternative neutron detectors are currently available and their 
effectiveness, and (2) the status of research on alternative neutron 
detector technologies under development for future availability. GAO 
reviewed agency documents and interviewed agency officials and 
detector developers. With assistance from the National Academy of 
Sciences, GAO also assembled a group of experts to review and advise 
on this study. 

What GAO Found: 

Science facilities and federal agencies are working to determine the 
effectiveness of currently available alternative neutron detector 
technologies for use in large-area detectors and radiation portal 
monitors (RPM)—the two neutron detector applications that have created 
the greatest demand for helium-3. An international collaboration of 
science facilities that plan to deploy large-area detectors for 
research and federal agencies that procure and deploy RPMs for security
have identified three alternative neutron detector technologies that 
are available and might satisfy requirements for use: boron-10 lined 
proportional detectors, boron trifluoride proportional detectors, and 
lithium-6 scintillators. These technologies use boron-10 and lithium-6 
rather than helium-3 to detect neutrons. The international 
collaboration has agreed on a plan to develop and test large area 
detectors using these technologies. Federal agencies, such as DHS, 
have been directing the testing of these technologies for use in RPMs, 
and field testing of RPMs using boron-10 lined proportional detectors 
has been completed. According to agency officials, a boron-10 lined 
proportional detector may be available for domestic RPM deployments in 
early fiscal year 2012. GAO estimates this neutron detector is 
sufficiently mature such that a decision to use it in forthcoming 
portal monitor deployments can be made with confidence that the portals
will perform as required. Our estimate is based on our assessment of 
the technology readiness levels (TRL), which assess the maturity of an 
application on a scale of 1 to 9. We found these three currently 
available alternative neutron detector technologies range in TRL from 
5 to 7. 

Federal agencies are funding more than 30 research and development 
programs that may result in additional alternative neutron detector 
technologies. At varying stages of research, these programs focus on 
security applications but may eventually be applied to other neutron 
detector applications. Some of these technologies may become available 
for integration into deployable detector systems in less than two 
years and could potentially help reduce demand for helium-3. 

Figure: Neutron detectors: a large-area detector (left) and a 
radiation portal monitor (right)] 

[Refer to PDF for image: 2 illustrations] 

Source: GAO. 

[End of figure] 

We provided a draft of this report to Commerce, DOD, DOE, and DHS. 
They generally provided technical comments that we included as 
appropriate. 

View [hyperlink, http://www.gao.gov/products/GAO-11-753 or key 
components at [hyperlink, http://www.gao.gov]. For more information, 
contact Timothy Persons at (202) 512-6412 or personst@gao.gov or Gene 
Aloise at (202) 512-3841 or aloisee@gao.gov. 

Report multimedia: 

Neutron detector animation: 

[hyperlink, http://www.gao.gov/products/GAO-11-753. 

[End of section] 

United States Government Accountability Office: 

September 29, 2011: 

The Honorable Donna Edwards: 
Ranking Member: 
Subcommittee on Investigations and Oversight: 
Committee on Science, Space, and Technology: 
House of Representatives: 

The Honorable Brad Miller: 
Ranking Member: 
Subcommittee on Energy and Environment: 
Committee on Science, Space, and Technology: 
House of Representatives: 

Neutron detectors are used to detect neutron radiation in science and 
security applications. For example, large-area detectors support 
materials research, and radiation portal monitors use neutron 
detectors to screen vehicles and cargo at ports and border crossings. 
Helium-3 is a critical component of neutron detectors and, in 2008, 
the U.S. government became aware of a shortage of helium-3 gas. 
Responding to your request that we conduct a technology assessment on 
alternative neutron detection technologies that could be used in place 
of neutron detectors utilizing helium-3, we assessed what alternative 
technologies are currently available. We also discuss the status of 
alternative technologies under development that may be available for 
future use. 

As agreed with your offices, unless you publicly announce the contents 
of this report earlier, we plan no further distribution of it until 30 
days from the report date. At that time, we will send copies to
the appropriate congressional committees, Secretaries of Commerce, 
Defense, Energy, and Homeland Security; and other interested parties. 
The report will also be available at no charge on the GAO Web site
at [hyperlink, http://www.gao.gov]. 

If you or your staffs have any questions about this report, please 
contact Timothy M. Persons at (202) 512-6412 or personst@gao.gov, or 
Gene Aloise at (202) 512-3841 or aloisee@gao.gov. Contact points
for our Offices of Congressional Relations and Public Affairs are 
named on the last page of the report. GAO staff who made key 
contributions to this report are listed on page 47. 

Signed by: 

Timothy M. Persons, Ph.D. 
Chief Scientist: 
Director, Center for Science, Technology, and Engineering: 

Signed by: 

Gene Aloise: 
Director: 
Natural Resources and Environment: 

[End of section] 

Contents: 

Highlights: 

Letter: 

1. Introduction: 

2. Background: 

2.1. How neutron detectors operate: 

2.2. Three main categories of neutron detectors: 

2.3. Using helium-3 in neutron detection applications: 

2.4. Production of and demand for helium-3: 

3. Alternative neutron detector technologies are currently available: 

3.1. Three current alternative technologies may meet requirements for 
large-area detectors and radiation portal monitors: 

3.1.1. Boron-lined proportional detectors: 

3.1.2. Boron trifluoride gas proportional detectors: 

3.1.3. Lithium-6 scintillator detectors: 

3.2. Testing process for the three alternative technologies for use in 
large-area detectors: 

3.3. Testing process for the three alternative technologies for use in 
radiation portal monitors: 

3.4. Maturity of radiation portal monitors using the three alternative 
detector technologies: 

4. Additional neutron detector technologies are being developed: 

5. Conclusions: 

6. External comments: 

6.1. Agency comments: 

6.2. Expert comments: 

7. Appendices: 

7.1. Objectives, scope, and methodology: 

7.2. Boron and lithium isotopes are alternatives to helium-3: 

7.3. Requirements for alternative neutron detectors for use in large-
area detectors and radiation portal monitors: 

7.4. Technology readiness levels: 

7.5. Expert participation in the engagement: 

7.6. Comments from the Department of Energy: 

7.7. Comments from the Department of Homeland Security: 

8. References: 

GAO contacts and staff acknowledgments: 

Other GAO technology assessments: 

Figures: 

Figure 2.1. Neutron detector operation: 

Figure 2.2. Tubes used in a proportional detector for large-area 
detectors: 

Figure 2.3. Neutron detectors—radiation portal monitors and a handheld 
detector: 

Figure 2.4. Neutron detectors—a large-area detector: 

Figure 2.5. Helium-3 demand and annual U.S. production, 2011–18, as 
projected in 2009 and 2011: 

Figure 2.6. Helium-3 demand by application, 2011-18, as projected in 
2011: 

Figure 3.1. A Li-6/ZnS scintillator detector module for SNS: 

Figure 3.2. DHS’s steps to test radiation portal monitors using 
alternative neutron detectors: 

Figure 4.1. Cherenkov radiation illuminating reactor fuel assemblies 
immersed in water: 

Tables: 

Table 3.1. Key characteristics of alternative neutron detector 
technologies for use in radiation portal monitors: 

Table 3.2. Testing of radiation portal monitors using alternative 
neutron detector technologies: 

Table 4.1. Examples of alternative neutron detector technology 
programs: 

Table 7.1. Three isotopes commonly used as conversion materials in 
neutron detectors: 

Table 7.2. TRLs as defined by NASA: 

Abbreviations: 

ANSI: American National Standards Institute: 

ASP: Advanced spectroscopic portal: 

BF3: Boron trifluoride: 

Commerce: Department of Commerce: 

DHS: Department of Homeland Security: 

DOD: Department of Defense: 

DOE: Department of Energy: 

DNDO: Domestic Nuclear Detection Office: 

Isotope Program: DOE Isotope Development and Production for Research 
and Applications Program: 

LANL: Los Alamos National Laboratory: 

Li-6/ZnS: Lithium-6 zinc sulfide: 

MRI: Magnetic resonance imaging: 

NAS: National Academy of Sciences: 

NASA: National Aeronautics and Space Administration: 

NIST: National Institute of Standards and Technology: 

NNSA: National Nuclear Security Administration: 

ORNL: Oak Ridge National Laboratory: 

PNNL: Pacific Northwest National Laboratory: 

SLD: Second Line of Defense: 

SNL: Sandia National Laboratories: 

SNS: Spallation Neutron Source: 

TRL: Technology readiness level: 

[End of section] 

Natural Resources and Environment:

1. Introduction:

Neutron detectors are used in research, security, and industrial 
applications to detect neutron radiation, a type of ionizing radiation 
composed of neutron particles.[Footnote 1] One critical component of 
many such neutron detectors is helium-3 gas--a rare, nonradioactive 
isotope of helium that is a byproduct of the radioactive decay of 
tritium, a key component of the nation's nuclear weapons that is used 
to enhance their power.[Footnote 2] 

Helium-3 became the favored material for neutron detectors beginning 
in the 1980s. In May 2011, we reported that weaknesses in the 
Department of Energy's (DOE) management of helium-3 contributed to a 
shortage of the gas in 2008.[Footnote 3] The shortage has affected 
scientific research because helium-3 is in great demand for large-area 
neutron detectors, which are critical for conducting materials 
research that benefits fields, such as medicine, energy, and 
transportation at facilities worldwide, such as at DOE's Spallation 
Neutron Source (SNS).[Footnote 4] The Department of Homeland 
Security's (DHS), the Department of Defense's (DOD), and DOE's future 
deployments of radiation detection portal monitors, which incorporate 
neutron detectors, have also been affected. 

Neutron detectors are used in more than 1,400 radiation detection 
portal monitors deployed domestically at ports and border crossings 
for security purposes to screen cargo and vehicles for potentially 
dangerous nuclear material that could be used by terrorists in a 
nuclear weapon. According to a 2009 DHS document, more than 9 million 
containers are offloaded annually at U.S. seaports, and 103 million 
trucks and personal vehicles entered the United States through land 
border crossings in 2010. In addition, neutron detectors are used 
overseas in about 2,000 U.S.-deployed radiation portal monitors. The 
helium-3 shortage also affected commercial applications of neutron 
detectors, which include helping to determine the presence of oil and 
gas deposits and measuring the moisture content of soil during road 
construction.

Federal agencies and DOE's national laboratories are collaborating to 
acquire or develop alternative neutron detection technologies in order 
to mitigate the effect of the shortage of helium-3 on its largest 
consumers--large-area detectors in research facilities and radiation 
detection portal monitors at ports and border crossings. To support 
programs developing alternative neutron detection technologies and 
their testing, DHS, DOD, DOE, and the Department of Commerce awarded 
about $16 million in fiscal year 2009 and about $20 million in fiscal 
year 2010 to industry, academia, and national laboratories. 
Alternative technologies could also free the limited helium-3 supply 
for use in applications for which there are no helium-3 alternatives. 
[Footnote 5]

In this context, you asked us to review the availability and 
effectiveness of alternative neutron detector technologies that do not 
use helium-3. To do so, we assessed (1) what alternative neutron 
detectors are currently available and their effectiveness; and (2) the 
status of research on alternative neutron detector technologies under 
development for future availability.

To meet these objectives, we reviewed test and evaluation documents 
supplied by manufacturers of neutron detection technologies and by 
DOE's Oak Ridge National Laboratory (ORNL) and Pacific Northwest 
National Laboratory (PNNL). We visited the neutron detection test 
facilities at PNNL, as well as SNS at ORNL, which relies on helium-3 
in large-area detectors for conducting scientific research.[Footnote 
6] With the assistance of the National Academy of Sciences (NAS), we 
identified and contacted a group of experts from academia and 
federally funded research and development centers with a range of 
knowledge and expertise in technology development, nuclear physics and 
engineering, and neutron detector applications. These experts helped 
us identify alternatives neutron detector technologies now available 
or being developed for future use in research and security 
applications, as well as the considerations involved with the 
selection of technologies to replace neutron detectors using helium-3. 
This group of experts also reviewed and commented on a draft of this 
report. Further, we reviewed information from and interviewed 
officials at Commerce's National Institute of Standards and Technology 
(NIST), DHS, DOD, and DOE, and we contacted officials at the national 
laboratories involved in developing and testing neutron detectors, 
including DOE's Los Alamos National Laboratory (LANL), ORNL, PNNL, and 
Sandia National Laboratories (SNL). We interviewed representatives of 
companies that manufacture or research alternative neutron detection 
technologies. Based on available test reports, we estimated the 
technology readiness levels (TRLs) for the currently available 
alternative neutron detector technologies.[Footnote 7] Appendix I 
contains additional details on our scope and methodology.

We conducted our work from July 2010 to September 2011 in accordance 
with all sections of GAO's quality assurance framework that are 
relevant to technology assessments. The framework requires that we 
plan and perform the engagement to obtain sufficient and appropriate 
evidence to meet our stated objectives and to discuss any limitations 
to our work. We believe that the information and data obtained, and 
the analysis conducted, provide a reasonable basis for any findings 
and conclusions in this product. 

