i 

A Primer on the Detection of

Nuclear and Radiological

Weapons

Gary W. Phillips, David J. Nagel, and Timothy Coffey 

Center for Technology and National Security Policy 

National Defense University 

July 2005 
ii 

The views expressed in this article are those of the authors and do not reflect the official policy 

or position of the National Defense University, the Department of Defense, or the U.S. 

Government. All information and sources for this paper were drawn from unclassified materials. 

The presentation and discussion of individual commercial and research products does not 

represent a Department of Defense endorsement, nor is it intended to be an exhaustive 

survey of such devices and systems. 

Gary W. Phillips is an Adjunct Associate Professor at Georgetown University Medical Center 

in Washington, DC, performing advanced research in nuclear radiation detection and neutron 

dosimetry. From 1973 until his retirement in 2001, he conducted research in radiation detection 

at the U.S. Naval Research Laboratory, Washington, DC. Dr. Phillips has served on U.S. 

advisory and review panels and has over 70 publications in refereed journals and proceedings. 

David J. Nagel obtained a Ph.D. in Engineering Materials from the University of 

Maryland. He worked at the Naval Research laboratory for 36 years and led the 

Condensed Matter and Radiation Sciences Division for one-third of that time. He served 

for 30 years as a naval officer on active and reserve duty and retired with the rank of 

Captain. He is now a Research Professor at The George Washington University 

specializing in applications of micro- and nano-technologies. 

Timothy Coffey holds the Edison Chair for Technology in the Center for Technology 

and National Security Policy. He is also a Senior Research Scientist at the University of 

Maryland. He was the Director of Research at the U.S. Naval Research Laboratory from 

1982 to 2001. He has published extensively in the scientific literature and has served on 

numerous advisory panels. 

Defense & Technology Papers are published by the National Defense University Center for Technology and National Security 

Policy, Fort Lesley J. McNair, Washington, DC. CTNSP publications are available online at 

http://www.ndu.edu/ctnsp/publications.html . 
iii 

Contents 

Executive Summary........................................................................................................................ v 

1. Introduction................................................................................................................................. 1 

2. Nuclear Radiation and Interactions............................................................................................ 3 

3. Nuclear and Radiation Effects .................................................................................................... 5 

A. Effects of Nuclear Weapons ......................................................................................... 5 

B. Human Effects of Radiation.......................................................................................... 6 

4. Characteristics of Nuclear and Radiological Weapons............................................................. 7 

A. Nuclear Weapons .......................................................................................................... 7 

B. Nuclear Weapon Materials............................................................................................ 8 

C. Radiological Weapons................................................................................................. 16 

D. Radiological Sources................................................................................................... 18 

5. Background Radiation ...................................................................................................... 20

A. Gamma-Ray Background............................................................................................ 20 

B. Neutron Background ................................................................................................... 24 

C. Man-Made Background............................................................................................... 24 

D. Detection Capabilities in the Natural Radiation Environment.................................... 24 

6. Nuclear Radiation Detection................................................................................................... 27 

A. General Aspects of NRWMD Detection...................................................................... 27 

B. Characteristics of Radiation Detectors......................................................................... 29 

C. Employment Scenarios................................................................................................ 31 

7. Available Point Detector Technologies ................................................................................... 32 

A. Gamma-Ray Detectors................................................................................................ 32 

B. Gamma-Ray Detector Electronics............................................................................... 34 

C. Neutron Detectors ....................................................................................................... 40 

D. Survey Meters and Portable MCAs ............................................................................ 42 

E. Portals and Search Systems......................................................................................... 46 

8. Imaging Technologies.............................................................................................................. 46 

9. New Detector Technologies...................................................................................................... 50 

A. Integrated Detection Systems....................................................................................... 50 

B. Detector Technologies in Development...................................................................... 51 

10. Summary and Conclusions ..................................................................................................... 52 

Appendix A. Calibration and Operating Curves.......................................................................... 55 

Appendix B. Practical Factors ..................................................................................................... 61 

References..................................................................................................................................... 63 
iv 
v 

Executive Summary 

Terrorist use of radioactive nuclear materials is a serious threat for mass destruction or 

disruption of civil and military activities. Most worrisome is the use of nuclear devices to cause 

massive casualties to people and damage to structures. Fortunately, the procurement of adequate 

material and the engineering design, construction, and transportation and triggering of a nuclear 

weapon are all difficult problems for terrorist organizations. More likely is a device that 

combines radioactive materials with conventional explosives to make a radiological dispersion 

device, commonly called a "dirty bomb". The procurement of nuclear materials for this purpose, 

the construction of the bomb and its use are all easier than for a nuclear weapon. Fortunately, the 

effects from the use of a radiological weapon would be much smaller than from a nuclear device, 

although they could still be very disruptive. Thus, it is important to detect the transport of nuclear 

weapons and radiological dispersion devices and the materials for their construction. These 

materials emit gamma rays or neutrons, which can be detected to show the presence and amounts 

of such materials. 

Radiation detectors have two applications toward nuclear and radiological weapons of mass 

destruction. The first is to intercept nuclear materials and devices prior to a terrorist attack. The 

second is for assessment and attribution after an attack. Such detectors represent relatively 

mature technologies. Portable radiation instruments were developed over fifty years ago in 

response to the use of nuclear weapons in WW II. Modern electronics and detector materials 

have made them much more capable. Now, new materials are being developed to enable even 

more efficient detection of gamma rays and neutrons with simpler devices. In recent years, large 

systems for imaging of gamma rays from nuclear materials and devices have been developed. 

Many passive and active, fixed and portable instruments for the detection of gamma rays or 

neutrons are available commercially. 

Geometry, air attenuation, and background radiation from natural and man-made sources 

determine the limits of detection of these materials. For realistic source strengths and available 

gamma-ray and neutron detectors, nuclear materials and devices can be detected at ranges of a 

few meters up to a few tens of meters, at most. Detection from fixed-wing aircraft or satellites is 

impossible. Examples of detection limits for generic detectors are given in this report. It is 

necessary to quantify the limits of utility for specific nuclear radiation detectors under the 

circumstances in which they are likely to be used. 
vi 
1 

1. Introduction 

 The detection of materials or devices for nuclear or radiological weapons of mass destruction 

(NRWMD) is fundamentally important to both homeland security and to military operations. 

Detection technologies are necessary both to find and to verify the location of materials, 

components and systems for NRWMD. They are also central to actions taken after deployment 

of a NRWMD. The study upon which this report is based was undertaken because of the large 

and growing importance of detection technologies for NRWMD. While this report will focus on 

detection of NRWMD, the NRWMD problem does not have a purely technical solution. The 

reasons for this will become clear in the report. Detectors are necessary but not sufficient for 

dealing with this problem. 

 This report seeks to be pragmatically comprehensive in its coverage. It takes a level 

intermediate between policy, on one hand, and technology details, on the other hand. The goal is 

to provide part of the basis for “higher level” policy considerations, as well as a framework for 

“lower level” more detailed technical concerns. Potential readers of this report are expected to 

fall into a few classes. They include policy makers, managers responsible for implementing 

policies and putting in place the combinations of technologies and people for effective and 

timely detection of NRWMD materials and devices, users of the technologies, technology 

developers and also students. This report includes the fundamentals of detection technologies, as 

well as insights into the current and coming state-of-the-art in such technologies. As such, it 

should be of use to most of these readers. Researchers involved in developing new technologies 

will not find the latest details of their field documented in this report, since this is not a scientific 

review article. However, researchers might find this overview of the technologies of interest. 

 The detection of NRWMD shares many features of the more general case of the detection of 

WMD. The variety of materials and devices relevant to defense against WMD can be arrayed 

against the diverse detection 

technologies to form a twodimensional matrix. It would show 

that (a) multiple detection 

technologies are relevant to most of 

the different WMD materials and 

devices, and (b) a given detection 

technology can be used to detect more 

than one kind of WMD material or 

device. However, such a spreadsheet 

is insufficient to describe the 

complexity of the overall field. This 

is the case because any combination 

of one material or device with one 

detection technology can be used in a 

variety of scenarios. For example, 

security at ports dealing with the 

contents of arriving containers is 

radically different than the actions of 

first responders coming on the scene of an urban radiological emergency. Hence, we are really 

Figure 1. Schematic of the three dimensional 

“space” defined by different WMD subjected to 

various detection technologies in a variety of 

employment scenarios. 
2 

concerned with a quasi three-dimensional space, such as that sketched in Figure 1. Some boxes 

in the plane relating WMD and detection technologies are not relevant. That is, some detection 

instruments do not apply to some WMD materials and devices. Even those that are relevant do 

not come into play in all scenarios. However, there are indeed many combinations of the three 

factors that are significant. Hence, the detection of WMD materials is complex due to its variety 

as well as because of technical and practical factors. 