[End of section] 

2. Background:

A neutron detector operates by detecting the signal generated when a 
neutron interacts with certain parts of the detector and can be 
classified by how the detection process occurs. Because helium-3 has 
characteristics that made it effective for use in neutron detectors, 
it was considered a "gold-standard" for neutron detection. However, 
helium-3 is a rare material, and production in the United States is 
declining while demand has been increasing, requiring helium-3 users 
to take action to reduce their consumption of the gas.

2.1 How neutron detectors operate:

Neutron detectors operate in an environment where they may be exposed 
to two main types of ionizing radiation--neutron radiation and gamma 
radiation. Neutron radiation typically encountered can be categorized 
as being composed of fast (higher energy) or thermal (lower energy, 
also known as slow) neutrons. Fission reactions and fissionable 
nuclear material, such as plutonium that might be used in a nuclear 
weapon, emit fast neutrons, while thermal neutrons result after fast 
neutrons have lost much of their energy by interacting with materials. 
Gamma radiation, which is similar to high-energy x-ray radiation and 
is emitted by a variety of sources, can be present where neutron 
detectors are used and can cause false positive results for the 
neutron detectors.[Footnote 8] Radiation portal monitors incorporate 
both gamma radiation detectors and neutron radiation detectors.

Neutron detectors can take many forms, but how they detect neutrons is 
generally similar. Neutrons originate from a source containing a fast 
neutron emitter, such as fissionable nuclear material. When some of 
these neutrons come in contact with a nearby neutron detector (see 
figure 2.1), they first strike a component of the detector called the 
moderator, which is designed to slow the fast neutrons so they can be 
more readily detected. This is an important component because neutron 
detectors are designed to detect thermal neutrons because the 
likelihood that a neutron is absorbed increases as the energy of the 
neutron decreases; the moderator will also absorb or reflect some 
neutrons. After passing through the moderator, the neutron strikes the 
detector component that contains a conversion material, such as helium-
3. 

A neutron absorption reaction takes place in the conversion material 
and emits energetic charged particles that interact with the rest of 
the detector to generate a signal, which is collected and processed to 
determine if it was caused by a neutron reaction or if it is a false 
positive due to other causes, such as background radiation (e.g., 
ambient neutron or gamma radiation due to natural sources, such as 
cosmic rays or minerals in soil). Carefully designed neutron detectors 
can minimize the likelihood of false positive signals while maximizing 
the likelihood that a neutron will interact with the conversion 
material with detectable results. Because false positive signals often 
differ from a signal created by a neutron absorption reaction--for 
example, by being a smaller, lower amplitude signal--improved signal 
processing can further decrease the likelihood of false positives. 

Figure 2.1: Neutron detector operation: 

[Refer to PDF for image: illustration] 

Depicted on the illustration: 

Neutron source: 
Misses detector; 
Passes through detector without detection; 
Signal collection. 

Fast neutron; 
Slow/thermal neutron; 
Background radiation. 

Detector:
Moderator; 
Conversion material; 
Signal processing. 

Source: GAO. 

Note: An animated depiction may be accessed at [hyperlink, 
http://www.gao.gov/products/GAO-11-753]. 

[End of figure] 

2.2 Three main categories of neutron detectors:

Though specific neutron detector designs can vary, based on the way in 
which the conversion material is arranged and how the neutron 
absorption reaction products are detected, they can typically be 
classified into one of three main categories of neutron detectors: 
proportional, scintillator, and semiconductor detectors.

Proportional detectors use a gas to amplify the charge from the 
original charged particles generated by a neutron absorption reaction 
in the conversion material--the amplified charge is proportional to 
the original charge. A helium-3 proportional detector uses helium-3 
gas as both the conversion material and for the gas amplification. 
Other proportional detectors use a layer of solid material as a 
conversion material for the detector with argon gas that provides the 
charge amplification. These detectors are sealed gas-filled tubes with 
electronic connections. 

Figure 2.2 shows an example of proportional detector tubes used in 
large-area detectors--helium-3 tubes used in proportional detectors 
are commercially available in diameters from about 0.4 to 2 inches (10 
to 50.8 mm) and lengths from about 2.5 to 78.5 inches (63.5 to 1994 mm).

Figure 2.2: Tubes used in a proportional detector for large-area 
detectors: 

[Refer to PDF for image: illustration] 

Source: Oak Ridge National Laboratory. 

[End of figure] 

Scintillator detectors use solid or liquid scintillating materials, 
which are materials that emit light when struck by an incoming 
particle. In scintillator-based neutron detectors, the conversion 
material is placed in, on, or near the scintillator. When neutrons are 
absorbed by the conversion material, the resulting charged particles 
deposit energy in the scintillating material, which causes the 
scintillator to emit light that can be converted to an electric signal.

Semiconductor detectors consist of semiconductor chips with conversion 
material that can be incorporated into the chip, applied in a flat 
layer on the chip, or applied to a three-dimensional structure on the 
chip--these can be referred to as bulk/intrinsic semiconductor, 
coated/ layered semiconductor, and three-dimensional semiconductor 
detectors, respectively. The charged particles from a neutron 
absorption reaction in the conversion material deposit energy in the 
semiconductor, creating an electric signal.

The three main categories of neutron detectors--proportional, 
scintillator, and semiconductor--can use isotopes other than helium-3 
as conversion material to absorb and detect neutrons. Boron-10 and 
lithium-6 are the most common alternative isotopes to helium-3 for use 
in neutron detectors, and have greater natural abundance than helium- 
3. DHS considers both boron-10 and lithium-6 to be in sufficient 
supply for neutron detectors needed for future radiation portal 
monitor deployments. Furthermore, according to DOE officials, the U.S. 
stockpile of lithium-6 is sufficient to meet neutron detector demand. 
Both boron-10 and lithium-6 are export-controlled materials, meaning 
that licenses are required when exporting boron or lithium enriched in 
these isotopes to certain countries. See Appendix II for additional 
information about boron-10 and lithium-6.

2.3 Using helium-3 in neutron detection applications:

Beginning in the 1980s, when helium-3 became available to DOE's 
Isotope Development and Production for Research and Applications 
Program (Isotope Program) to sell, demand for helium-3 for use in 
neutron detectors grew, in part, because of the unique characteristics 
of detectors that use helium-3.[Footnote 9] The characteristics that 
led these detectors containing helium-3 to become, according to 
experts, the "gold standard" for neutron detection include:[Footnote 10]

* high neutron detection efficiency--the likelihood that a helium-3 
neutron detector will absorb a neutron and produce a detection signal;

* good gamma radiation discrimination--the ability to minimize false 
positives by determining whether a signal is due to neutron radiation 
or from gamma radiation;

* nontoxicity--helium-3 gas is nontoxic, so neutron detectors 
containing it do not pose a health hazard; and:

* low cost--before the shortage, helium-3 ranged in cost from about 
$40 to $85 per liter, so neutron detectors containing it were low in 
cost compared to alternatives.[Footnote 11]

One type of neutron detector that uses helium-3, the large-area 
detector, is used by scientific research programs. For example, at ORNL,
large-area detectors at SNS are used for neutron scattering 
experiments and in materials research for a variety of applications. 
These large-area detectors typically have a surface area of about 160 
to 430 square feet (15 to 40 square meters) and can each require 
hundreds to thousands of liters of helium-3.[Footnote 12] They do not 
have a single set of established requirements because each detector 
could be unique and designed for a specific research purpose. However, 
an international collaboration of science facilities that builds and 
uses such detectors has developed guidelines for the development of 
large-area detectors. For example, they should be available with 
spatial and time resolution and be capable of operation in ultra-low 
temperatures and in a vacuum.[Footnote 13] They should also be 70 
percent efficient at detecting thermal neutrons, with gamma radiation 
discrimination of less than 10-6 (Technical Working Group 2010a). 
[Footnote 14] Section 7.3 has additional information on neutron 
detector requirements. 

Another type of neutron detector containing helium-3 is a component in 
radiation portal monitors, which use the gas for security 
applications. These radiation portal monitors are used to scan cargo 
at ports and border crossings for fissionable nuclear materials and 
radioactive materials, which may be used for nuclear weapons, by using 
both gamma radiation and neutron radiation detectors. Radiation portal 
monitors use neutron detectors to search for fast neutrons emitted by 
certain fissionable nuclear materials. Such nuclear materials could be 
shielded by water or other hydrogen-containing material in the vehicle 
to try to block neutrons from being detected, but because neutrons are 
generally difficult to block, some will still reach the detector. Such 
radiation portal monitors contain neutron detectors with a surface 
area of about 7.5 square feet (0.7 square meters) and require about 44 
liters of helium-3 each. 

Radiation portal monitors are acquired and deployed in large numbers, 
and their neutron detector components have established requirements. 
These requirements define: (1) performance requirements, including 
specifications such as neutron detection efficiency and gamma 
radiation discrimination; (2) environmental requirements, specifying 
conditions (such as temperature, humidity, rain, and electric 
discharge) the neutron detector must operate under; and (3) system- 
level requirements, specifying how the neutron detectors must work 
within the overall radiation portal monitor system. For example, DHS's 
radiation portal monitor requirements include DHS-defined requirements 
as well as incorporate standards from the American National Standards 
Institute (ANSI).[Footnote 15] 

There are three primary federal agencies that acquire radiation portal 
monitors:

* In DHS, the Domestic Nuclear Detection Office (DNDO) acquires 
radiation portal monitors and U.S. Customs and Border Protection (CBP) 
operates them. DHS has deployed about 1,400 radiation portal monitors 
and is currently revisiting the additional number needed; it had plans 
to deploy about 700 more through 2015 to scan vehicles and cargo at 
domestic ports and border crossings.[Footnote 16]

* In DOE, the Second Line of Defense (SLD) program acquires radiation 
portal monitors and deploys them overseas to be operated by the host 
countries. According to DOE officials, SLD has deployed about 2,000 
radiation portal monitors and plans to deploy about 2,900 more through 
2018 to scan cargo at ports overseas.[Footnote 17]

* In DOD, the Guardian program acquires radiation portal monitors to 
scan vehicles and cargo entering some military facilities. According 
to DOD officials, as of March 2011, Guardian has deployed 24 radiation 
portal monitors that use helium-3 and has plans to deploy an 
additional 12 using an alternate neutron detector technology to 
military facilities worldwide. 

Besides large-area detectors and radiation portal monitors, neutron 
detectors using helium-3 are used in several other applications. 
Smaller and intermediate-sized detectors, such as handheld and 
backpack detectors, are used for mobile security-related applications. 
Each of these detectors uses a small quantity of helium- 3, as 
compared to radiation portal monitors, with a total annual demand 
across the U.S. government from 2011 to 2015 of about 7,000 liters of 
helium-3. Industrial uses include measuring the moisture content in 
rock and soil in support of activities like road construction. The oil 
and gas industry uses neutron detectors in exploratory drill shafts to 
determine the likelihood of oil or gas. Annual demand for helium-3 for 
moisture gauges is estimated at about 500 liters, and annual demand 
for oil and gas exploration is estimated at about 1,000 liters. Figure 
2.3 and Figure 2.4 illustrate the relative scales of different neutron 
detectors. 

Figure 2.3: Neutron detectors--a large-area detector, radiation portal 
monitors, and a handheld detector: 

[Refer to PDF for image: illustration] 

Source: GAO. 

[End of figure] 

Figure 2.4 Neutron detectors—a large-area detector. 

[Refer to PDF for image: illustration] 

Source: GAO. 

[End of figure] 

2.4 Production of and demand for helium-3: 

Helium-3 is a rare isotope of helium; naturally occurring helium-3 
constitutes a few parts per million of helium gas (the rest of the 
helium gas is composed of the common helium-4 isotope) (Coursey et al. 
2010). Today’s U.S. supply of helium-3 comes from the radioactive 
decay of tritium[Footnote 18] in the U.S. tritium stockpile, which is 
maintained by the National Nuclear Security Administration (NNSA). 
[Footnote 19] Helium-3 can also be extracted from natural sources of 
helium gas, such as subterranean natural gas deposits, but these 
sources have not been pursued commercially because extracting the very 
low concentrations of helium-3 has not been economically viable. 

Until 1988, tritium was manufactured to support the U.S. nuclear 
weapons program because it is a key component used to enhance a weapon’s
power. In maintaining the tritium stockpile, NNSA removes the helium-3 
that accumulates as tritium decays, because the helium-3 can diminish 
the effectiveness of the nuclear weapons. In the past, NNSA and its 
predecessor agencies considered helium-3 to be a waste product of the 
weapons program and vented it to the atmosphere, but from about 1980 
through 1995, and again from 2003 through 2008, those agencies 
provided helium-3 to DOE’s Isotope Program to sell.[Footnote 20] The 
minimum price for helium-3 was set to recover the cost of extracting 
it from the tritium and the administrative cost of selling it, which 
until 2009 typically ranged from $40 to $85 per liter. 