 Detection technologies provide information that has various uses. Very often the output of an 

instrument is locally and immediately useful. An example is the detection of an explosive 

material in the backpack of a person seeking admission to a political convention. Other times, the 

information from an instrument for detecting WMD materials is used remotely as a basis of 

action. The latter situation will be increasingly relevant as responsible officials employ 

distributions of diverse manned and unmanned sensors to obtain larger area views of complex 

scenarios. The release of a radiological material in the heart of a city would be an example. In 

such cases, simulations of the expected flow for the prevailing wind conditions might be used to 

provide brief, but crucial, warnings to people downwind from the release. Figure 2 sketches the 

sequences of actions in such cases. Information from the detection is combined with simulations 

under various scenarios. The results are communicated to decision makers who determine 

appropriate actions. The detection technologies themselves are but a part of a much more 

complex undertaking. 

 This study was motivated by increases around the world in the frequency of terrorist attacks 

and by the importance of WMD, in general, as asymmetric weapons, which can have impacts 

very disproportionate to their costs. These events and factors come at a time when technologies, 

generally, and detection technologies, specifically, are improving with unprecedented rapidity. 

Advances in diverse miniaturization technologies, including micro-electronics, -magnetics, -

optics and -mechanics, are being applied to detection instrumentation. 

 There are many technologies for the detection of people and vehicles that might be 

transporting materials or devices for WMD. They exploit acoustic, seismic, optical, radiofrequency and other mechanisms, which indirectly provide information on WMD. While such 

non-specific technologies can be very important for overall defense against WMD, they are not 

within the scope of this report, which focuses on detection technologies that can directly detect 

the materials or devices that might become, or are already part of a WMD. 

 Section 2 describes nuclear radiations and how they interact with matter, including detectors. 

The effects of nuclear explosions and the effects of radiation on people are surveyed in Section 

3. Section 4 briefly describes the design of nuclear weapons and details the observable 

radiations from nuclear weapons and nuclear materials. This section includes a discussion of the 

effects of radiological weapons or “dirty bombs” and the observable radiations from radiological 

Figure 2. Illustration of the generation, fusion and utilization of information from detection 

and simulation technologies. 
3 

materials. Section 5 focuses on the background radiation from gamma rays, cosmic rays and 

neutrons, which often determines detection thresholds. It goes on to discuss the challenges of 

weapons detection in the natural radiation environment. Section 6 gives a general discussion of 

radiation detectors and detector deployment scenarios as background for the following material. 

Section 7 reviews available point detector technologies for gamma rays and neutrons, and 

Section 8 surveys imaging technologies. Section 9 discusses new detector technologies in 

development and new integrated detection systems. The concluding Section 10 summarizes the 

major characteristics and useful ranges of detectors for nuclear and radiological weapons. 

Appendix A provides a unified view of the calibration curves and receiver operating 

characteristics that determine detector performance. Appendix B summarizes the practical 

aspects of nuclear radiation detectors, and includes a check list for managers making 

procurement decisions. 

2. Nuclear Radiation and Interactions 

All nuclear detection technologies are designed to detect emissions from the decay of 

radioactive nuclides, which can occur naturally, such as uranium and thorium, or are manmade, 

such as plutonium and various fission products produced in a nuclear reactor. The French 

physicist Henri Becquerel discovered radioactivity in 1896, when he observed images on 

photographic plates wrapped in black paper and exposed to uranium. For this, he shared the first 

Nobel Prize in Physics 1903. Following Becquerel’s discovery, early investigators identified 

three types of radioactivity which they called α, β and γ rays. Alpha particles were found to be 

4

He nuclei, consisting of two protons and two neutrons tightly bound. Beta particles can be 

either positively or negatively charged. The β−

 was eventually identified as the common electron 

and the β+ as its anti-particle, the positron. Gamma rays were found to be energetic photons. To 

these three emissions one can add x-rays and neutrons. X-rays and gamma rays are both highenergy photons, with gamma rays emitted during the decay of radioactive nuclei and x-rays 

during the realignment of atomic electrons following radioactive decay or by an electron beam 

striking a target. Neutrons are emitted during spontaneous fission by uranium and plutonium. 

The primary long-range observables from nuclear materials are gamma rays and neutrons, 

which have mean free paths of the order of a hundred meters in air and only 10 cm in water 

(Table 1). Alpha particles have a short range of about 50 mm in air but only about 60 μm in 

water. Beta particles have a range of a few meters in air, but are stopped by a few mm of water. 

X-rays and gamma rays have overlapping energy ranges of a few hundred eV to about 100 keV 

for x-rays and tens of keV to several MeV for gamma raysa

. Both are strongly attenuated by high 

atomic number materials, such as lead. 

 

a

 The electron volt (eV) is the energy gained by an electron when accelerated through a 1 volt potential. Energies of 

gamma rays and other nuclear particles are commonly given in keV (103

 eV) or MeV (106 eV). 
4 

Table 1. Range of Nuclear Particles 

energy (keV) range (m) 

 in air in water in 

aluminum 

in lead 

alpha particles 5000 0.05 6x10-5 3x10-5 2x10-5

beta particles 1000 4 0.004 0.002 7x10-4

x-rays 10 1.9 0.002 1.4x10-4 7x10-6

 30 30 0.03 0.004 3x10-5

gamma raysa 100 50 0.06 0.02 1.7x10-4

 400 80 0.09 0.04 0.004 

 1000 120 0.14 0.06 0.013 

neutronsb

 1000 200 0.10 0.10 0.08 

references: [Attix, 1976; Lamarsh, 1975; NIST, 2003] 

In addition to attenuation, the signal from a point source of radiation decreases with an 

inverse square geometrical dependence with distance. The detection range of nuclear materials 

is also limited by interference of the background radiation due to man-made and naturally 

occurring radionuclides and to cosmic rays. Nuclear materials near the earth's surface are not 

observable from high-flying aircraft or from satellites, despite occasional claims to the contrary. 

The fissile nuclides, 235U and 239Pu, in subcritical mass amounts, decay primarily by alpha 

particle emissions followed by gamma rays and x-rays. This results in a chain of unstable nuclei 

that build up and decay over time, leading eventually to lead. Some of the decay products emit 

gamma rays whose energies are characteristic signatures of the fissile material. There is also a 

small probability of spontaneous fission for 235U and 239Pu, resulting in emission of 2.5 to 3 

energetic neutrons per fission. These fission neutrons can induce additional nuclei to fission 

leading to a chain reaction. A critical mass is the amount of fissionable material just necessary 

for a self-sustaining nuclear chain reaction, as occurs in a nuclear reactor. Rapid assembly of a 

supercritical mass results in a nuclear explosion. Weapons grade plutonium typically contains 

6% of the 240Pu nuclide, which has a relatively high spontaneous fission probability resulting in 

significant neutron emission, but which reduces the efficiency for a nuclear explosion. 

Industrial radionuclide sources of gamma rays include 137Cs and 60Co. Industrial neutron 

sources include 252Cf and 241Am/Be. Linear accelerators are also used to produce neutrons or 

high-energy x-rays. Unlike sources, they have the advantage that they are emission free when 

powered down. Some of these accelerators are small enough to be considered portable or 

transportable. 

Fission neutrons from 235U and 239Pu have an average energy of about 1 MeV. At these 

energies, measured neutron attenuation lengths in most solids range from 6 to 10 cm [Attix, 

1976; Lamarsh, 1975]. To shield against these, a moderator is used in which the fast neutrons 

slow down primarily by elastic scattering. This is usually combined with a material with a high 

capture rate for “thermal” neutrons (neutrons which have slowed down until their energies are in 

 

a

 X-rays and gamma rays do not have a well-defined range. The table gives the average attenuation length or mean 

free path. 

b

 The table gives the measured attenuation length for fission energy neutrons. 
5 

equilibrium with the thermal environment, about 0.025 eV.) It takes a large number of 

scatterings to slow down a fast neutron to thermal energies, so that it can be absorbed or 

“captured.” The most effective moderators are those with high hydrogen content such as water, 

paraffin or polyethylene. Graphite is also used as a moderator in some reactors. The mean 

distance to slow down fission energy neutrons to thermal energies is about 12 cm in 

polyethylene, 13 cm in water and 47 cm in graphite [Lamarsh, 1975]. Materials with high 

thermal neutron capture probabilities include 3

He, 6Li, 10B, 113Cd and 157Gd. After capture, 

energetic electrons or heavy charged particles (protons, alphas, etc.) are emitted, which can be 

detected by a particle detector. 

3. Nuclear and Radiation Effects 

A. Effects of Nuclear Weapons 

Nuclear weapons have the greatest destructive potential of all weapons of mass destruction. 

The 20-kiloton (kT) bomb dropped on Hiroshima (Figure 3) destroyed almost everything out to a 

radius of one mile (1.6 km) [Manhattan, 1946]. An unshielded person exposed to the radiation 

from a 20 kT burst would receive a lethal dose exceeding 10 Gya

 out to about 1.2 km, about 

equally due to gamma rays and neutrons [Glasstone, 1977]. Because both are strongly attenuated 

by a few hundred meters of air, a factor of a thousand increase in yield only extends the lethal 

dose radius by about a factor of two; a 20 MT thermonuclear explosion would deliver a lethal 

dose out to about 3.0 km. A study by the Congressional Office of Technology Assessment of the 

effects of a 25 MT blast over Detroit estimated about 3.2 million casualties and 1.8 million 

deaths and predicted serious structural damage out to a radius of 11 km [OTA, 1979]. Fallout, 

while highly variable depending on local weather conditions, would likely extend to much 

greater distances. The early (1st day) fallout pattern from a 20 MT surface burst, for a wind 

speed of 24 km/hr, would cut a swath producing a dose rate of 10 mGy/hr or greater downwind 

for a distance of 245 km and with a maximum width of 20 km [Glasstone, 1977]. 

 

a

 The International System (SI) unit of dose is the gray (Gy) defined as the energy deposition of one J/kg. The 

sievert (Sv) is the equivalent dose adjusted for biological effects. (Neutrons have a biological effect about 10 times 

that of gamma rays.) Still in use are the older units the rad = 0.01 Gy and the rem = 0.01 Sv. 
6 

B. Human Effects of Radiation 

The effects of radiation on people can be divided into a) acute physical effects, which occur 

at high doses and lead to debilitation or death, and b) longer term low-dose effects such as cancer 

and genetic effects, which lead to birth defects in offspring. The effects of acute doses vary 

widely depending on the organs exposed. Doses as low as 0.1 Gy (10 rad) to the testes can cause 

temporary sterility in men. Doses of 0.5 to 2 Gy to the eye can lead to an increase in cataracts. 

Red bone marrow is affected by doses as low as 0.5 Gy but the damage at these dose levels is 

usually reversible. Higher doses of 3-5 Gy result in 50% mortality after 1 to 2 months due to 

bone marrow damage. Doses of 10 to 50 Gy kill within 1 to 2 weeks due to gastrointestinal 

damage, while doses of 100 Gy kill in a few hours or days due to nervous system damage [UN, 

1986]. 

Biological effects vary greatly depending on the type of radiation. Short-range alpha 

particles are stopped by the dead skin layer, where they are not harmful. However, when 

ingested or inhaled they can cause leukemia or lung cancer. Beta radiation can penetrate several 

mm of skin and cause painful burns. The more penetrating gamma rays and neutrons can 

damage internal organs at high doses. The average background dose to people in the U.S. is 

about 2 mSv per year. Medical x-rays add another 0.4 mSv on average. 

There is little data on hereditary effects of radiation. Extrapolating from high dose 

Hiroshima and Nagasaki atomic bomb survivors in Japan, the UN estimates an eventual increase 

in birth defects of about 1.5% for a cumulative population dose of 1 Gy per generation. Cancer 

risks have proven even harder to quantify. The incidence of leukemia peaked at about 6 years in 

the atomic bomb survivors, while the rates of all other cancers rose slowly and peaked much 

later at about 30 years. There is little data available on the effects of much lower doses received 

Figure 3. Ruins of Hiroshima [Papalia, 2003]. 
7 

by the general population and little agreement on the validity of extrapolating from the high-dose 

effects seen in the atomic bomb survivors. 

The U.S. annual regulatory exposure limit for radiation workers is 0.05 Sv. The limit for 

exposure to the general public is 5 mSv. 

4. Characteristics of Nuclear and Radiological Weapons 

A. Nuclear Weapons

The first types of nuclear weapons developed during WWII were fission weapons relying on 

the sudden assembly of a supercritical mass of 235U or 239Pu. Two methods can be used to 

achieve this: a) gun assembly where an explosive charge is used to propel two subcritical masses 

together into a supercritical mass, and b) implosion, where a shaped charge is used to compress a 

subcritical mass into a supercritical condition (Figure 4). Modern fission weapons are almost all 

of the implosion type. This results in explosions with yields in the range of 1 kiloton (kT) to 

hundreds of kilotons of TNT equivalent. 

Thermonuclear fusion weapons, first developed in the 1950’s, rely on a first stage fission 

explosion to provide the energy needed to trigger a second thermonuclear stage. The second 

stage makes use of high pressure and energy from the fission explosion to initiate the fusion of 

deuterium or lithium, resulting in the release of large amounts of energy with explosive yields in 

the 1 to 100 megaton (MT) range. Detection of the transport of either type of weapon depends 

on detecting high-energy gamma rays or neutrons emitted by the fission fuel. More details on 

nuclear weapon design can be found in [FAS, 1998]. 

Figure 4. a.) Diagram of a gun assembled weapon. b.) Diagram of an 

implosion weapon. [Glasstone, 1977] 
8 

B. Nuclear Weapon Materials 

Two fissile materials, 235U and 239Pu, which are fissionable by neutrons of all energies, are 

commonly used in nuclear weapons. Natural uranium contains only 0.7% of fissile 235U and is 

mostly (99.3%) 238U, which is fissionable only by high-energy neutrons. For a fission weapon, 

uranium must be enriched to about 93% 235U. This is known as weapons grade uranium (WGU) 

or highly enriched uranium (HEU). If natural uranium is used as the feedstock for enrichment, 

the remaining 7% will be 238U. However, if reprocessed uranium from a reactor is used, there 

will be small amounts of other uranium isotopes, one of which, 232U, has decay products that 

build up over several years and are strong gamma-ray emitters. After enrichment, the natural 

uranium feedstock is reduced to less than 0.3% 235U. This material, known as depleted uranium 

(DU), thus contains greater than 99.7% 238U. DU by itself is not fissile but it is often found in 

fission weapons as a tamper, which acts to prevent premature expansion and also to reflect 

neutrons back into the fissile material. DU is also employed as a radiation casing and fuel in 

thermonuclear weapons, where it is fissionable by high-energy neutrons from fusion. Because of 

its high density, DU is also used in heavy armor-piercing munitions. 

Weapons grade plutonium (WGPu) contains about 93% of the fissile nuclide 239Pu, about 6% 

of 240Pu and less than 1% of other isotopes. Most of the neutron activity of both WGU and 

WGPu is due not to the fissile isotopes but to the other isotopes contained in lesser amounts in 

the materials. Table 2 summarizes the composition and neutron activity from WGU and WGPu. 
9 

Table 2. Neutron Activities from Nuclear Materials 

Weapons Grade Uranium Neutron Activity 

 

 spontaneous spontaneous alpha- WGU 

mass fission fission induced WGU neutron percent

number half-life neutron activity activity composition activity activity

(A) (years) (n/kg/s) (n/kg/s) (n/kg/s) 

 

234 1.50x1016 9.43 50 1.0% 0.594 38.3%

235 1.00x1019 0.01 0.012 93.3% 0.024 1.6%

238 8.20x1015 16.95 0.001 5.5% 0.932 60.1%

other < 0.2% 

 

 Total activity 1.550 

 

Weapons Grade Plutonium Neutron Activity 

 

 spontaneous spontaneous alpha- WGPu 

mass fission fission induced WGPu neutron percent

number half-life neutron activity activity composition activity activity

(A) (years) (n/kg/s) (n/kg/s) (n/kg/s) 

 

238 4.75x1010 2.93x106 2.20 x105 0.005% 160 0.2%

239 8.00x1015 17 630 93.3% 600 0.8%

240 1.14x1011 1.21x106 2,300 6.0% 72,700 98.6%

241 6.00x1016 2.3 22 0.4% 0.1 <0.01%

242 6.77x1010 2.02x106 33 0.015% 300 0.4%

other <0.2% 

 

 Total activity 73,800 

 

references: [Browne, 1986], [Fetter, 1990], [Holden, 2000] 

The dominant decay modes of the uranium and plutonium isotopes listed in Table 2 are by 

alpha-particle emission, except for 241Pu, which decays primarily by beta decay to 241Am. The 

neutron activities arise mostly from spontaneous fissions, which have relatively long half-lives 

compared to alpha or beta emissions. In both uranium and plutonium, the spontaneous fission 

neutron activities are much greater for the even isotopes than for the odd isotopes. In some 

cases, alpha-particle reactions with light elements in the weapon (primarily carbon and oxygen) 

also result in significant neutron emissions [Fetter, 1990]. WGPu is a relatively strong neutron 

emitter, with most of the neutrons coming from 240Pu. For this reason, plutonium weapons are 

generally much more easily detected than uranium weapons. In a hypothetical weapon design 

study using unclassified information [Fetter, 1990], a plutonium weapon containing 4 kg of 

WGPu with a 52 kg DU tamper was found to emit 400,000 neutrons per second. WGU emits 

very few neutrons, so the primary observables are low-energy gamma rays, which are easily 

shielded. 
10 

There has been some debate recently about whether a terrorist weapon could be made from 

reactor grade plutonium (RGPu), which tends to be less tightly guarded than WGPu [Mark, 

1993]. RGPu has a higher percentage than WGPu of plutonium isotopes other than 239Pu. It 

contains 240Pu at a level of 10% or greater, which increases the neutron activity. A crude weapon 

made from RGPu could ignite prematurely due to the excess neutrons, resulting in a “fizzle” 

yield of only a few percent of the expected yield [CCNR, 1997]. Such an explosion would still 

spread highly radioactive material over a wide area with consequences similar to a “dirty bomb” 

as discussed in the following section. 