Since the end of the Cold War, the United States has been reducing its 
nuclear weapons stockpile, resulting in less tritium and, therefore, 
less helium-3. Meanwhile, demand for helium-3 rose over the past 10 
years primarily because it was used increasingly in neutron detectors 
for research and security applications. In 2008, the U.S. government 
abruptly learned that it faced a severe shortage of helium-3 because 
of this reduction in supply and increase in demand. In 2009, the 
National Security Staff established an interagency policy committee 
consisting of officials from Commerce, DHS, DOE, and the Department of 
State to address the helium-3 shortage.[Footnote 21] This policy 
committee established criteria and a process for allocating the 
limited supply of helium-3 to government and nongovernment customers, 
reducing the amount of helium-3 available for large-area detectors and 
radiation portal monitors.[Footnote 22] According to officials, SNS, 
which has plans to expand in 2018 by adding a second instrument hall, 
will be able to support about 25 additional large-area detectors; 
alternative neutron detectors will be needed for these new detectors. 
Radiation portal monitor deployments have not yet been affected by the 
helium-3 shortage, as according to agency officials, DHS and DOE have 
sufficient radiation portal monitors using helium-3 awaiting 
deployment to maintain their deployment plans through 2011 and 2013, 
respectively, and DOD’s Guardian—-a relatively small radiation portal 
monitor program-—has also met its deployment plan.

Figure 2.5 Helium-3 demand and annual U.S. production, 2011–18, as 
projected in 2009 and 2011: 

[Refer to PDF for image: vertical bar graph] 

Projected annual U.S. production of helium-3: 8,000-10,000 liters. 

Fiscal year: 2011; 
U.S. helium-3 demand as projected in 2009: 71,216; 
U.S. helium-3 demand as projected in 2011: 13,644. 

Fiscal year: 2012; 
U.S. helium-3 demand as projected in 2009: 85,085; 
U.S. helium-3 demand as projected in 2011: 13,228. 

Fiscal year: 2013; 
U.S. helium-3 demand as projected in 2009: 90,214; 
U.S. helium-3 demand as projected in 2011: 12,292. 

Fiscal year: 2014; 
U.S. helium-3 demand as projected in 2009: 86,052; 
U.S. helium-3 demand as projected in 2011: 12,445. 

Fiscal year: 2015; 
U.S. helium-3 demand as projected in 2009: 98,397; 
U.S. helium-3 demand as projected in 2011: 11,227. 

Fiscal year: 2016; 
U.S. helium-3 demand as projected in 2009: 42,550; 
U.S. helium-3 demand as projected in 2011: 11,236. 

Fiscal year: 2017; 
U.S. helium-3 demand as projected in 2009: 33,050; 
U.S. helium-3 demand as projected in 2011: 16,032. 

Fiscal year: 2018; 
U.S. helium-3 demand as projected in 2009: 101,000; 
U.S. helium-3 demand as projected in 2011: 18,816. 

Source: GAO analysis of information from the interagency policy 
committee.

[End of figure] 

NNSA estimates that it will make available about 8,000-10,000 liters 
of helium-3 per year for distribution, less than the demand created by 
neutron detectors and other applications. To address this shortage, 
the policy committee eliminated allocations of helium-3 for domestic
radiation portal monitors beginning in fiscal year 2010 because, 
according to committee documents, it had determined that alternatives
to helium-3 for neutron detectors in radiation portal monitors would 
suffice (GAO 2011). To further address this shortage, 10 research 
facilities around the world (including DOE’s SNS and several 
facilities in Europe and Japan) that have plans to deploy large-area 
detectors to support their research programs have agreed to form an
international collaboration to coordinate their development of 
alternative detectors using a multi-year development and testing plan. 
[Footnote 23] The two U.S. member facilities of this international
collaboration—NIST and ORNL—are conducting some testing of alternative 
detectors and will likely benefit from the knowledge gained from the 
testing conducted at other facilities. 

The policy committee’s allocation process, along with efforts by the 
international collaboration of neutron science facilities, has reduced 
the projected demand for helium-3. Figure 2.5 illustrates the total 
estimated demand for helium-3 as projected in 2009, before steps were 
taken to reduce demand, and as projected in 2011, after the policy 
committee and others took measures to better align demand for helium-3
with its available supply. The substantial difference between the 
helium-3 demand projections in 2009 and 2011 is largely based on the 
assumed availability of alternative neutron detector technologies used 
for neutron detectors in large-area detectors (for science) and 
radiation portal monitors (for security). 

Projected demand still exceeds the expected annual U.S. production of 
helium-3, but some inventory of the gas remains-—about 31,000 liters 
in February 2011, but the amount changes daily-—and the feasibility of 
utilizing other sources of helium-3 is being explored by U.S. agencies 
(GAO 2011). Figure 2.6 illustrates the projected demand for helium-3 
by application category, as projected in 2011-—because the interagency 
policy committee had eliminated allocations of helium-3 for domestic 
radiation portal monitors beginning in fiscal year 2010, the demand 
for helium-3 in figure 2.6 does not include demand due to radiation 
portal monitors. 

Figure 2.6 Helium-3 demand by application, 2011-18, as projected in 
2011: 

[Refer to PDF for image: 4 vertical bar graphs] 

Projected annual U.S. production of helium-3: 8,000-10,000 liters. 

Neutron detectors in security, excluding neutron detectors for 
radiation portal monitors, but including portable neutron detectors: 
Liters of helium-3: 
Fiscal year 2011: 9,252; 
Fiscal year 2012: 7,713; 
Fiscal year 2013: 6,782; 
Fiscal year 2014: 6,924; 
Fiscal year 2015: 5,696; 
Fiscal year 2016: 5,696; 
Fiscal year 2017: 5,287; 
Fiscal year 2018: 5,046. 

Neutron detectors in science, including large-area and smaller 
detectors: 
Liters of helium-3: 
Fiscal year 2011: 315; 
Fiscal year 2012: 304; 
Fiscal year 2013: 300; 
Fiscal year 2014: 310; 
Fiscal year 2015: 320; 
Fiscal year 2016: 320; 
Fiscal year 2017: 5,500
Fiscal year 2018: 8,500. 

Other research applications and industry applications, such as oil and
gas exploration: 
Liters of helium-3: 
Fiscal year 2011: 2,091; 
Fiscal year 2012: 3,191; 
Fiscal year 2013: 3,191; 
Fiscal year 2014: 3,191; 
Fiscal year 2015: 3,191; 
Fiscal year 2016: 3,200; 
Fiscal year 2017: 3,225; 
Fiscal year 2018: 3,250. 

Other U.S. government-funded applications that use small quantities
of helium-3, including NASA-associated missions: 
Liters of helium-3: 
Fiscal year 2011: 1,986; 
Fiscal year 2012: 2,020; 
Fiscal year 2013: 2,020; 
Fiscal year 2014: 2,020; 
Fiscal year 2015: 2,020; 
Fiscal year 2016: 2,020; 
Fiscal year 2017: 2,020; 
Fiscal year 2018: 2,020. 

Source: GAO analysis of information from the interagency policy 
committee. 

[End of figure] 

[End of section] 

3. Alternative neutron detector technologies are currently available: 

Three different neutron detector technologies are currently available 
that utilize different technologies and different isotopes, and 
additional testing is being performed to determine their effectiveness 
at meeting the requirements for use in large-area detectors and 
radiation portal monitors. We determined the maturity of these 
technologies for use in radiation portal monitors by reviewing the 
available test results and estimating the TRLs of these technologies. 

3.1. Three current alternative technologies may meet requirements for 
large-area detectors and radiation portal monitors: 

Three alternative neutron detector technologies, currently available 
and in use for other detector applications, were identified by agency 
officials and experts as potentially meeting the requirements for 
large-area detectors and radiation portal monitors, although they have 
not yet been fully tested for these applications. The three 
technologies—-boron-lined proportional detectors, boron trifluoride 
(BF3) proportional detectors, and lithium-6 scintillators-—have
characteristics similar to helium-3 detectors, based on testing done 
to date. Each is described in table 3.1 and in the text below. 

3.1.1. Boron-lined proportional detectors: 

Several vendors produce boron-lined proportional detectors with 
different designs, but they are all sealed tubes with boron-10 placed 
in a thin layer on the interior surface of the tube to form the 
conversion material; the tube is then filled with a mix of gases. 
Similar to helium-3 tubes, these tubes are typically less than 1 to 2 
inches in diameter (25.4 to 50.8 mm) and have an appearance similar to 
those depicted in figure 2.2. A variation of this technology, known as 
boron-lined straw tubes, uses thin tubes with very small diameters 
(about 1/6 inch, or 4.2 mm). The boron-lined proportional detector 
tubes typically are about 10 to 15 percent as efficient at detecting 
neutrons as a helium-3 tube. Boron-lined tubes are used in arrays of
tubes to achieve detector efficiency comparable to a neutron detector 
using a single helium-3 tube. (Ginhoven et al. 2009; Kouzes et al. 
2010a; Woodring et al. 2010) 

The international collaboration of science facilities has identified 
boron layer detectors-—of which boron-lined proportional counters are
a type-—as an alternative neutron detector technology that could be 
tested and used for large-area detectors by 2014. It notes that
the boron-lined straw tubes are promising but expensive, although 
options may exist for making them more cost-effective. One of the
international collaboration’s working groups is looking at a variety 
of configurations for Boron-lined counters and will be studying 
different coating techniques, tube shapes and sizes, and performance 
characteristics (Technical Working Group 2010b). 

Table 3.1 Key characteristics of alternative neutron detector 
technologies for use in radiation portal monitors: 

Technology: Boron-lined proportional detectors; 
Expected cost[A]: High; 
Meets required detection efficiency?[B] Yes; 
Meets required gamma radiation discrimination?[C] Yes; 
Factors to consider: Boron-10 is an export controlled material. 

Technology: Boron trifluoride (BF3) proportional detectors; 
Expected cost[A]: Low; 
Meets required detection efficiency?[B] Yes; 
Meets required gamma radiation discrimination?[C] Yes; 
Factors to consider: Hazardous material. 

Technology: Lithium-6 scintillators; 
Expected cost[A]: High; 
Meets required detection efficiency?[B] Yes; 
Meets required gamma radiation discrimination?[C] Yes; 
Factors to consider: Lithium-6 is an export controlled material. 

Source: GAO analysis of agency documents and test results from 
national laboratories. 

Note: This table addresses tests using a particular technology as 
incorporated into a radiation portal monitor system and does not 
address testing in non-portal monitor systems, such as vehicle-
portable or backpack neutron detectors. 

[A] For the purposes of this comparison, the cost of acquiring a 
single neutron detector module for one radiation portal monitor panel
was considered, with “low cost” being less than $15,000, and “high cost”
being more than $15,000, based on estimates provided by detector 
manufacturers and by DHS officials. This cost comparison does not 
include any potential differences in lifecycle costs due to 
maintenance, safety, etc. 

[B] Required detection efficiency for neutron detectors in radiation 
portal monitors is 2.5 counts per second per nanogram of
Californium-252 in a defined testing geometry. 

[C] Required gamma radiation discrimination in radiation portal 
monitors is 10-6 in a defined gamma radiation field. 

[End of table] 

3.1.2. Boron trifluoride gas proportional detectors: 

Boron trifluoride (BF3) gas is composed of fluoride atoms and boron 
atoms—the boron in BF3 is enriched in boron-10, which allows BF3 to be 
used as a conversion material. BF3 gas proportional detectors were 
widely used as neutron detectors before helium-3 became a commonly 
used conversion material, but they would still require testing in 
specific systems for large-area detectors and radiation portal 
monitors. BF3 proportional detectors are similar in construction to 
helium-3 proportional detectors, but contain BF3 gas instead of helium-3
gas. According to test results, BF3 tubes are about 30 to 50 percent 
as efficient at detecting neutrons as helium-3, but multiple tubes can 
achieve the desired detector efficiency, and BF3 detectors can provide 
better gamma discrimination than helium-3 detectors (Knoll 2000; 
Kouzes et al. 2009b). These tubes have an appearance similar to the 
detector tubes shown in figure 2.2. 

Neutron detectors using BF3 are the least expensive of the three 
alternatives for acquisition purposes, but BF3 is a toxic material 
that must be handled and shipped in accordance with Department of 
Transportation regulatory requirements.[Footnote 24] Industry 
representatives and national laboratory scientists have expressed
concern about its use, because of both the transportation issues 
entailed in regulatory requirements and possible exposure to the public
in the event of a leak. While a sealed BF3 tube would pose little risk 
by itself, damage to the tube could result in a leak of the toxic 
material, although the tubes are filled to slightly less than 
atmospheric pressure, potentially mitigating the effects of a leak. 

The international collaboration of science facilities has identified 
BF3 as the “most direct and probably the fastest way” to replace 
helium-3 in large area detectors and expressed concern about its 
toxicity (Technical Working Group 2010a).[Footnote 25] According to an 
international collaboration document, the collaboration is exploring 
the safety requirements associated with using BF3 in detectors, in 
addition to whether the detectors can provide adequate spatial 
resolution. Concern about the toxicity of BF3 may limit its use in 
radiation portal monitors because of potential exposure of the public 
to BF3 in the event of a leak—one vendor has rejected the use of BF3 
for use in radiation portal monitors because of such concerns. According
to DNDO, to support the evaluation of all possible alternatives, the 
agency is supporting work to improve radiation portal monitor designs 
to mitigate the risks of a BF3 leak from a radiation portal monitor. 
For example, a DNDO requirements document stated that the hazards 
posed by BF3 could be mitigated by requiring that a radiation portal 
monitor using BF3 should have multiple layers of containment, material 
to absorb the gas in the event of a leak, and an improved detector 
housing to minimize the likelihood of a leak (DHS 2010). A decision to 
use BF3 in radiation portal monitors will need to consider the risks 
associated with using it, as well as any increased costs associated 
with implementing additional safeguards and compliance with 
regulations governing BF3 handling and shipping. 