A hypothetical uranium weapon design containing 12 kg of WGU surrounded by a 79 kg DU 

tamper would emit about 1,400 neutrons per second, while a tungsten tamper would reduce this 

to only 30 neutrons per second [Fetter, 1990]. Thermonuclear weapons with yields in the MT 

range use a fission first stage to trigger thermonuclear fusion in the second stage. Construction 

of a fusion weapon is probably beyond the technical capabilities of a terrorist group, but they 

could conceivably steal a poorly guarded one. In a thermonuclear weapon, the gamma-ray and 

neutron emissions from the fission fuel in the first stage may be significantly shielded and harder 

to detect, while the fusion fuel in the second stage consists of light elements, which do not emit 

detectable radiation. However, the second stage will also likely contain significant quantities of 

238U, which is used as a secondary fuel and is often used as an outer radiation casing. 238U emits 

a penetrating 1001 keV gamma ray that is easily detectable and difficult to shield. 

Table 3 lists the primary gamma ray activities greater than 110 keV for each component of 

WGU. The most prominent gamma rays for each uranium isotope in the table are bolded. 

Gamma rays below 110 keV are easily shielded, and are also obscured by strong x-rays from 

uranium and its decay products. 235U has a strong gamma ray at 186 keV. However, at this 

relatively low energy, it suffers from self-attenuation and is easily blocked by a few mm of lead. 

The 5-6% of 238U in WGU has more penetrating, but relatively weak, gamma ray emission at 

1001 keV. Commonly, the most observable emissions from WGU are gamma rays from the 

small 232U impurity, which is present in reprocessed uranium. Even assuming a 232U 

composition as small as one part per million, the 239, 511, 583 and 2614 keV gamma rays from 

the 232U decay chain are the strongest activities in the table, and they are difficult to shield due to 

their relatively high energies. 
11 

Table 3. Gamma-Ray Activities per Kilogram of Weapons Grade Uranium (the most prominent gamma-rays are bolded) mass number 232a 233 234 235 236 half-life (years) 68.9 1.59x1005 2.45x1005 7.04x1008 2.34x1007 

activity (Bq/kg) 8.28x1014 3.57x1011 2.30x1011 8.00x1007 2.39x1009 WGU Compositionc 0.0001% 0.01% 1.00% 93.30% 0.20% 

 energy activity energy activity energy activity energy activity energy activity  (keV) (kg-s)-1 E(keV) (kg-s)-1 E(keV) (kg-s)-1 E(keV) (kg-s)-1 E(keV) (kg-s)-1 

Strongest 238.59 3.61x1008 117.16 9.99x1002 454.97 5.99x1002 143.79 7.84x1006 112.75 9.10x1002 Gammas 277.28 5.63x1007 118.97 1.14x1003 508.2 3.39x1002 163.38 3.51x1006 > 110 keV 510.61 1.79x1008 120.81 7.49x1002 581.78 2.77x1002 185.74 3.96x1007 

583.02 7.12x1008 135.33 7.85x1002 202.14 7.47x1005 727.25 5.51x1007 145.29 5.71x1002 205.33 3.51x1006 

860.3 9.93x1007 146.35 2.25x1003 221.4 8.96x1004 

2614.35 8.26x1008 164.51 2.35x1003 279.5 2.02x1005 

 187.94 7.14x1002  208.15 8.92x1002  217.13 1.25x1003

 245.29 1.25x1003 291.32 1.86x1003

317.13 3.14x1003

 320.51 1.11x1003

a 232U in equilibrium with 228Th (1.9 years half-life) and subsequent short-lived decay products. b 238U in equilibrium with 234mPa (1.17 minutes half-life) c representative compositions for WGU, actual compositions will vary 

reference: [Browne,1986] 
12 

In the design study referred to above, a weapon made with 235U enriched from natural 

uranium (thus containing no 232U) with a DU tamper would emit about 105

 1001 keV gamma 

rays per second from 238U. If a tungsten tamper were used, the emission would be only about 50 

gamma rays per second [Fetter, 1990], which would be virtually undetectable above background. 

This is clearly a worst-case scenario for detection and not at all typical. A crude weapon 

constructed by a terrorist organization is likely to contain considerably larger amounts of fissile 

material, thus making it more readily detectable. 

Table 4 lists the primary gamma ray activities greater than 110 keV for each component of 

WGPu, with the most prominent gamma rays bolded. 241Am builds up due to decay of 241Pu, 

which has a relatively short half-life of 14.4 years. The table shows the 241Am activities after 3 

years of ingrowth from 241Pu. The strength of the gamma-ray emissions from this isotope is 

useful as an indicator of the age of the plutonium. 241Am also emits a very intense 59 keV 

gamma ray, which may be detectable in lightly shielded material. 
13 

Table 4. Gamma-Ray Activities per Kilogram of Weapons Grade Plutonium 

(the most prominent gamma-rays are bolded) mass number 238 239 240 241 241(Am)a 

half-life (years) 87.7 2.41x1004 2.56x1003 14.4 432.7 

activity (Bq/kg) 6.34x1014 2.30x1012 2.15x1006 3.81x1015 5.91x1014 WGU Composition 0.005% 93.3% 6.0% 0.40% 0.062%* 

 energy activity energy activity energy activity energy activity energy activi

 (keV) (kg-s)-1 (keV) (kg-s)-1 (keV) (kg-s)-1 (keV) (kg-s)-1 (keV) (kg-

Strongest 152.69 3.20x1005 129.28 1.33x1008 160.31 5.43x1006 148.54 2.78x1007 146.55 2.34x1Gammas 742.82 2.47x1003 332.81 1.08x1007 212.46 3.88x1005 159.93 1.01x1006 208.01 4.01x1> 110 keV 766.41 1.05x1004 344.94 1.22x1007 642.35 1.29x1005 322.54 7.72x1

786.29 1.49x1003 375.02 3.39x1007 687.59 3.49x1004 368.59 1.17x1

 808.25 5.45x1002 380.17 6.58x1006 376.58 7.01x1 851.72 6.43x1002 382.68 5.57x1006 662.43 1.83x1 883.24 3.52x1002 392.50 2.49x1006 722.7 6.60x1

1001 4.34x1002 393.12 9.51x1006

413.83 3.24x1007

 451.44 4.11x1006 645.98 3.11x1005 769.37 2.36x1005

a 241Am activity and composition given after 3 years ingrowth from 241Pu 

reference: [Browne, 1986] 
14 

Figure 5 shows computer simulations of high-resolution gamma-ray spectra from WGU and 

WGPu. The 186 and 1001 keV peaks are prominent in the WGU spectrum. There is a complex 

series of peaks in WGPu from 239Pu with energies in the range 300 to 500 keV. The strongest of 

these at 375 and 414 keV comprise a characteristic signature for WGPu. For each material, the 

top spectrum is for a bare source and the bottom spectrum is attenuated by 1 cm of iron and 50 

cm of polyethylene to represent the spectrum observed outside a typical cargo container. In a 

real measurement, these spectra would be superimposed over the background due to 

environmental radiation (see Section 6). 

Figure 5. Computer simulations of high-resolution gamma-ray spectra of WGU 

(left) and WGPu (right). The most prominent peaks are labeled with their energies 

in keV. The top spectrum includes self-attenuation only. The bottom spectrum is 

the result of typical additional attenuation for a source inside a “generic” cargo 

container. Not included are the effects of the environmental background, which 

would obscure all but the strongest peaks. The WGPu peak labeled 0.59 is from 

the decay of 241Am [Geelhood, 2002]. 
15 

Figure 6 shows a high-resolution gamma-ray spectrum purportedly of a Soviet nuclear cruise 

missile in its launcher, taken by a group of US researchers on a Soviet cruiser in the Black Sea 

[Fetter, 1990A]. The spectrum was taken for 10 minutes with a large germanium detector on 

top of the cruise missile launcher. The experimenters were surprised by the “unexpectedly low” 

1001 MeV gamma ray from 238U, indicating very highly enriched uranium with “only about 4% 

238U”, and by the prominent low-energy 235U gamma rays, indicating “that there is almost no 

heavy-metal shielding between the 235U and the detector.” There is a strong possibility that this 

was a mockup rather than an actual weapon, but it does give some indication of what could be 

observed under fairly ideal conditions. The large peak at 2614 keV labeled 208Tl, a decay 

product of 232U, indicates that the HEU comes from reprocessed uranium. 

Figure 6. Spectrum taken with a germanium gamma-ray detector, purportedly of a 

Soviet cruise missile in its launcher. The spectrum was taken for 10 minutes with the 

detector directly on top of the launcher. [Fetter, 1990A]. 
16 

A hypothetical WGPu weapon with a heavy tungsten tamper would strongly attenuate the 

gamma rays below 600 keV [Fetter, 1990 (Appendix B)]. In this case the strongest gamma-ray 

emissions would likely be the 662 and 722 keV gamma rays from 241Am and the 646 and 769 keV 

gamma rays from 239Pu. Even if these gamma rays were heavily shielded, a weapon constructed 

from WGPu would likely still be detectable via its strong neutron emissions, which are difficult to 

shield. 

An on-line catalog of gamma-ray spectra taken with Ge and NaI detectors for sources of 

individual radionuclides is available at [INEEL, 2004]. Included are tables of gamma-ray decay 

energies and intensities. Links are also given to other compilations of nuclear data. 

C. Radiological Weapons 

It is unlikely, but within the realm of possibility, that a terrorist group or rogue state would be 

able to acquire and deliver a nuclear weapon. There are many obstacles to overcome, including 

a) circumventing security measures at nuclear facilities and storage sites to acquire a weapon or 

sufficient material to build one, b) smuggling the weapon or material to a safe site, c) 

engineering and building a weapon or unlocking the safeguards on a stolen weapon, d) 

smuggling the weapon into the country to be attacked and e) successfully detonating the weapon 

[Washington Post, 2004]. 

Much more likely is the possibility of an attack using a radiological weapon or “dirty bomb”. 

Such a weapon would combine a chemical explosive with radioactive material diverted from a 

medical or industrial facility or with highly radioactive spent reactor fuel. The blast effects 

would be confined to nearby buildings. However, the radioactive material could be dispersed 

over an area of a few city blocks to many square miles, depending on the characteristics of the 

explosion, the type of radioactive material and the weather conditions at the time. Any 

immediate deaths would be due to the effects of the blast. In most cases, the population exposed 

to the radioactive material could be evacuated before a lethal dose was received. The primary 

damages of such an attack would be the psychological effects on the population and the 

economic costs due to loss of use of the buildings in the contaminated area, plus the costs of 

cleanup. A recent study concludes that if the winds were right, an explosion at the tip of 

Manhattan (Figure 7) using a stolen industrial cobalt source could cause long-term contamination 

of an area up to the middle of Central Park [FAS, 2002]. 
17 

The use of spent reactor fuel in a radiological attack could have far-ranging effects. The US 

spent nuclear fuel inventory has an average activity of about 150 Ci/kg (5x1012 Bq/kg)a [EPA, 

2003]. A computer model indicates that fallout from a weapon using spent nuclear fuel could 

deliver a lethal dose in a 24 hour period over a broad area extending as far as 400 km [Nichelson, 

1999]. Figure 8 shows a simulation using historical wind patterns on March 2, 1999. A shift in 

the winds could have delivered a 24-hour lethal dose to Washington, DC and spread serious 

contamination as far as Philadelphia and New York City. However, the difficulties in making, 

handling and delivering such a weapon without receiving a lethal or debilitating dose could limit 

its use, while the radioactivity from such a device would be easily detectable and difficult to 

shield during transport. 

 

a

 The SI unit for radioactivity is the becquerel (Bq), named after the discoverer of radioactivity, and defined as one 

disintegration per second. The traditional unit is the curie (Ci) defined as 3.7 x1010 Bq. Both are in common use 

since the Bq is inconveniently small in many contexts, while the Ci is often inconveniently large. 

Figure 7. Simulation of a dirty bomb using an industrial cobalt source normally 

used for food irradiation. The inner contour shows contamination at radiation 

levels equal to the permanent exclusion zone at Chernobyl. The middle contour 

equals the permanently controlled zone and the outer contour the periodically 

controlled zone. [FAS, 2002]. 
18 

In addition to explosive devices, a recent comprehensive study also considered non-explosive 

dispersion scenarios, which have the potential of exposing a large population to lethal amounts 

of radiation before local authorities become aware of the danger and begin evacuations 

[Zimmerman, 2004]. The worst-case scenario assumes that terrorists devise an efficient means 

of aerosol dispersion of a stolen Russian mobile seed irradiator containing up to 250,000 Ci of 

137Cs. If this device were set off in a large city, the death toll could reach hundreds of thousands 

with economic costs in the billions of dollars. Most of these deaths, however, could be averted 

by installation of radioactivity monitors, allowing prompt evacuation in the case of such an 

attack. 

D. Radiological Sources 

The primary potential sources of material for radiological weapons are medical, industrial 

and research sources, and spent nuclear fuel. The most hazardous of these from a radiation 

safety perspective are ranked in category 1 by the IAEA [Ferguson, 2003]. These are listed in 

Figure 8. Simulation of a radiological attack at Langley AFB, Virginia using 10.27 kg 

of spent nuclear fuel. Cumulative dose contours after 24 hours are given in rem, which 

is the biological equivalent dose. A dose of 600 rem (6 Sv) or greater during a 24-hour 

period is usually fatal [Nichelson, 1999]. 
19 

Table 5 along with their typical activity levels. Table 6 lists the half-lives, types of radiation 

emitted and their energies for these sources. By their nature, the sources used for irradiation all 

produce energetic and penetrating gamma rays. They are hard to shield and difficult to transport 

safely. 90Sr, used for radioisotope thermal power generators, emits only low-energy betas that 

are easily shielded. However, its short-lived decay product, 90Y, emits a high-energy beta that, 

when stopped in the shielding material, produces a continuous bremsstrahlunga photon spectrum 

peaked below 200 keV [McCain, 2002; Brodzinski, 1980]. 

Table 5. Medical, Industrial and Research Radiation Sources 

Application Radioisotope Activity Level (Ci) 

Radioisotope Thermal Generators 90Sr 30,000 -- 300,000 

Radiotherapy 60Co 1,350 -- 27,000 137Cs 13,500 

Blood irradiation 137Cs 50 – 2700 

Industrial irradiation 192Ir 3 – 250 

Sterilization and Food Irradiation 60Co 2,700 -- 11x106 

137Cs 2,700 -- 11x106 

Research Irradiators 60Co 27 -- 27,000 137Cs 27 -- 27,000 

Reference: [Ferguson, 2003] 

Table 6. Medical, Industrial and Research Source Characteristics 

isotope half-life type of radiation energy (keV) 

90Sr 28.8 years beta 546* 

 → 90Y 2.67 days beta 2228* 

bremsstrahlung continuous 137Cs 30.1 years beta 1175* 

 gamma 662 

192Ir 74 days beta 669* 

 gamma 296, 308, 316, 468 

60Co 5.3 years beta 669* 

 gamma 1173, 1332 

 

 * endpoint energy 

reference: [Browne, 1986] 

Spent nuclear fuel is a strong source of alphas, betas, gamma rays and neutrons. Table 7 

gives the activities of the primary isotopes in the US spent nuclear fuel inventory [EPA, 2003]. 

As discussed earlier, its high radiation level makes spent nuclear fuel a difficult material for 

construction of a radiological weapon, but one that could have dire consequences if dispersed in 

an explosion. 

 

a

 The word bremsstrahlung comes from German and literally means braking radiation. 
20 

Table 7. US Radionuclide Inventory 

 activities (> 100 Ci per metric ton) 

 half-life Commercial DOE/DOD weighted

isotope (years) BWRa PWRb HLWc

 average 137Cs 30.2 51,800 69,600 29,700 60,000 90Sr 28.8 37,900 49,100 29,300 43,600

241Pu 14.4 30,900 46,100 349 36,700 241Am 433 2,500 3,670 1,050 3,040 238Pu 87.7 1,610 2,870 977 2,280 244Cm 18.1 3,520 9,860 27.70 689 240Pu 6,570 464 53,400 7.77 459 63Ni 100 132 544 8.13 357

151Sm 90 280 366 466 349 239Pu 24,100 297 353 11.30 301 

 a

 BWR: boiling water reactor (units of Ci per initial metric ton of heavy metal) 

b

 PWR: pressurized water reactor (units of Ci per initial metric ton of heavy metal) 

c

 HLW: high level waste (units of Ci per metric ton of heavy metal) 

reference [EPA,2000] 

Neutron sources such as 252Cf or 241Am/Be are also used for industrial and research 

applications. These usually have higher average neutron energy than WGPu, but they may be 

moderated in energy by neutron scattering materials such as polyethylene. These sources are 

also strong gamma-ray emitters. 252Cf has a continuous high-energy gamma-ray spectrum with 

no distinct peaks. 241Am/Be makes use of capture by beryllium of high-energy alpha particles 

from 241Am, followed by neutron emission. This reaction also emits high-energy 4.43 MeV 

gamma rays in addition to low-energy 59 keV gammas from 241Am. 