3.1.3 Lithium-6 scintillator detectors: 

Several vendors produce lithium-6 scintillators with different 
designs. One type uses lithium-6 zinc sulfide (Li-6/ZnS) scintillating 
material coated on fiber optics. In this design, an incoming neutron 
is absorbed by the lithium-6, creating charged particles that generate 
light in the scintillating material that is then detected by photo-
detectors via the fiber optics (Kouzes et al. 2010b). The photo-
detectors generate a signal that undergoes signal processing to 
determine the source of the scintillation light. The other type of
lithium-6 scintillator uses lithium-6 embedded directly into a 
scintillating glass fiber (Ginhoven et al. 2009). Figure 3.1 shows a 
detector panel under construction with these fibers arranged in a flat 
sheet for use in a large-area detector, which requires spatial 
resolution. These types of detectors, when assembled for use in 
detectors that do not require spatial resolution, such as radiation 
portal monitors, will be simpler in design than the panel in figure 
3.1, because a grid of fibers will not be necessary. 

Figure 3.1: A Li-6/ZnS scintillator detector module for SNS: 

[Refer to PDF for image: photograph] 

Note: This detector module is for use in a large-area detector and 
provides spatial resolution by creating a grid of fibers; the location 
of a neutron absorbed by this detector will therefore be known by 
identifying which fiber scintillates. 

Source: Oak Ridge National Laboratory. 

[End of figure] 

3.2. Testing process for the three alternative technologies for use in 
large-area detectors: 

The international collaboration of science facilities agreed upon a 
development and test process that will determine the suitability of 
alternative detector technologies for large-area detectors. The 
participating U.S. organizations are supporting the collaboration’s 
work with boron-lined proportional detectors and lithium-6 
scintillators. According to collaboration planning documents, the 
knowledge gained by those efforts and by work done by the other 
members of the international collaboration will help inform both large-
area detector design and the design of smaller research-oriented 
neutron detectors using alternative technologies. According to the 
collaboration planning documents, NIST will be examining some of the 
light detection equipment used in lithium-6 scintillator detectors, and
ORNL will be developing a lithium-6 detector and will test the 
performance of a boron-10 lined straw tube detector (Technical Working 
Group 2010b). Because of the unique nature of each large-area detector 
and the long design process associated with building such detectors, the
planning documents indicate that this process will continue for the 
next several years and will involve the development of demonstration
detectors using each currently available alternative technology under 
consideration (Technical Working Group 2010a): 

* Boron-lined proportional detectors-—the international collaboration 
plans to demonstrate that boron-10 detectors can perform nearly as 
well as helium-3, to optimize the design and fabrication of boron-10
detectors, and to demonstrate the feasibility of large scale 
detectors. International collaboration members reported that one small 
demonstrator detector using boron-lined proportional detectors is now 
being built and that they anticipate that more complete demonstrator
detectors may become available from March 2012 to March 2014 
(Technical Working Group 2010a). 

* Boron triflouride proportional detectors-—the international 
collaboration plans to explore how to build and operate BF3 detectors in
compliance with safety rules, to assess their performance, and to 
build and evaluate a fullscale demonstrator detector. The international
collaboration plans to have a large-area prototype, with demonstration 
tests, by March 2012 (Technical Working Group 2010a). According to a 
member of the international collaboration, current testing of a 
prototype shows that BF3-based positional sensitive detectors have 
proper position resolution, gamma discrimination, and theoretical 
detection efficiency. 

* Lithium-6 scintillators—-the international collaboration identified 
scintillators with fiber optic light guides as a potential replacement 
technology, and collaboration member facilities will conduct testing 
to determine the suitability of lithium-6 scintillators. The 
collaboration has several concerns about these scintillators-—for 
example, gamma radiation discrimination may not be good enough, and 
the detector may not be able to count neutrons at a high enough rate. 
Furthermore, the materials used may not be suitable for the vacuum 
environment experienced by large-area detectors, and the high cost of 
these detectors remains a concern. The international collaboration 
plans for 2 to 2-1/2 years of detector development and evaluation in 
the participating facilities, with as much as another year to conduct 
another iteration of evaluation for the detectors if needed, with a 
final year to transfer techniques to industry, for a total of 3 to 4-
1/2 years (Technical Working Group 2010a). 

According to SNS officials, because the helium-3 shortage occurred 
after the construction of SNS’s large-area detectors was completed or 
well under way, SNS does not have an immediate need for additional 
helium-3; however, a planned SNS expansion will need alternative 
neutron detectors in 2018. Some new large-area detectors may use 
alternative technologies rather than helium-3; the planned development 
and testing of these alternative technologies can provide additional 
information regarding the characteristics of the alternatives, 
informing decisions on what technology to utilize. This testing 
appears to be appropriate to help mitigate the effects of the shortage 
on U.S. neutron science efforts—-any new detectors that can utilize an 
alternative technology will help reduce demand for helium-3. Other 
detectors may have requirements for very high neutron detection 
efficiencies that can only be satisfied by using helium-3. In such
cases, a decision to use the limited supply of helium-3 for these 
detectors or to accept reduced performance using an alternative 
technology will need to be made based on the anticipated costs and 
benefits of each option. 

3.3. Testing process for the three alternative technologies for use in
radiation portal monitors: 

According to officials from the acquisition agencies (DNDO, DOE, and 
DOD), the suitability of alternative technologies for radiation portal 
monitors will be determined by a series of tests the acquisition 
agencies will conduct. This process, as DNDO describes it, is outlined 
in figure 3.2 and in the text below. 

After a vendor develops and tests a detector, DNDO may direct certain 
national laboratories (or other laboratories qualified to do so) to 
conduct performance testing, verifying that the detector satisfies 
performance requirements (described in section 7.3). If the performance
testing is successful, the laboratories will then conduct 
environmental and system integration testing to verify that the other 
requirements are satisfied. The government then field tests the 
radiation portal monitors using the alternative neutron detector. 
Depending on when in this process DNDO makes an acquisition decision,
these tests may be used to inform an acquisition or as acceptance 
testing for radiation portal monitors following the acquisition. 

Figure 3.2: DHS’s steps to test radiation portal monitors using 
alternative neutron detectors: 

[Refer to PDF for image: illustration] 

Vendor develops and tests detector; 

Performance testing by qualified laboratories; 

Environmental and systems testing by qualified laboratories; 

Field testing of the portal system. 

Source: GAO analysis of DNDO information. 

[End of figure] 

Boron-10 lined proportional detectors, BF3 proportional detectors, and 
lithium-6 scintillators have undergone performance testing at PNNL, 
according to testing documents (Kouzes et al. 2009b; Kouzes et al. 
2010a; Kouzes et al. 2010b). Furthermore, DNDO awarded contracts to 
five vendors in February 2011 to acquire radiation portal monitors 
using alternative neutron detectors—-of these five vendors, one uses 
boron-10 lined straw tubes as its alternative neutron detector 
technology and the four others use lithium-6 scintillators of varying 
designs. DNDO is conducting performance tests and limited 
environmental tests on these detectors in 2011.[Footnote 26] In a 
separate effort, DNDO has completed performance, environmental, and
systems tests for one radiation portal monitor design using a boron-10 
lined proportional detector, and completed field testing of this
design in July 2011. Preliminary results of the field tests indicate 
this radiation portal monitor design successfully passed all test 
objectives and, according to agency officials, DNDO anticipates having 
this alternative available for deployment in early fiscal year 2012. 

DOD’s Guardian program, which deploys radiation portal monitors to 
scan vehicles and cargo entering military facilities, directed PNNL to 
verify the vendor-reported performance of one radiation portal monitor 
using a boron-10 lined proportional detector, according to DOD 
officials. DOD acquired 12 of these radiation portal monitors and, and 
according to these officials, plans to conduct field tests during 
deployment in 2011. 

DOE’s SLD program, which deploys radiation portal monitors overseas to 
help prevent nuclear smuggling, is currently proceeding to acquire 
radiation portal monitors that use alternative neutron detectors (GAO 
2011; GAO 2006). According to DOE officials, as part of the 
procurement, prototypes will undergo performance tests at a national 
laboratory. Following award of the contract but prior to deployment of 
the radiation portal monitors, they will undergo exhaustive testing at 
a national laboratory. 

3.4. Maturity of radiation portal monitors using the three alternative 
detector technologies: 

To determine the maturity of radiation portal monitors using the 
currently available alternative neutron detector technologies, we have 
estimated the TRL for radiation portal monitors using each technology. 
TRLs are commonly used to assess the maturity of a technology for a 
specific application (see section 7.4 for additional details on TRLs) 
and range from TRL 1 to TRL 9: TRL 1 indicates a technology for which 
only the basic principles have been observed, and TRL 9 indicates a 
technology fully integrated into a system that has been fully tested and
demonstrated through successful operational use. A higher TRL 
indicates a system has better demonstrated suitability relative to a 
specific set of criteria, and a decision to proceed with an 
acquisition of the system will accordingly be lower risk. According to 
DNDO officials, radiation portal monitors using alternative neutron 
detectors will typically be transitioned to an acquisition group when 
the radiation portal monitors are at TRL 6 to 7. 

To estimate the TRLs, we used the TRL scale developed by the National 
Aeronautics and Space Administration (NASA) (NASA 2008). We use this 
scale because the NASA scale is the basis for other TRL scales, such 
as DOD’s TRL calculator. Furthermore, according to DNDO officials, a 
research directorate in DNDO typically uses the NASA TRL scale when it 
performs TRL assessments. Our TRL estimates are based on assessing the 
quality of the radiation portal monitor test objects containing 
alternative neutron detectors and comparing the performance of the 
test objects to DNDO radiation portal monitor requirements, using the 
current status of testing of alternatives for radiation portal 
monitors. These estimates do not represent the TRLs for each 
technology when integrated into other neutron detector systems, such 
as portable handheld or backpack detectors. Table 3.2 summarizes the 
testing that has been completed for radiation portal monitors. 

* Boron-lined proportional detectors-—based on performance testing of 
prototypes from multiple vendors at PNNL, this detector technology can 
provide the required sensitivity and gamma radiation discrimination. A
prototype from one vendor passed DNDO performance tests and limited 
environmental tests in summer 2011. Another vendor has produced a 
radiation portal monitor that is a finalized production model that, at 
the direction of DNDO, has passed all performance, environmental, and 
systems testing and has completed field testing in its intended 
operational environment (a CBP port of entry). As a result, we 
estimate this radiation portal monitor using a boron-lined 
proportional detector is at a TRL 7 and could advance to TRL 8 upon 
successful completion of field tests.[Footnote 27] Upon reaching TRL 
8, this radiation portal monitor will have been proven to work in its 
final form and under expected conditions and could be acquired for
deployment with reduced risk. 

* Boron triflouride gas proportional detectors—-based on performance 
testing conducted at PNNL, BF3 proportional detectors can provide a 
suitable replacement for helium-3 proportional detectors for radiation 
portal monitors. However, full tests of a BF3 radiation portal monitor 
prototype are still needed-—for example, environmental and systems 
testing—-and BF3 radiation portal monitors have yet to demonstrate 
performance under operational conditions. BF3 proportional detectors, 
while generally considered a mature technology for other detector 
applications, have not been integrated into mature prototypes of 
radiation portal monitors, and their testing for use in radiation 
portal monitors has demonstrated only basic performance capabilities. 
We estimate radiation portal monitors using BF3 proportional detectors 
are at a TRL 5 for use in radiation portal monitors; they could 
advance to higher TRLs upon the completion of additional testing and the
development of a more mature prototype. 

* Lithium-6 scintillator detectors-—based on performance testing of 
prototypes from multiple vendors at PNNL, some of the tested systems 
can provide the required neutron detection efficiency and gamma 
radiation discrimination. Prototypes from three different vendors 
passed DHS performance tests and limited environmental tests in summer 
2011. We estimate these radiation portal monitors using lithium-6 
scintillators are at a TRL 6 and could advance to a TRL 7 upon the 
successful completion of testing in an operational environment. Based 
on the duration of testing of the boron-lined proportional detector that
recently completed field tests, if DNDO decides to direct one of these 
prototypes to complete the remainder of the environmental tests and to 
undergo systems tests, such tests could require less than four months. 

According to DNDO, and supported by the testing of radiation portal 
monitors using alternative neutron detectors, it appears that a boron-
10 lined proportional detector will be available for use in early 
fiscal year 2012. BF3 detectors are under development at national 
laboratories at the direction of DNDO, but no vendor has yet made a 
prototype available for test and evaluation. Lithium-6 scintillator 
detectors may be available later in fiscal year 2012. 

Table 3.2 Testing of radiation portal monitors using alternative 
neutron detector technologies: 

Technology: Boron-lined proportional detector; 
Performance testing by qualified labs: Production models satisfied DHS
performance requirements in 2010; 
Environmental and systems testing by qualified laboratories: Full 
environmental and systems tests completed at national laboratories in 
2011; 
Estimated technology readiness level (TRL)[A]: 7. 