5. Background Radiation 

A. Gamma-Ray Background 

The natural gamma-ray background is a combination of terrestrial, atmospheric and cosmic 

ray induced gamma rays. A typical gamma-ray background spectrum is shown in Figure 9 with 

the most prominent background peaks marked. 

The terrestrial background is constant at a given location unless there is a substantial 

change in nearby structures. This background has three main components, from the decay of 

232Th, 238U and 40K, generally referred to as thorium, uranium and potassium. Thorium and 

uranium have lengthy decay chains through short-lived “daughter” nuclei, primarily by alpha or 

beta particle emissions, which are not detectable. However, some of the intermediate decay 

products are also strong gamma-ray emitters. Table 8 gives the gamma rays from the decay 

chains of the “parent” background components assuming equilibrium with their decay products. 

The most prominent gamma rays from each component are bolded. The branching fraction is the 

equilibrium gamma-ray intensity per decay of each parent nuclide. 
21 

• Thorium activity is due to decay products from 232Th, which has a half-life of 1.4x1010

years and is found in most rocks, soils and building materials, such as concrete and brick. 

It decays through a series of short-lived isotopes ending in stable 208Pb. The most 

prominent gamma rays are 239 keV from 212Pb, 511, 583 and 2614 keV from 208Tl, and 

911 and 969 keV from 228Ac. The 232Th and 232U decay chains are similar with the 

exception that 228Ac occurs only from 232Th decay. Thus, the observation of the 228Ac 

gamma-rays serves to distinguish the spectrum of 232Th from that of 232U. 

• Uranium activity is due to decay products from 238U, which has a half-life of 4.5x109

years and, like thorium, is found in most rocks, soils and building materials. It decays 

through a series of shorter-lived isotopes ending in 206Pb. The most prominent gamma 

rays are 609, 1120, and 1764 keV from 214Bi. 

• Potassium activity is due to decay of 40K, which has a half-life of 1.28x109

 years. It 

has a single very prominent 1461 keV gamma ray. 

The open ocean background is similar to the terrestrial background but has about one tenth 

the strength of the background over land. Over fresh water and over the ocean near the shore, 

the background intensity depends on how much suspended sediment is in the water. 

The atmospheric background can vary considerably due to wind direction and 

meteorological conditions. This activity is mostly due to short-lived decay products from 222Rn 

gas (3.8 day half-life), which is emitted from decay of soil deposits of 226Ra (1600 year half-life), 

a member of the 238U decay chain. Radon gas often builds up in the soil and can then be 

released in a burst, which may travel tens of kilometers with the wind as a “radon cloud”. The 

prominent gamma rays in Figure 9 at 609, 1120, and 1764 keV are emitted by the radon decay 

product 214Bi. 

The cosmic-ray background is characterized by a 511 keV gamma ray induced by cosmicray interactions. This comes about when high-energy cosmic rays (mostly muons at sea level 

with average energies of 100 MeV or greater) interact with matter producing primarily neutrons 

and pairs of fast moving positive and negative electrons. The positive electron or “positron” is 

the antiparticle of the ordinary negative electron. It eventually slows down enough to be 

attracted by and annihilate with a negative electron, producing two 511 keV gamma rays. The 

intensity of the cosmic-ray background increases rapidly above sea level and dominates the 

gamma-ray background spectrum at aircraft cruising altitudes. 

Underlying the gamma-ray peaks in Figure 9 is a strong continuous background spectrum, 

which is highest at low energies. This is due primarily to higher-energy gamma rays that are only 

partially absorbed by the detector. At higher altitudes there is also a strong contribution to the 

background continuum from high-energy cosmic rays, which produce a continuous 

bremsstrahlung spectrum as they slow down in the material in the vicinity of the detector. 
22 

Figure 9. A typical high-resolution gamma-ray background spectrum, taken for 

4096 seconds with a 15% relative efficiency detector. The most prominent peaks 

are labeled with their energies in keV. The scatter in the spectrum is due to 

random statistical variations. 
23 

Table 8. Terrestrial Background Gamma Rays (the most prominent gamma-rays are bolded) background nuclide 

232Th 238U 40K 

half-life (years) 1.41x1010 4.47x1009 1.28x

activity (Bq/kg) 4.06x1006 1.24x1007 2.59x

 energy 

(keV) 

branching 

fraction

decay 

product 

 energy 

(keV)

branching 

fraction 

decay 

product 

 energy 

(keV)

branc

fra

strongest 238.58 43.60% 212Pb 241.92 7.46% 214Pb 1460.83 10.

gammas 240.76 3.90% 224Ra 351.87 37.10% 214Pb 

> 110 keV 277.28 6.80% 208Tl 609.31 46.10% 214Bi 

 300.03 3.34% 212Pb 665.44 1.56% 214Bi 

338.42 12.40% 228Ac 768.35 4.88% 214Bi 

 463.1 4.60% 228Ac 785.83 1.09% 214Pb 

510.61 21.60% 208Tl 806.16 1.23% 214Bi 

583.02 86.00% 208Tl 934.04 3.16% 214Bi 

 727.25 6.65% 212Bi 1120.27 15.00% 214Bi  763.06 1.64% 208Tl 1555.18 1.69% 214Bi  794.79 4.60% 228Ac 1238.11 5.92% 214Bi 

860.3 12.00% 208Tl 1280.95 1.47% 214Bi 

911.16 29.00% 228Ac 1377.66 4.02% 214Bi 

 964.64 5.80% 228Ac 1401.48 1.39% 214Bi 

968.97 17.40% 228Ac 1407.97 2.48% 214Bi 

 1496 1.05% 228Ac 1509.22 2.19% 214Bi  1588.23 3.60% 228Ac 1661.26 1.15% 214Bi  1620.66 1.51% 212Bi 1729.58 3.05% 214Bi 

2614.35 99.79% 208Tl 1764.49 15.90% 214Bi 

 2118.53 1.21% 214Bi  2204.09 4.99% 214Bi 

 2447.68 1.55% 214Bi reference: [Browne,1986] 
24 

B. Neutron Background 

The natural neutron background is mostly due to cosmic-ray interactions with the 

atmosphere, the ground and massive objects such as buildings, ship superstructures and cargo (a 

phenomenon known as the “ship effect” since it was first observed in the neutron signal from 

large ships.) It peaks in energy at about 1 MeV and drops off rapidly above this. At sea level, 

the average cosmic-ray neutron production is about 20 neutrons per kg of material per second. 

This results in a neutron flux of 100-300 neutrons/m2

/s. The average neutron background varies 

with geomagnetic latitude and is highest above 45 degrees, dropping to a low point at the 

equator. It also varies with solar activity and is approximately a factor of two higher during the 

solar minimum, when the shielding effect of the sun’s magnetic field is lowest. During solar 

flares, the neutron background at high latitudes increases dramatically due to atmospheric 

interactions with energetic charged particles emitted by the flare. This variability must be 

considered when using a neutron detector. In addition, an inspector must know the expected 

amplitude of the ship effect, or cosmic-ray induced neutron signature, from any massive cargo 

container, in order to avoid mistaking it for a suspect source. 

C. Man-Made Background 

Since the cessation of atmospheric nuclear testing, man-made background due to fallout has 

declined to levels well below the natural background. Except in regions contaminated by nuclear 

accidents, such as Chernobyl, or by an occasional lost medical or industrial source, man-made 

background will not be an appreciable contribution to the radiation background. 

D. Detection Capabilities in the Natural Radiation Environment 

Several factors come into play in determining whether a nuclear weapon or nuclear material 

is detectable above the natural radiation background. These include the configuration of the 

weapon or material, the amount of shielding, the type of detector, the level of the radiation 

background, the distance from the source and the counting time. In general, the instrumental 

noise in the detector is negligible compared to the random statistical noise due to variations in 

the counts due to the background and the source. 