Technology: Boron trifluoride gas proportional detector; 
Performance testing by qualified labs: Laboratory prototypes satisfied
DHS performance requirements in 2009; 
Environmental and systems testing by qualified laboratories: None; 
Estimated technology readiness level (TRL)[A]: 5. 

Technology: Lithium-6 scintillator; 
Performance testing by qualified labs: Production prototypes satisfied 
DHS performance requirements in 2011; 
Environmental and systems testing by qualified laboratories: 
Production prototypes satisfied limited environmental requirements in 
2011[B]; 
Estimated technology readiness level (TRL)[A]: 6. 

Note: This table addresses tests using a particular technology as 
incorporated into a radiation portal monitor system and does not 
address testing in non-portal monitor systems, such as handheld or 
backpack neutron detectors. The test status is based on the particular 
design in each technology type that has successfully undergone the 
most testing. 

[A] TRLs based on the neutron detector prototype with the highest TRL 
in each technology classification. 

[B] Limited environmental tests: temperature, humidity, temperate and 
freezing rain, microphonics and impact, and electrostatic discharge

Source: GAO analysis of testing conducted by ORNL and PNNL.

[End of table] 

[End of section] 

4. Additional neutron detector technologies are being developed: 

Four federal agencies are funding research and development projects to 
develop alternative neutron detector technologies. According to agency 
officials and documents from the interagency policy committee formed 
to address the helium-3 shortage, Commerce, DNDO, DOD, and DOE are 
coordinating more than 30 different research and development projects in
industry, academia, and national laboratories that may result in 
alternative technologies for neutron detection applications-—funding 
for these projects was about $20 million in fiscal year 2010. Agency
documents indicate that some of these alternative technologies may be 
sufficiently advanced with laboratory tests to begin integration into
prototype detector systems in less than two years, with deployable 
detector systems following the integration of these technologies. 

These agency-funded projects are not limited to developing alternative 
neutron detectors for radiation portal monitors-—they are also 
developing neutron detectors for use in smaller, portable detectors, 
such as handheld or backpack detectors, to further reduce demand for
helium-3. Because the agencies are developing technologies for 
security applications that could have overlap in research program 
goals, they are acting to reduce duplication of effort by formally
coordinating the research and development projects through the 
interagency policy committee by sharing information on the projects
each agency is funding. According to agency officials, there is also 
informal coordination through contact between program managers at each 
agency. 

Our review of agency documents for these federally funded programs 
indicates that these research projects are working to improve the 
performance of existing technologies or to develop new forms of 
scintillator and semiconductor technologies, as well as to develop
new neutron detector technologies that do not fit within these 
categories. Table 4.1 provides examples of some of the technologies 
being developed. 

For scintillator detectors, some research and development projects are 
focused on optimizing the more mature technologies, like DOD and DNDO 
projects supporting the improvement of lithium-6 scintillators. Other 
programs are working toward developing new scintillator materials, 
like the DNDO-supported work on CLYC crystals.[Footnote 28] The 
development of new scintillator materials includes exploring families of
materials for scintillation properties, identifying detection 
capabilities, and developing techniques to fabricate the materials in 
quantities sufficient to support the development of prototype 
detectors. 

For semiconductor detectors, research and development projects are 
working to improve detection efficiencies and fabrication techniques. 
Coated/layered semiconductors, consisting of a layer of conversion 
material on an electronic chip, have neutron detection efficiencies
that are usually low compared to other types of detectors—-projects 
looking to improve semiconductor neutron detectors can improve 
efficiencies through methods such as creating three-dimensional 
surfaces to increase the amount of conversion material that can be 
applied to the chip. One such project is the DNDO-funded effort to 
develop semiconductors using boron-10 in 3-D pillar and trench 
structure. Other projects work to incorporate conversion material into
the semiconductor material itself, such as the DNDO-supported program 
to develop boron triselenide (B2Se3) detectors.[Footnote 29] While 
these projects often focus on developing ways to improve detection 
efficiencies, they also attempt to improve the fabrication techniques 
to produce new semiconductor detectors.

Table 4.1 Examples of alternative neutron detector technology programs. 

Detector type: Scintillator; 
Technology program[A]: Sodium iodide or cesium iodide scintillators 
with lithium-6; 
Research stage[B]: Early; 
Potential security application[C]: Portal: [Check]; 
Potential security application[C]: Portable: [Check]. 

Detector type: Scintillator; 
Technology program[A]: Zinc oxide with lithium-6, boron-10, or 
gadolinium coatings; 
Research stage[B]: Early; 
Potential security application[C]: Portal: [Check]; 
Potential security application[C]: Portable: [Check]. 

Detector type: Scintillator; 
Technology program[A]: CLYC (Cs2LiYCl6:Ce) scintillator crystals; 
Research stage[B]: Late; 
Potential security application[C]: Portal: [Empty]; 
Potential security application[C]: Portable: [Check]. 

Detector type: Semiconductor; Boron triselenide (B2Se3); 
Technology program[A]: 
Research stage[B]: Late; 
Potential security application[C]: Portal: [Empty]; 
Potential security application[C]: Portable: [Check]. 

Detector type: Semiconductor; 
Technology program[A]: Lithium-6 semiconductor compounds; 
Research stage[B]: Early; 
Potential security application[C]: Portal: [Empty]; 
Potential security application[C]: Portable: [Check]. 

Detector type: Semiconductor; 
Technology program[A]: Boron loaded 3-D pillar and trench structured 
semiconductors; 
Research stage[B]: Late; 
Potential security application[C]: Portal: [Check]; 
Potential security application[C]: Portable: [Check]. 

Detector type: Other[D]; 
Technology program[A]: Water-based detector; 
Research stage[B]: Mid; 
Potential security application[C]: Portal: [Check]; 
Potential security application[C]: Portable: [Empty]. 

Detector type: Other[D]; 
Technology program[A]: Noble gas excimers; 
Research stage[B]: Mid; 
Potential security application[C]: Portal: [Check]; 
Potential security application[C]: Portable: [Check]. 

Source: GAO analysis of information from the interagency policy 
committee. 

[A] Examples are selected to convey the range of technologies under 
development. 

[B] ”Research stage” estimates how advanced the research and 
development program is, based on information from the funding
departments through the interagency policy committee. “Late” indicates 
the technology may become available for integration into
detector systems in less than 3 years; “Mid”, 3–5 years; “Early”, more 
than 5 years. 

[C] Potential applications are limited to radiation portal monitors 
and portable detectors (handheld or backpack) because the funding
agencies generally support neutron detector development for security 
applications. Information for “Potential application” is from
the funding departments through the interagency policy committee. 

[D] ”Other” indicates these programs are developing technologies that 
do not fit within the three main categories of neutron detector
technologies—-proportional, scintillator, and semiconductor detectors.

[End of table] 

Proportional detectors, scintillators, and semiconductors are the 
major types of neutron detectors, but there are other, less common 
detector types. A few agency research and development projects are 
developing these other types of detectors. For example, a DOE-funded
project is developing water-based neutron detectors—-when conversion 
material, such as gadolinium, is in water, the water can be used
as a neutron detector by observing the blue glow of Cherenkov 
radiation emitted when a neutron is absorbed by the conversion 
material.[Footnote 30] Cherenkov radiation is light emitted when a 
charged particle passes through a medium at a speed greater than the 
velocity of light in that medium (see figure 4.1)—in this detector, 
the neutron absorption results in fast electrons that generate this 
Cherenkov radiation. According to NIST officials, the agency is 
supporting the development of a neutron detector that observes the 
ultraviolet light emitted by noble gas excimers-—molecules in an 
excited electronic state-—that form after boron-10 or lithium-6
absorbs a neutron in a noble gas, such as argon or xenon. 

According to agency officials, after these technologies are explored 
in laboratory settings, they may become available for integration into
neutron detector designs. They could then provide additional options 
for neutron detector designs and help to further reduce helium-3 
demand. 

Figure 4.1. Cherenkov radiation illuminating reactor fuel assemblies 
immersed in water. 

[Refer to PDF for image: photograph] 

Source: Photo courtesy of U.S. Department of Energy. 

[End of figure] 

[End of section] 

5. Conclusions: 

Adopting alternative neutron detector technologies for research, 
security, and other applications is becoming increasingly important as 
the nation’s helium-3 supply continues to decrease. Since the helium-3 
shortage was first realized in 2008, federal agencies have 
collaborated to mitigate its effects by identifying or developing 
alternative neutron detector technologies that do not use helium-3. 
Based on performance tests, three alternative detector technologies—-
boron-lined proportional detectors, boron trifluoride proportional 
detectors, and lithium-6 scintillators-—appear to be potential 
replacement technologies for use in both large-area detectors and 
radiation portal monitors, although additional testing is under way. 

Successful integration of alternative technologies in large-area 
detectors and successful testing of these detectors will allow science 
programs requiring additional large-area detectors to construct them 
using the alternative technologies, minimizing the impact of the 
helium-3 shortage on their research. We estimate one alternative 
detector technology-—boron-10 lined proportional detectors-—is at TRL 
7 for use in radiation portal monitors, indicating such radiation 
portal monitors could be acquired and deployed with confidence that 
they will perform as required. Federal agencies should therefore be 
able to continue the deployment of radiation portal monitors with 
minimal additional program delays and with minimal use of additional 
helium-3. As these technologies are undergoing additional testing, 
federal agencies are funding the development of additional neutron 
detector technologies for security applications that may have broader 
application for research and industry and provide a greater range of 
neutron detector technologies to choose from. 

[End of section] 

6. External comments: 

6.1. Agency comments: 

We provided a draft of this report to the Secretaries of Commerce, 
Defense, Energy, and Homeland Security for their review and comment. 

Neither Commerce nor DOD provided written comments. DOD provided  
technical comments that we incorporated as appropriate throughout the 
report. 

DOE provided written comments, which are included in section 7.6. DOE 
generally agreed with the report but noted that BF3 is a proven, 
inexpensive, and reliable alternative with minimal development cost, 
that detectors containing BF3 use small quantities of the gas, which 
mitigates the impacts of leaks from such detectors, and that the 
Department of Transportation does not consider less than 1 gram of BF3
in portable instruments hazardous. We agree with these statements but 
note that while BF3 has been successfully used in other applications, 
detectors using BF3 must still undergo testing for the specific 
applications explored in this report to verify they can satisfy all 
applicable requirements in their expected operational environments.
Furthermore, the hazardous nature of BF3—-and the ways in which its 
effects can be mitigated—-will still need to be considered by agencies 
selecting a detector. DOE also noted the instruments at SNS will be 
less efficient if they use an alternative to helium-3. We have 
modified the report to reflect that while alternative technologies may 
be viable for some large-area detectors, others may be planned for 
with requirements for very high neutron detection efficiency that may 
only be satisfied by using helium-3. Such high performance detectors
will require a decision based on the cost-benefit analysis of using 
the limited supply of helium-3 or accepting the lower performance 
provided by an alternative technology. DOE provided technical comments 
that we incorporated as appropriate throughout the report. 

DHS provided written comments, which are included in section 7.7. In 
its comments, DHS further highlighted the role of its DNDO in 
addressing the helium-3 shortage and its interaction with the 
interagency policy committee that was formed to address the shortage.
DHS provided technical comments that we incorporated as appropriate 
throughout the report. 

6.2. Expert comments: 

We provided a draft of this report to our group of external experts 
for their review and comment—all eight responded (see section 7.1 for 
a description of this group’s contributions and section 7.5 for a list 
of the experts). The majority of them expressed general agreement with 
the draft, and none expressed disagreement. One reviewer expressed 
concern about the drafts balance between short- and long-term neutron
detector solutions. We note that this report focuses on alternative 
technologies currently available because they could provide the most
immediate reduction in demand for helium-3. The eight respondents also 
provided technical comments that we incorporated as appropriate 
throughout the report. 

[End of section] 

7. Appendices: 

7.1. Objectives, scope, and methodology: 

In this report, our objectives were to assess (1) what alternative 
neutron detectors are currently available and their effectiveness and 
(2) the status of alternative neutron detector technologies that are 
being researched for future availability. To address these objectives, 
we reviewed program documents from the various agencies, including 
test results for all the currently available alternatives, contract 
information, and proposals; product information from vendors; and 
scientific literature. We also attended two symposiums on alternative 
neutron detectors, including the 2010 Institute of Electrical and 
Electronics Engineers Nuclear Science Symposium, Medical Imaging 
Conference, and 17th International Workshop on Room-Temperature 
Semiconductor X-ray and Gamma-ray Detectors in Knoxville, Tennessee,
and the 2010 Symposium on Radiation Measurements and Application at 
the University of Michigan in Ann Arbor. We also visited the neutron 
detector test facilities at the Department of Energy’s (DOE) Pacific 
Northwest National Laboratory (PNNL), as well as DOE’s Spallation 
Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL), which 
relies on helium-3 in large-area detectors for conducting scientific
research. 