Figure 10 shows the calculated gamma-ray counts versus distance for the signal from a 

hypothetical WGU nuclear weapon [Fetter, 1990] with a thick depleted uranium tamper, which 

emits 100,000 gamma rays per second with energy 1001 keV. The calculation assumes the use 

of a large 100% relative efficiency Ge detector with detection efficiency at 1001 keV of 

1.35x10-4 for a point source one meter from the detector. It takes into account the geometrical 

1/R2

 decrease with range R for the signal from a point source, as well as the air attenuation of the 

gamma rays. The solid line shows the calculated counts per 1000 seconds versus distance for the 

1001 keV gamma-ray peak from the WGU source. The dashed line shows the expected counts 

in the region of the 1001 keV peak for a typical gamma-ray background and the dotted line 

shows the three standard deviation level “3-sigma” due to random variations in the total (peak 

plus background) countsa

. A good rule of thumb is that the signal from the weapon is detectable 

 

a

 Sigma is the statistical standard deviation in the total counts, which is equal to the square root of the sum of the 

peak counts plus background counts in the region of the peak. 
25 

if it is greater than three times the standard deviation in the signal. Using this rule, the gammaray signal is detectable out to a range of about 19.2 meters. 

Figure 11 shows the calculated neutron counts versus distance for the signal from a 

hypothetical WGPu weapon that emits 400,000 neutrons per second. The calculation assumes 

the use of a large 1 square meter area neutron detector with an absolute efficiency of 10-4 per 

neutron. This gives a detection efficiency of about 8 x 10-6 for a point source one meter from the 

detector. The solid line shows the calculated counts for 1000 seconds for neutrons from the 

WGPu source. The dashed line shows the expected counts for a typical background flux of 300 

m

-2

s

-1 and the dotted line shows the 3-sigma level. Using the above rule of thumb, the neutron 

signal is detectable out to about 11.8 meters. 

Figure 10. Peak gamma-ray counts (solid curve), background counts (dashed line) 

and 3-sigma standard deviation level (dotted line) plotted versus distance for a 1000 

second observation of a hypothetical WGU weapon using a 100% relative efficiency 

Ge detector. The signal is detectable as long as the peak counts are above the 3-

sigma level. 

Peak Gamma-Ray Counts from a 

Hypothetical Nuclear Weapon

1

10

100

1000

10000

100000

0 10 20 30 40

distance (m)

counts/1000 s (100% G

e

peak counts

background

3 sigma
26 

Table 9 shows how the detection range for WGU and WGPu weapons varies with counting 

time, detector size (or number of detectors) and source strength. A rough approximation is that 

an N-fold increase in source strength increases the detection range by N1/2, while an N-fold 

increase in counting time or detector size, which raises both the source counts and the 

background counts, increases the detection range by only N1/4. The last row of the table shows 

the detection range if the WGU gamma-ray source is shielded by 1 cm lead and the WGPu 

neutron source by 10 cm polyethylene. This reduces the detection range from 19.2 to 13.4 

meters for the WGU weapon and from 11.8 to 6.7 meters for the WGPu weapon. For a given 

source, in order to double the ratio of the signal to 3-sigma level, one would need to increase the 

product of the counting time and detector size by a factor of four. If possible, the best detection 

strategy is to get closer, which increases the signal without increasing the background. 

Figure 11. Neutron counts (solid curve), background counts (dashed line) and 3-

sigma standard deviation level (dotted line) plotted versus distance for a 1000 second 

observation of a hypothetical WGPu weapon emitting 400,000 neutrons per second, 

using a 1 square meter neutron detector. The signal is detectable as long as the 

counts from the weapon are above the 3-sigma level. 

Neutron Counts from a 

Hypothetical Plutonium Weapon

0.1

1

10

100

1000

10000

0 10 20 30 40

distance (m)

counts/1000 s (1 m2 detector)

counts

background

3 sigma
27 

Table 9. Detection ranges for hypothetical nuclear weapons 

WGU gamma ray emitting weapon WGPu neutron emitting weapon 

number of 

detectorsa

counting 

time (s) 

gamma-ray 

source (s-1)

range 

(m)

detector 

area (m2)

counting 

time (s)

neutron 

source (s-1)

range 

(m)

one 100 100,000 9.5 1 100 400,000 5.2 

one 1000 100,000 19.2 1 1000 400,000 11.8 

two 1000 100,000 23.1 2 1000 400,000 14.5 

ten 1000 100,000 34.1 10 1000 400,000 22.4 

one 1000 1,000,000 53.0 1 1000 4,000,000 35.2 

one 1000 46,300b

 13.4 1 1000 126,000c 6.7 

 a 100% relative efficiency Ge detectors b 100,000 s-1 source shielded by 1 cm lead c 400,000 s-1 source shielded by 10 cm polyethylene 

6. Nuclear Radiation Detection

A. General Aspects of NRWMD Detection 

The detection technologies relevant to locating WMD are generally complex in both their 

design and employment. They almost always involve the tight integration of “cross-cutting” 

capabilities from multiple scientific and engineering disciplines. Production of a detection 

instrument requires exploitation and optimization of one or more generally sophisticated 

physical, chemical or biological mechanisms. It also requires expertise in materials and in 

electrical and mechanical engineering. Many engineering tradeoffs are needed for designing such 

systems in order to insure adequate performance and reliability at reasonable costs. An 

instrument that offers such performance at acceptable costs is inevitably a tight integration of 

electrical and mechanical components. Instruments intended for use in the field require attention 

to thermal, vibrational and other environmental factors. Design and performance trade-offs are 

virtually unavoidable. Care needs to be taken that the complexity of design and fabrication does 

not translate into a complexity of operation. Attention to the user interface and training is 

important to minimize operator error. Provision of needed supplies and spare parts is a logistical 

requirement. Successful utilization of any of the detection technologies for NRWMD hinges on 

proper care for the instrument, including routine maintenance and re-calibration. 

There are two fundamental classes of means for detecting and assaying the materials that 

may be made into or already constitute an NRWMD. The first class is technologies to find and 

exploit some signature, which indicates the presence of nuclear or radiological material. 

Typically these exploit spontaneous radioactive emissions from nuclear materials, or emissions 

stimulated by x-rays, gamma rays or neutrons. The second class of detection technologies 

involves finding NRWMD devices. They often involve the acquisition of images that reveal 

these devices from their shape or from surrounding materials. Very large radiography systems, 

using either high energy x-rays or gamma rays, can image the contents of an entire truck or sea 

container. 
28 

Whatever the WMD materials of interest, there are some fundamental and general aspects of 

the instruments that will detect, identify and quantify these substances. In the most general terms, 

a detection technology is embodied in an instrument that accepts signals or samples molecules 

from nuclear materials and turns them into information. This is indicated schematically in 

Figure 12. 

 

 The front end of a detection device may include filters or masks for nuclear radiation. 

Ultimately, the external energy is sensed by one or more sensors that transform the input into 

data. The computer, which is commonly built into an instrument, turns the data into information 

for display, storage, processing or transmission. The system must provide power, communication 

of the components and be contained in a protective housing or case. 

The detection of nuclear materials by their natural or stimulated radiation requires the 

employment of a detector system with a few subsystems based on physical interactions. The 

general layout of such systems is shown in Figure 13. 

 The key element is the detector itself, which consists of materials of sufficient size and 

proper geometry to absorb energy from incident radiation and convert that energy into some 

other measurable form. In general, an electrical signal results, which is usually processed by 

electronics, such as pulse amplifiers and counters that register the number of arriving pulses with 

defined characteristics. Some detector systems are insensitive to the energy of the incident 

 Figure 12. Diagram of the most basic components of a detection technology 

Figure 13. General layout of systems for detection of nuclear radiations 
29 

radiation. If so, it may be possible to determine the radiation energy range by absorbers between 

the source and detector, or by using detectors of varying thicknesses. 

 The problem of detection of an NRWMD usually involves low count rates and the use of 

detector systems that yield pulses that vary in size with the energy of the incident radiation. In 

this case, a spectrum results from the display of the number of counts for each incident energy 

increment. Examples of such spectra are shown in Figures 6 and 9. Whatever the detector 

behavior, the resulting digitized information is stored in the computer. This information is 

available for display, further manipulation or transmission. Special purpose software is required 

to perform both the control functions and the data handling. A common function is to apply 

corrections to the data to account for energy-dependent responses of the materials and the 

detector. After making these corrections, the energies and intensities of gamma-ray peaks 

observed in the spectra can be compared with stored values for materials of interest. 

 Nuclear detection instruments share a common set of characteristics with detection 

technologies for all WMD. The use of calibration curves, which relate count rates to the amount 

of material, is one of them. The threshold behavior near the limit of detection for weak nearby 

radiation sources, or strong sources at a greater distance, is another. Threshold detection is very 

dependent on the environmental and operating conditions, as well as the radiation source and the 

detection instrument. Calibration and operating curves are discussed in Appendix A. There are 

also practical considerations that apply to the choice of detection technologies for nuclear 

materials and other WMD. These are reviewed in Appendix B. 