With the assistance of the National Academy of Sciences (NAS), we 
identified a group of eight experts from academia and federally funded 
research and development centers who had relevant expertise and 
knowledge in one or more of the following areas: (1) technology 
development, particularly with respect to neutron detectors; (2) 
nuclear physics and nuclear engineering; (3) neutron detector 
applications, including for security, science, and medicine. They 
helped us identify alternative neutron detector technologies currently 
available or being developed for future use in research and security 
applications. This group of experts also reviewed and commented on a 
draft of this report. 

For the first objective, we limited our assessment to alternative 
detectors for large-area detectors for science applications and for 
security applications, specifically radiation portal monitors. These 
two applications create the greatest demand for helium-3, and 
alternative detector technologies for these applications would 
therefore have the greatest impact on alleviating helium-3 demand. We 
further restricted our review to technologies that are “currently 
available”-—those that are already commercially available in other
neutron detection applications and available for integration into 
large-area detectors or radiation portal monitors. Accordingly, we
reviewed information from and interviewed officials at the two U.S. 
facilities participating in an international collaboration of facilities
using large-area detectors-—the Department of Commerce’s National 
Institute for Standards and Technology (NIST) and DOE’s ORNL. We 
interviewed officials at NIST and ORNL who worked with the 
international collaboration of large science facilities that are 
examining alternative technologies. We limited our discussions with 
members of this working group to U.S. members because of their knowledge
of how U.S. facilities would be affected. We reviewed the 
international collaboration’s plans for developing and testing 
detectors using alternative technologies, which included guideline
criteria for detectors. 

To determine what alternative neutron detectors are currently 
available for integration into deployable products for security 
applications, we interviewed and received documentation from officials 
in Commerce, DOD, DOE, and DHS; and officials from the national 
laboratories conducting work in this area: PNNL, ORNL, Los Alamos 
National Laboratory (LANL), and Sandia National Laboratories (SNL). To
determine the criteria for the effectiveness of neutron detectors used 
for radiation portal monitors, we reviewed the relevant American 
National Standards Institute (ANSI) standards and the PNNL-developed 
radiation portal monitor requirements used for procuring DHS’s 
radiation portal monitors. 

To determine the relative gamma radiation discrimination and neutron 
detection efficiency capabilities of the neutron-detecting isotopes 
helium-3, boron-10, and lithium-6, for each isotope we obtained the 
energy released by the neutron-absorbing reactions and the cross-section
of those reactions from the scientific literature. We used these 
values to compare the capabilities of these isotopes. To determine the 
performance characteristics of potential alternative detector 
technologies, such as their detection efficiencies or gamma radiation 
discrimination capabilities, we reviewed relevant research and testing
documentation from PNNL and ORNL on each potential technology we 
identified. We also interviewed detector developers and manufacturers 
producing the three technologies we identified in the first objective. 
We determined whether the alternative technologies have demonstrated 
capabilities that met or exceeded the primary performance requirements
of neutron detection efficiency and gamma radiation discrimination. 

To estimate the technology readiness levels (TRL) of radiation portal 
monitors using potential alternative detector technologies, we 
reviewed relevant research and testing documentation from DHS and the 
national laboratories on each technology that we identified in the 
first objective. We estimated the TRL of each currently available 
technology based on this research and testing documentation by 
determining, for each technology, the type of prototype that was 
tested (for example, whether it was a relatively immature laboratory 
system or a more mature production model), the testing that was 
performed (for example, whether it involved laboratory simulations of 
environments or field testing in operational environments), and the 
results of this testing, using the DHS radiation portal monitor 
requirements as a metric. We then compared each technology’s prototype
and testing status to the NASA TRL scale. We used this scale because 
it is the basis for other TRL scales, such as DOD’s TRL calculator. 
Furthermore, according to DNDO officials, DNDO typically uses the NASA 
TRL scale when it performs TRL assessments. Because multiple vendors 
have developed multiple designs using these alternative technologies, 
the TRL we estimate for each technology is the highest TRL we estimate 
for a design using that technology. We discussed our TRL estimates 
with DNDO officials, who generally concurred with our estimates based 
on our application of the NASA TRL scale, but who also noted that these
technologies have been successfully used for other neutron detector 
applications and might have higher TRLs when considered for these other
applications. 

For the second objective, we assessed technologies that are not 
readily available for any application, but are under development. We 
focused on technologies funded by Commerce, DHS, DOD, and DOE. The 
interagency policy committee addressing the helium-3 shortage 
identified these Departments’ programs as part of the committee’s work 
to mitigate demand for helium-3. To determine the status of 
alternative detector technologies being researched for future use, we 
interviewed and requested information from officials from Commerce, 
DHS, DOD, and DOE, and the national laboratories conducting work in 
this area—LANL, ORNL, PNNL, and SNL. We reviewed program documentation
on alternative technologies, including contract information, technical 
proposals, scientific literature, and program updates. 

We conducted our work from July 2010 to September 2011 in accordance 
with all sections of GAO’s quality assurance framework that are 
relevant to technology assessments. The framework requires that we 
plan and perform the engagement to obtain sufficient and appropriate
evidence to meet our stated objectives and to discuss any limitations 
to our work. We believe that the information and data obtained, and the
analysis conducted, provide a reasonable basis for any findings and 
conclusions in this product. 

7.2. Boron and lithium isotopes are alternatives to helium-3: 

The three main categories of neutron detectors-—proportional, 
scintillator, and semiconductor—-can use isotopes other than helium-3 
as conversion material to absorb and detect neutrons. Boron-10 and 
lithium-6 are the most common alternative isotopes to helium-3 for use 
in neutron detectors.[Footnote 31] These isotopes can be chemically 
combined with other elements to create molecules that also act as 
conversion materials. For example, boron-10 and fluorine can be used 
to create boron trifluoride gas, which can be used in sealed tubes in 
much the same way as helium-3 has been used in proportional detectors. 
DHS considers both boron-10 and lithium-6 to be in sufficient supply 
for neutron detectors needed for future radiation portal monitor 
deployments. Furthermore, according to DOE officials, the U.S. 
stockpile of lithium-6 is sufficient to meet neutron detector demand. 
Both boron-10 and lithium-6 are export-controlled materials, meaning 
that licenses are required when exporting boron or lithium enriched in
these isotopes to certain countries. 

The suitability of a detector for an application derives from both the 
characteristics of the isotope and the detector’s design. Factors such 
as how a neutron absorbing isotope is integrated into the conversion 
material, the arrangement of the moderator relative to the conversion
material, and the signal processing for the detector influence the 
detector’s characteristics. For example, while lithium-6 can have 
excellent gamma radiation discrimination, when it is used in a 
scintillator, the resulting detector must use good signal processing 
techniques to achieve good gamma radiation discrimination because 
scintillator materials are generally sensitive to gamma radiation. 
Table 7.1 compares the performance characteristics of the two 
alternative isotopes, with helium-3 included for comparison, and the 
categories of detectors in which they are typically used. 

Table 7.1 Three isotopes commonly used as conversion materials in 
neutron detectors: 

Isotope: Helium-3 (for comparison); 
Potential gamma radiation discrimination[A]: Good (0.76 MeV); 
Potential neutron detection efficiency[B]: Excellent (5,333 barns); 
Typical detector type: Proportional. 

Isotope: Boron-10; 
Potential gamma radiation discrimination[A]: Very good (2.3 MeV); 
Potential neutron detection efficiency[B]: Very Good (3,835 barns); 
Typical detector type: Proportional, scintillator, or semiconductor. 

Isotope: Lithium-6; 
Potential gamma radiation discrimination[A]: Excellent (4.78 MeV); 
Potential neutron detection efficiency[B]: Good (940 barns); 
Typical detector type: Scintillator or semiconductor. 

Source: GAO analysis based on agency documents and physical properties 
of these isotopes. 

[A] Qualitative assessments of the energies of the neutron absorption 
reaction products (which are fast-moving charged particles),
with energies measured in mega-electronvolts (MeV) and indicated in 
parentheses in this column. The qualitative assessments are
based on the energy released by a neutron interaction, but without 
consideration of additional factors such as the specifics of a
particular detector design. The higher the product energy, the more 
energy that the detector can use to determine if a neutron
is detected; thus, higher-energy products can prove beneficial in 
determining that a signal is not due to a low-energy source of
radiation, such as a gamma radiation source. We define “good” at 0.5 
to 2 MeV; “very good”, 2 to 4 MeV; and “excellent”, greater
than 4 MeV. 

[B] Qualitative assessments of the thermal neutron reaction cross-
section, which is a measure of how likely an incoming neutron will
be absorbed by a conversion material atom. The cross-section is 
measured in barns, where one barn is 10-28 square meters, and
is indicated in parentheses in this column. The higher the cross-
section, the more likely an absorption reaction can occur for an
incoming neutron, which is beneficial when considering a detector’s 
efficiency at detecting neutrons. We define “good” at 500 to
2000 barns; “very good”, 2000 to 4000 barns; and “excellent”, greater 
than 4000 barns. 

[End of table] 

7.3. Requirements for alternative neutron detectors for use in large-
area detectors and radiation portal monitors: 

Neutron detectors have several characteristics that are important in 
judging performance for particular applications, including large-area
detectors and radiation portal monitors. These include: 

* Neutron detection efficiency, used for both large-area detectors and 
radiation portal monitors, specifies how well a neutron detector 
should detect neutrons. It can be defined in different ways—-for 
example, for a specified detector size and shape, and with a specified
type of neutron source, the absolute neutron detection efficiency 
describes the number of neutrons a detector will count per second. 
Neutron detection efficiency can also be defined as the likelihood 
that any particular neutron passing through a detector will be 
detected (as opposed to not being detected at all)-—this is the 
intrinsic neutron detection efficiency. Large-area detectors need higher
neutron detection efficiencies, while neutron detectors used in 
radiation portal monitors can be effective with lower neutron detection
efficiencies. 

* Gamma radiation discrimination (or gamma radiation rejection) 
specifies the maximum fraction of incident gamma rays that a detector 
can falsely identify as neutrons. Both large-area detectors and 
radiation portal monitors use a requirement that the gamma radiation 
discrimination be less than 10-6, which indicates that less than one 
in a million gammas can result in a false positive neutron detection. 

* Gamma absolute rejection ratio for neutrons (GARRn), used for 
radiation portal monitors, specifies the neutron detector response in
the presence of both neutrons and gamma radiation. It is the ratio of 
the absolute neutron detector efficiency (defined above), as measured 
with simultaneous neutron and gamma radiation sources, to the absolute
neutron detection efficiency, as measured with only a neutron source. 
A GARRn of 1.0 would indicate these two measurements are the same and, 
therefore, that the gamma radiation source had no effect on the 
neutron detection efficiency of the detector. 

* Detector size and spatial resolution establish physical requirements 
for the detectors. Large-area detectors must have larger surface 
areas, while neutron detectors for radiation portal monitors must fit 
within a specified volume. Large-area detectors must have spatial 
resolution, which is the capability of a detector to determine where 
on a detector a neutron is detected. 

* Time resolution, used for large-area detectors, specifies how well a 
detector must determine when neutrons are detected. Large-area 
detectors must have a time resolution of a microsecond (one millionth 
of a second). 

* Environmental requirements determine what conditions a detector must 
operate under. For example, large-area detectors may be used at ultra-
low temperatures or in vacuum. Radiation portal monitors are deployed
outdoors in potentially harsh conditions, requiring operation in 
environments with high humidity, rain, ice, dust and sand, and seasonal
temperature variations. 

7.4. Technology readiness levels: 

Technology readiness levels (TRLs) are commonly used to assess the 
maturity of a technology for a specific application. They range from 
TRL 1 to TRL 9, and the levels, as defined by the National Aeronautics 
and Space Administration (NASA), are described in table 7.2. 
Terminology used with TRLs is described after table 7.2. 

Table 7.2: TRLs as defined by NASA: 

Technology readiness level: 1. Basic principles observed and reported; 
Description: Lowest level of technology readiness. Scientific research 
begins to be translated into applied research and development. Examples
might include paper studies of a technology’s basic properties; 
Hardware/software: None (paper studies and analysis); 
Demonstration environment: None. 

Technology readiness level: 2. Technology concept and/or application 
formulated; 
Description: Invention begins. Once basic principles are observed, 
practical applications can be invented. The application is speculative
and there is no proof or detailed analysis to support the assumption. 
Examples are still limited to paper studies; 
Hardware/software: None (paper studies and analysis); 
Demonstration environment: None. 

Technology readiness level: 3. Analytical and experimental critical 
function and/or characteristic proof of concept; 
Description: Active research and development is initiated. This includes
analytical studies and laboratory studies to physically validate 
analytical predictions of separate elements of the technology. Examples
include components that are not yet integrated or representative; 
Hardware/software: Analytical studies and demonstration of nonscale 
individual components (pieces of subsystem); 
Demonstration environment: Lab. 

Technology readiness level: 4. Component and/or breadboard validation in
laboratory environment; 
Description: Basic technological components are integrated to establish
that the pieces will work together. This is relatively “low fidelity” 
compared to the eventual system. Examples include integration of “ad 
hoc" hardware in a laboratory; 
Hardware/software: Low-fidelity breadboard. Integration of nonscale 
components to show pieces will work together. Not fully functional or 
form or fit but representative of technically feasible approach 
suitable for flight articles; 
Demonstration environment: Lab. 

Technology readiness level: 5. Component and/or breadboard validation
in relevant environment; 
Description: Fidelity of breadboard technology increases 
significantly. The basic technological components are integrated with
reasonably realistic supporting elements so that the technology can be 
tested in a simulated environment. Examples include “high fidelity”
laboratory integration of components; 
Hardware/software: High-fidelity breadboard. Functionally equivalent 
but not necessarily form and/or fit (size weight, materials, etc). 
Should be approaching appropriate scale. May include integration of
several components with reasonably realistic support elements/subsystems
to demonstrate functionality; 
Demonstration environment: Lab demonstrating functionality but not 
form and fit. May include flight demonstrating breadboard in surrogate 
aircraft. Technology ready for detailed design studies. 

Technology readiness level: 6. System/subsystem model or prototype
demonstration in a relevant environment; 
Description: Representative model or prototype system, which is well 
beyond the breadboard tested for TRL 5, is tested in a relevant 
environment. Represents a major step up in a technology’s demonstrated 
readiness. Examples include testing a prototype in a high fidelity 
laboratory environment or in simulated realistic environment; 
Hardware/software: Prototype. Should be very close to form, fit and 
function. Probably includes the integration of many new components and 
realistic supporting elements/subsystems if needed to demonstrate full
functionality of the subsystem; 
Demonstration environment: High-fidelity lab demonstration or limited/
restricted flight demonstration for a relevant environment. 
Integration of technology is well defined. 

Technology readiness level: 7. System prototype demonstration in a 
realistic environment; 
Description: Prototype near or at planned operational system. 
Represents a major step up from TRL 6, requiring the demonstration of 
an actual system prototype in a realistic environment, such as in an 
aircraft, vehicle or space. Examples include testing the prototype in
a test bed aircraft; 
Hardware/software: Prototype. Should be form, fit and function 
integrated with other key supporting elements/subsystems to 
demonstrate full functionality of subsystem; 
Demonstration environment: Flight demonstration in representative 
realistic environment such as flying test bed or demonstrator aircraft.
Technology is well substantiated with test data. 

Technology readiness level: 8. Actual system completed and “flight
qualified” through test and demonstration; 
Description: Technology has been proven to work in its final form and 
under expected conditions. In almost all cases, this TRL represents 
the end of true system development. Examples include developmental 
test and evaluation of the system in its intended weapon system to 
determine if it meets design specifications; 
Hardware/software: Flight-qualified hardware; 
Demonstration environment: Developmental Test and Evaluation (DT&E) in 
the actual system application. 

Technology readiness level: 9. Actual system "flight proven” through
successful mission operations; 
Description: Actual application of the technology in its final form 
and under mission conditions, such as those encountered in operational 
test and evaluation. In almost all cases, this is the end of the last 
“bug fixing” aspects of true system development. Examples include 
using the system under operational mission conditions; 
Hardware/software: Actual system in final form; 
Demonstration environment: Operational Test and Evaluation (OT&E) in
operational mission conditions. 

Source: GAO and its analysis of National Aeronautics and Space
Administration data. 

[End of table] 

Proof of Concept: Analytical and experimental demonstration of 
hardware/software concepts that may or may not be incorporated into
subsequent development and/or operational units. 

Breadboard: A low fidelity unit that demonstrates function only, 
without respect to form or fit in the case of hardware, or platform in 
the case of software. It often uses commercial and/or ad hoc 
components and is not intended to provide definitive information 
regarding operational performance. 

Brassboard: A medium fidelity functional unit that typically tries to 
make use of as much operational hardware/software as possible and 
begins to address scaling issues associated with the operational 
system. It does not have the engineering pedigree in all aspects, but is
structured to be able to operate in simulated operational environments 
in order to assess performance of critical functions. 

Prototype Unit: The prototype unit demonstrates form, fit, and 
function at a scale deemed to be representative of the final product
operating in its operational environment. A subscale test article 
provides fidelity sufficient to permit validation of analytical models 
capable of predicting the behavior of full-scale systems in an 
operational environment. 

Engineering Unit: A high fidelity unit that demonstrates critical 
aspects of the engineering processes involved in the development of the
operational unit. Engineering test units are intended to closely 
resemble the final product (hardware/software) to the maximum extent
possible and are built and tested so as to establish confidence that 
the design will function in the expected environments. In some cases, 
the engineering unit will become the final product, assuming proper 
traceability has been exercised over the components and hardware 
handling. 

Mission Configuration: The final architecture/system design of the 
product that will be used in the operational environment. If the 
product is a subsystem/component, then it is embedded in the actual 
system in the actual configuration used in operation. 

Laboratory Environment: An environment that does not address in any 
manner the environment to be encountered by the system, subsystem, or 
component (hardware or software) during its intended operation. Tests 
in a laboratory environment are solely for the purpose of 
demonstrating the underlying principles of technical performance 
(functions), without respect to the impact of environment. 

Relevant Environment: Not all systems, subsystems, and/or components 
need to be operated in the operational environment in order to 
satisfactorily address performance margin requirements. Consequently, 
the relevant environment is the specific subset of the operational 
environment that is required to demonstrate critical “at risk” aspects 
of the final product performance in an operational environment. It is 
an environment that focuses specifically on “stressing” the technology 
advance in question. 

Operational Environment: The environment in which the final product 
will be operated. In the case of space flight hardware/software, it is 
space. In the case of ground-based or airborne systems that are not 
directed toward space flight, it will be the environments defined by 
the scope of operations. For software, the environment will be defined 
by the operational platform. 

[End of section] 

7.5. Expert participation in the engagement: 

At our request, the following individuals helped us to identify 
currently available alternative neutron detector technologies and 
those under development. They also reviewed and provided comments on 
the draft of this report: 

Yacouba Diawara, Oak Ridge National Laboratory, Oak Ridge, Tennessee; 

Glenn F. Knoll, University of Michigan, Ann Arbor, Michigan; 

Richard T. Kouzes, Pacific Northwest National Laboratory, Richland, 
Washington; 

Craig Marianno, Texas A&M University, College Station, Texas; 

Keith Marlow, Sandia National Laboratories, Albuquerque, New Mexico; 

Stanley G. Prussin, University of California, Berkeley, California; 

Tor Raubenheimer, Stanford Linear Accelerator Center, Menlo Park, 
California; 

George Thompson, Homeland Security Studies & Analysis Institute, 
Arlington, Virginia. 

[End of section] 

7.6. Comments from the Department of Energy: 

Department of Energy: 
National Nuclear Security Administration: 
Washington, DC 20585: 

September 13, 2011: 

Mr. Gene Aloise: 
Director: 
Natural Resources and Environment: 
Government Accountability Office:
Washington, DC 20458: 

Dear Mr. Aloise: 

The Department of Energy (Department) and National Nuclear Security 
Administration (NNSA) appreciates the opportunity to review the 
Government Accountability Office's (GAO) report, Technology 
Assessment: Alternatives to using Helium-3 for Neutron Detectors,
GAO-i 1-753. In response to a request made by the Subcommittee on 
Investigations and Oversight, Committee on Science and Technology, 
U.S. House of Representatives, GAO was asked to review the 
availability and effectiveness of alternative neutron detector 
technologies that do not use helium-3 and to assess (1) what 
alternative neutron detectors are currently available and their 
effectiveness, and (2) the status of research on alternative neutron 
detector technologies under development for future availability. 

We generally agree with the report; however, we do have the following 
concerns: 

* This report does not accurately reflect the possible use of boron 
trifluoride (BF3) as a U.S. domestic alternative to 3He. 
- BF3 is a proven, inexpensive, and reliable alternative with minimal 
development costs. BF3 detectors were in widespread use before the 
availability of 3He in the 1990s. They are still commercially available.
-	BF3 is a hazardous gas. However, detectors use small quantities and 
operate at less than atmospheric pressure, mitigating the impact of 
any leak. The DOT does not consider less than 1 gm of BF3 shipped 
inside portable instruments hazardous. 
-	We do note that for international deployments, such as in the NNSA 
Second Line of Defense program, BF3 presents significant cost and 
program risks compared to other portal monitors. Hazardous material 
transportation requirements, handling rules, and training requirements 
will vary greatly from country to country leading to indeterminate 
life-cycle costs and significant programmatic risks. 

*	The instruments at the Department's Spallation Neutron Source (SNS) 
were designed for neutron pass energies that are only efficiently 
detected by 3He and any alternatives will cut the overall counting 
efficiency by 30 to 50 percent. 

Enclosed are comments that we believe will help clarify and improve 
the report in areas that may be confusing or misleading.

If you have any questions concerning this response, please contact 
JoAnne Parker, Director, Office of Internal Controls, at 202-586-1913. 

Sincerely, 

Signed by: 

Kenneth W. Powers: 
Associate Administrator for Management and Budget: 

Enclosure: 

[End of section] 

7.7. Comments from the Department of Homeland Security: 

U.S. Department of Homeland Security: 
Washington, DC 20528: 

August 31, 2011: 

Timothy M. Persons, Ph.D. 
Mr. Gene Aloise:
U.S. Government Accountability Office: 
441 G Street NW: 
Washington, DC 20548: 

Re: Draft Report GAO-11-753,	"Technology Assessment: Alternatives to 
Using Helium-3 for Neutron Detectors" 

Dear Dr. Persons and Mr. Aloise: 

Thank you for the opportunity to review and comment on this draft 
report. The U.S. Department of Homeland Security (DHS) appreciates the 
U.S. Government Accountability Office's (GAO's) work in planning and 
conducting its review and issuance of this report. The Department is 
pleased to note the report recognizes the active and key role the 
Domestic Nuclear Detection Office (DNDO) has had in the discovery and 
mitigation of the shortage of Helium-3. 

In early 2009, DNDO spearheaded interagency efforts among the U.S. 
Departments of Commerce. Defense, State, and Energy to address the 
Helium-3 shortage. DNDO led efforts of Helium-3 stakeholders 
nationwide to establish and charter the Helium-3 Integrated Project Team
(IPT). The IPT established four subordinate working groups for: 

* Policy, which identified and addressed policy and procedural issues 
associated with the interagency activities required to mitigate the 
shortage; 

* Supply, which was chartered to identify- viable sources of Helium-3, 
nationally and internationally, and characterize them in terms of 
overall potential to provide Helium-3 in response to the shortage; 

* Demand, which compiled a list of valid claimants to the Helium-3 
inventory, both in the Government and industry; and; 

* Technology, which reviewed and, across the Government. coordinated 
the research and development alternatives to Helium-3 based neutron 
detection. 

Via these working groups, which still meet today, the IPT identified 
mutually acceptable paths forward. In July 2009, an Interagency Policy 
Committee (IPC) was formed by the National Security Staff, which 
adopted the structure put in place by the IPT. DNDO remains an active 
member of the ongoing oversight process via its continuing role as a 
member of the IPC', Chair of the Helium-3 IPT, and Lead for the 
Technology Working Group.

Again, thank you for the opportunity to review and comment on this 
draft report. We note the report does not contain any recommendations 
for DIIS. Technical and sensitivity comments were submitted under 
separate cover. We look forward to working with you on future homeland 
security issues. 

Sincerely, 

Signed by: 

Jim H. Crumpacker: 
Director: 
Departmental GAO/OIG Liaison Office: 

[End of section] 

8. References: 

ANSI (American National Standards Institute). 2007. American National 
Standard for Evaluation and Performance of Radiation Detection Portal
Monitors for Use in Homeland Security. ANSI N42.35-2006. New York: 
January 22. 

CBP (U.S. Customs and Border Protection). 2009. Performance and 
Accountability Report, Fiscal Year 2009. Washington D.C.: December 9.

Coursey, J., D. Schwab, J. Tsai, and R. Dragoset. 2010. Atomic Weights 
and Isotopic Compositions with Relative Atomic Masses. Gaitherburg, Md.:
National Institute of Standards and Technology. October 5. 

DHS (U.S. Department of Homeland Security). 2010. Helium-3 (3He) 
Neutron Detection Module Replacement Functional Requirements Document
(FRD). 600-NDRP-113410v3.00. October 3. 

GAO (U.S. Government Accountability Office). 2006. Combating Nuclear 
Smuggling: Corruption, Maintenance, and Coordination Problems 
Challenge U.S. Efforts to Provide Radiation Detection Equipment to 
Other Countries. GAO-06-311. Washington, D.C.: March 14. 

GAO (U.S. Government Accountability Office).2011. Managing Critical 
Isotopes, Weaknesses in DOE’s Management of Helium-3 Delayed the 
Federal Response to a Critical Supply Shortage. GAO-11-472. 
Washington, D.C.: May 12. 

Ginhoven, R., R. Kouzes, and D. Stephans. 2009. Alternative Neutron 
Detector Technologies for Homeland Security. PNNL-18471. Richland, 
Wash.: Pacific Northwest National Laboratory June 9. 

Knoll, G. 2000. Radiation Detection and Measurement. Hoboken, NJ: John 
Wiley & Sons, Inc. 

Kouzes, R., J. Ely, A. Lintereur, and D. Stephens. 2009a. Neutron 
Detector Gamma Insensitivity Criteria. PNNL-18903. Richland, Wash.: 
Pacific Northwest National Laboratory. October 28. 

Kouzes, R., J. Ely, A. Lintereur, E. Sicilianio, and M. Woodring. 
2009b. BF3 Neutron Detector Tests. PNNL-19050. Richland, Wash.: Pacific
Northwest National Laboratory. December 9. 

Kouzes, R., J. Ely, and D. Stromswold. 2010a. Boron-lined Straw Tube 
Detector Test. PNNL-19600. Richland, Wash.: Pacific Northwest National 
Laboratory. August 7. 

Kouzes, R. J. Ely, L. Erikson, W. Kernan, D. Stromswold, and M. 
Woodring. 2010b. Full Scale Coated Fiber Neutron Detector Measurements.
PNNL-19264. Richland, Wash.: Pacific Northwest National Laboratory. 
March 17. 

NASA (National Aeronautics and Space Administration). 2010 NASA 
Research and Technology Program and Project Management Requirements. 
Change 1. NPR 7120.8. dated November 24. 

Technical Working Group. 2010a. Alternative Techniques to Helium-3 
Based Neutron Detectors for Neutron Scattering Applications. Proposal 
for a Joint Development Programme. February 24. 

Technical Working Group. 2010b. Collaboration Agreement for the 
Development of Alternative Techniques to Helium-3 Based Neutron 
Detectors for Neutron Scattering Applications July 15. 

Woodring, M., J. Ely, R. Kouzes, and D. Stromswold. 2010. Boron-lined 
Multichamber and Conventional Neutron Proportional Counter Tests. PNNL-
19725. Richland, Wash.: Pacific Northwest National Laboratory. 
September 6. 

[End of section] 

GAO contacts and staff acknowledgments: 

GAO contacts: 

Timothy M. Persons, (202) 512-6412 or personst@gao.gov: 
Gene Aloise, (202) 512-3841 or aloisee@gao.gov: 

Staff acknowledgments: 

In addition to the persons named above, Ned H. Woodward, Assistant 
Director; R. Scott Fletcher; and Wyatt R. Hundrup made key 
contributions. Pille Anvelt, Kendall Childers, Cindy Gilbert, Rich
Hung, Mehrzad Nadji, Penny Pickett, and Kim Raheb also made important 
contributions. 

[End of section] 

Other GAO technology assessments: 

Technology Assessment: Climate Engineering: Technical status, future 
directions, and potential responses. [hyperlink, 
http://www.gao.gov/products/GAO-11-71]. Washington, D.C.: July 28, 
2011. 

Technology Assessment: Explosives Detection Technology to Protect 
Passenger Rail. [hyperlink, http://www.gao.gov/products/GAO-10-898]. 
Washington, D.C.: July 28, 2010. 

Technology Assessment: Securing the Transport of Cargo Containers.
GAO-06-68SU. Washington, D.C.: January 25, 2006. [Classification: For 
Official Use Only]. 

Technology Assessment: Protecting Structures and Improving 
Communications during Wildland Fires. [hyperlink, 
http://www.gao.gov/products/GAO-05-380]. Washington, D.C.: April 26, 
2005. 

Technology Assessment: Cybersecurity for Critical Infrastructure 
Protection. [hyperlink, http://www.gao.gov/products/GAO-04-321]. 
Washington, D.C.: May 28, 2004. 

Technology Assessment: Using Biometrics for Border Security. 
[hyperlink, http://www.gao.gov/products/GAO-03-174].
Washington, D.C.: November 14, 2002. 

[End of section] 

Footnotes: 

[1] Neutron radiation is indirectly ionizing radiation--the absorption 
of a neutron results in the creation of ionizing particles.

[2] Isotopes are varieties of a chemical element with the same number 
of protons but different numbers of neutrons; for example, helium-3 
has one less neutron than helium-4, the helium isotope that is 
commonly used in party balloons. An element's isotopes have nearly 
identical chemical properties, but their nuclear properties, like the 
ability to absorb neutrons, can differ significantly.

[3] GAO, Managing Critical Isotopes: Weaknesses in DOE's Management of 
Helium-3 Delayed the Federal Response to a Critical Supply Shortage, 
[hyperlink, http://www.gao.gov/products/GAO-11-472] (Washington, D.C.: 
May 12, 2011). 

[4] SNS is a research facility at Oak Ridge National Laboratory that 
includes the world's most powerful pulsed-neutron source. Constructed 
over 7 years at a cost of $1.4 billion, it can provide information 
about the structure and properties of materials that cannot be 
obtained by other means. 

[5] Helium-3 is also used in non-neutron detection applications. It is 
used in magnetic resonance imaging (MRI) to study pulmonary disorders 
such as chronic obstructive pulmonary disease, a lung disease in which 
the lungs are partially blocked, making it difficult to breathe. In 
this type of research, a patient inhales helium-3 during the MRI so 
that doctors may get a clear view of the entire pulmonary structure. 
Helium-3 is also important for scientific research involving ultra-low 
temperature refrigeration systems. Each of these applications was also 
affected by the current helium-3 shortage.

[6] According to an ORNL official, other national laboratories also 
use large neutron detectors, but the SNS has a planned expansion in 
2018, which will require the construction of additional new large-area 
detectors.

[7] The National Aeronautics and Space Administration and the Air 
Force Research Laboratory use TRLs to determine the readiness of 
technologies to be incorporated into a weapon or another type of 
system. Readiness levels are measured along a scale of one to nine, 
starting with paper studies of the basic concept, proceeding with 
laboratory demonstrations, and ending with a technology that has 
proven itself on the intended product.

[8] A false positive is a result that is incorrectly positive when the 
situation is normal. In the context of neutron detectors, a false 
positive is when a neutron detector indicates the presence of a 
neutron when no neutron is present.

[9] Both of these isotopes can be used in neutron detectors, but their 
strengths and weaknesses may determine their suitability for 
particular neutron detector applications. For example, an atom of 
boron-10 has a higher chance of absorbing a neutron than an atom of 
lithium-6, making boron-10 detectors potentially more efficient at 
detecting neutrons. A lithium-6 atom, after absorbing a neutron, emits 
charged particles with higher energies than boron-10, which makes 
lithium-6 detectors potentially better than boron-10 detectors at 
gamma radiation discrimination. Both of these isotopes can be 
chemically combined with other elements to create molecules that also 
act as conversion materials. For example, boron-10 and fluorine can be 
used to create boron trifluoride gas, which can be used in sealed 
tubes in much the same way as helium-3 has been used in proportional 
detectors. 

[10] The Isotope Program's mission is to produce and sell isotopes and 
related isotope services, maintain the infrastructure to do so, and 
conduct research and development on new and improved isotope 
production and processing techniques. DOE's Isotope Program produces 
and sells about 200 isotopes, although it does not control the supply--
the production or inventory--of all the isotopes is sells, such as 
helium- 3, which is extracted from tritium by the National Nuclear 
Security Administration (NNSA), a semiautonomous agency within DOE. 
Helium-3 can be extracted from natural sources of helium gas, such as 
subterranean natural gas deposits, but these sources have not been 
pursued commercially because extracting the very low concentrations of 
helium-3 has not been economically viable.

[11] Other physical properties of helium-3 make it practical for other 
applications. Spin polarization of the nucleus of a helium-3 atom 
aligns it magnetically, making it useful in magnetic resonance imaging 
for lung disease research. 

[12] Helium-3 prices in fiscal year 2011 range from $600 to $1,000 per 
liter when acquired from the U.S. government. For additional 
information, see [hyperlink, http://www.gao.gov/products/GAO-11-472].

[13] According to ORNL officials, the SNS can contain up to 25 large- 
area detectors, and has a planned expansion of the facility that will 
require additional detectors in 2018.

[14] Spatial and time resolution describe the ability to determine 
where on a neutron detector a neutron is detected, and when that 
detection occurs.

[15] Gamma radiation discrimination (or gamma radiation rejection) 
specifies the maximum number of gamma rays that a detector can falsely 
identify as neutrons. Both large-area detectors and radiation portal 
monitors use a requirement that the gamma radiation discrimination be 
less than 10-6, which indicates that less than one in a million gammas 
can result in a false positive neutron detection.

[16] ANSI, a private, not-for-profit organization with private and 
government membership, oversees the creation and use of a variety of 
consensus standards. General requirements for radiation portal 
monitors are established by ANSI, American National Standard for 
Evaluation and Performance of Radiation Detection Portal Monitors for 
Use in Homeland Security, ANSI N42.35-2006 (New York, NY: Jan. 22, 
2007). 

[17] A variation of radiation portal monitors, the Advanced 
Spectroscopic Portal (ASP) is designed to identify the material 
producing radiation. According to a July 2011 congressional testimony 
by the director of DNDO, the ASP program will end as originally 
conceived. Of the existing ASP systems, 13 will be deployed to ports 
of entry to gain operational familiarity with the systems and to 
gather data in support of a future acquisition program.

[18] DOE deploys radiation detection equipment at overseas facilities 
as part of its Second Line of Defense program to assist foreign 
governments in combating nuclear smuggling. For additional 
information, see GAO, Combating Nuclear Smuggling: Corruption, 
Maintenance, and Coordination Problems Challenge U.S. Efforts to 
Provide Radiation Detection Equipment to Other Countries, [hyperlink, 
http://www.gao.gov/products/GAO-06-311] (Washington, D.C.: March 14, 
2006).

[19] Tritium, an isotope of hydrogen, radioactively decays into helium-
3 at an annual rate of 5.5 percent. For further information about 
tritium production, see GAO, Nuclear Weapons: National Nuclear 
Security Administration Needs to Ensure Continued Availability of 
Tritium for the Weapons Stockpile, [hyperlink, 
http://www.gao.gov/products/GAO-11-100] (Washington, D.C.: Oct. 7, 
2010).

[20] Congress created NNSA as a semiautonomous agency within DOE under 
title 32 of the National Defense Authorization Act for Fiscal Year 
2000 (Pub. L. No. 106-65, § 3211, 113 Stat 512. 957 (1999)). NNSA is 
responsible for the management and security of the nation's nuclear 
weapons, nonproliferation, and naval reactors programs.

[21] The Isotope Program did not sell helium-3 from about 1995 through 
2001 because helium-3 was being stockpiled for use in NNSA's 
Accelerator Production of Tritium project. During this time, Russia 
was the primary source of commercially available helium-3.

[22] The National Security Staff, established under and reporting to 
the National Security Advisor, supports all White House policymaking 
activity related to international and homeland security matters. 

[23] Applications for which there is no alternative to helium-3 
receive the highest priority for helium-3 allocations, followed by 
programs for detecting nuclear material at foreign ports and borders, 
followed by programs for which substantial costs have already been 
incurred. For additional information, see [hyperlink, 
http://www.gao.gov/products/GAO-11-472]. 

[24] In 2010, an international group of 10 research facilities that 
use large-area detectors agreed to collaborate in the development of 
alternative neutron detectors. They include the Neutron Science 
Directorate at ORNL (which includes the SNS) and the NIST Center for 
Neutron Research in the United States, the Japan Proton Accelerator 
Research Complex (J-PARC), the Julich Centre for Neutron Science in 
Germany, and the Paul Scherrer Institute in Switzerland.

[25] Exposure to boron trifluoride can irritate or burn the skin and 
eyes. Inhalation can result in irritation of the upper respiratory 
tract or inflammation of the lungs, with potential chest pain and 
difficulty breathing.

[26] From Technical Working Group, Alternative techniques to 3He based 
neutron detectors for neutron scattering applications: Proposal for a 
Joint Development Programme, February 24, 2010. This proposal was 
authored by an international technical working group, assembled in 
response to a meeting of science facility directors in 2009, and is 
one document guiding the international collaboration in its work to 
develop alternative detector technologies for large-area detectors.

[27] At this stage in DHS's testing process, these prototypes do not 
undergo complete testing of the environmental requirements. They are 
tested over specified humidity and temperature ranges, with exposure 
to temperate and freezing rain, with a physical impact, and with 
exposure to an electrostatic discharge. They have not yet been tested 
for other environmental requirements, such as exposure to external 
electromagnetic fields, operation during/after vibrations, and 
operation during/after exposure to blowing sand, among other 
requirements.

[28] CLYC (Cs2LiYCl6) scintillating crystals detect both neutrons and 
gamma radiation, which would normally limit its effectiveness as a 
neutron detector due to poor gamma discrimination. However, neutrons 
and gamma radiation create different kinds of signals from the CLYC 
crystal, allowing it to function as an effective neutron detector.

[29] B2Se3 is a new semiconductor material that can be used as an 
efficient neutron detector because it contains boron-10 as a large 
fraction of its composition. 

[30] In addition to boron-10 and lithium-6, gadolinium can be used as 
a conversion material. However, detectors using it have relatively 
poor gamma radiation discrimination. 

[31] Other materials can be used for conversion materials, but are 
less common due to significant weaknesses. For example, gadolinium can 
be used, but it has poor gamma radiation discrimination. Fissile 
nuclear material can also be used, but such detectors typically have 
low efficiencies and can require safeguards to secure the detector 
itself. 

[End of section] 

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