B. Characteristics of Radiation Detectors 

The observable gamma rays and neutrons from nuclear weapons or materials have mean free 

paths of the order of hundreds of meters in air and one to ten centimeters in water and solids, as 

given in Table 1. This physical fact means that useful detectors cannot be too small or 

lightweight. Typical detectors have volumes of tens or hundreds of cubic centimeters and 

weights of several kilograms. Lightweight portable detectors exist, but they can only be used for 

relatively strong sources at close range. A general discussion of the detection process is given 

here. This is followed by more detailed descriptions of individual detector types. A useful 

source of information on radiation detectors can be found in [Knoll, 2000]. 

Gamma rays and neutrons are both long-range neutral particles, which do not produce an 

electrical signal when they pass through the detector. For detection, their energy must be 

transferred to short-range charged particles. Gamma-rays interact with the detector to produce 

energetic electrons. These lose energy in semiconductor detectors by producing particle-hole 

pairs, which are swept up by an applied voltage to give an electrical signal. Similarly, in gasfilled detectors, such as proportional counters and Geiger counters, the energetic electrons ionize 

the gas producing electron-ion pairs which are then swept up by an applied voltage. In 

scintillation detectors the electrons interact to excite the crystal structure which then decays by 

emitting photons. These are converted to an electrical signal by the use of photomultiplier tubes 

or photodiodes. 

Radiation exposure monitors, such as x-ray film or personnel dosimeters, record a physical 

change in the material caused by the radiation. When the film is developed or the dosimeter read 

out, a record of the cumulative radiation exposure is obtained. Radiation detectors convert the 

radiation in real time to an observable effect, such as electronic pulses or scintillator light. The 
30 

Geiger counter is the simplest type of detector, using a voltage applied across a gas-filled tube to 

convert radiation into electric pulses that can produce an audible click. The survey meter is a 

similar device, which is usually calibrated to give a meter reading in units of dose rate. More 

sophisticated detectors give additional information, such as the radiation energy, which can be 

used to identify the source. 

Gamma-ray detectors are available as hand-held scintillation crystals, such as thallium 

activated sodium iodide (abbreviated NaI(Tl) or often just NaI), which convert gamma radiation 

to light. The light is further converted by a photomultiplier to an electronic signal, whose pulseheight is proportional to the gamma-ray energy. These signals are recorded by a pulse-height 

analyzer (PHA), which displays a spectrum of counts versus gamma-ray energy. NaI detectors 

have a limited energy resolution of about 8% for the 662 keV gamma rays from 137Cs. 

Solid-state semi-conductor detectors convert the gamma-ray energy directly to an electronic 

pulse that is then recorded by a PHA. Germanium (Ge) detectors have an energy resolution of 

about 0.2%, or forty times better than NaI, and produce a high-resolution spectrum with sharp 

peaks rising above an underlying continuum. This superior resolution allows precise 

determination of the gamma-ray energy, separation of close lying peaks, and identification of 

weak peaks in the presence of a strong interfering background. The disadvantage of Ge detectors 

is that they require cooling to below 100 degrees Kelvin, either by liquid nitrogen or electromechanical coolers. This makes the detector system heavier than NaI and adds logistical 

problems in the field, requiring a source of liquid nitrogen or additional power to drive the 

cooler. For this reason, much effort has gone into developing room-temperature semiconductor 

detectors, such as cadmium zinc telluride (CZT) or mercuric iodide (HgI). These have 

resolutions down to about 2%, superior to NaI but no match for Ge detectors. To date they have 

not been produced in volumes of more than a few cm3

 compared to several hundred cm3 for Ge 

and several thousand cm3 for NaI. Their small volumes limit not only the sensitivity of roomtemperature semiconductor detectors but also their ability to stop high-energy gamma rays. 

Thus, Ge detectors remain the best choice for detecting and identifying gamma rays from nuclear 

materials. 

Both in semiconductor and scintillation detectors, the electrical signal is proportional to the 

energy deposited in the detector. By the use of specialized nuclear electronics, this signal is 

amplified and shaped into a short pulse whose height is proportional to the original signal 

strength. Pulse height converters digitize the signal, which is then sent to a computer and used to 

construct an energy spectrum that can be displayed on a monitor. Gamma-ray detector 

electronics are described in greater detail in Section 7. 

All neutron detectors work by detecting the charged particles produced when the neutrons 

interact with a suitable material. Neutron absorption is most efficient for low-energy thermal 

neutrons, so these detectors include a moderating material, which slows the neutron down by 

elastic scattering. After absorption, the excited nucleus decays emitting a charged particle, which 

can be detected by a scintillator, semiconductor diode or gas-filled counter. The neutron energy 

information is lost in the moderation process so the resulting display is typically count rate or is 

converted to dose rate. The most common neutron detectors are proportional counters containing 

a gas such as 3

He or BF3, which have high thermal neutron capture probabilities. Solid-state 

neutron detectors are in development, which contain a thermal neutron capture material such as 

10B or 6

Li followed by a semiconductor particle detector. All of these detectors require a 
31 

moderator to slow the fission energy neutrons emitted by nuclear materials. Fast neutron 

detectors, which do not contain a moderator, have several orders of magnitude lower sensitivity. 

C. Employment Scenarios 

The best detection equipment will not be effective unless it is in the right place at the right 

time and in the hands of trained inspectors. In an exercise reported by ABC News [Ross, 2002] a 

mock-up of a nuclear weapon, consisting of 15 lbs of depleted uranium shielded by a steel pipe 

with a lead lining inside a suitcase (Figure 14), was 

transported by rail from Austria to Turkey, passing 

through multiple border checkpoints without being 

inspected. An x-ray or gamma-ray scan of this 

mockup would have surely indicated something 

suspicious. It was then crated and shipped by sea 

from Istanbul to New York. There it passed through 

U.S. Customs on Staten Island without being 

stopped or the crate opened for inspection, although 

Customs reportedly has state-of-the-art x-ray and 

radioactivity detectors at this facility. 

In a warehouse situation, a survey meter is nondirectional and subject to false alarms. Portal 

detectors, or even a single collimated NaI detector, 

could be used as a first alarm, but a high-resolution germanium detector should then be brought 

in to confirm the identification of a suspect source. A neutron detector can supplement the 

gamma-ray detectors. 

Active interrogation such as a high-energy x-ray cargo scan is probably the only way to 

detect a well-shielded weapon inside a cargo container. However, an unshielded plutonium 

weapon emits about 105

 neutrons and gamma rays per second [Fetter, 1990] and would be much 

easier to detect. From Table 1, these radiations have mean free paths in air of the order of 100 

m. A mobile detector for such a weapon could be carried on a golf cart for dockside inspection 

or on small watercraft for harbor inspection. It would contain one or more large germanium 

detectors and gas-filled neutron tubes along with battery power, electronics and a computerbased MCA with analysis software. A similar system could be placed at border crossings and, 

with the addition of remote communication capabilities, under bridges leading to major ports 

such as the Golden Gate Bridge in San Francisco and the Verrazano Bridge in New York. 

A “dirty bomb” with enough activity to cause serious contamination would be easiest to 

detect. Small, cheap gamma-ray monitors with high enough thresholds to prevent frequent false 

alarms could be positioned at vulnerable locations in major cities and as perimeter monitors at 

nuclear facilities and military installations. Responders to such an alarm need to be equipped 

with NaI or germanium detectors and be trained to use them. 

Figure 14. Nuclear weapon mock-up 

in a suitcase [Ross, 2002]. 
32 

7. Available Point Detector Technologies 

A. Gamma-Ray Detectors 

Gamma-ray spectra from uranium and plutonium, illustrated in Figure 5, start out showing 

sharp peaks with energies characteristic of the radioactive material. However, some of these 

gamma rays undergo interactions in the source and in the environment, by which they lose 

energy and are then no longer characteristic of the source material. Those gamma rays that reach 

the detector with their full energy can be completely absorbed resulting in detection as a fullenergy peak or they can scatter and deposit only part of their energy. If the detector is large 

enough to have a significant probability of completely absorbing the scattered gamma rays then 

the combined energy again results in a full-energy peak. A thin detector will have a small 

probability of complete absorption and most of the detections will be in the continuum below the 

full-energy peak. 

There are two commonly used classes of detectors for gamma rays in the energy range of 

emissions from nuclear materials, a) scintillation crystals coupled to photodiodes or 

photomultiplier (PM) tubes and b) semiconductor detectors. The most common scintillation 

detector is NaI(Tl) usually coupled to a PM tube (Table 10). Other scintillators used in limited 

applications include thallium activated cesium iodide, CsI(Tl), and bismuth germanate, BGO. 

Plastic scintillators are used for large area applications such as portals, but they have little or no 

full-energy peak efficiency. 

 A common NaI configuration is a cylinder 3 in. diameter by 3 in. long (7.5 cm diameter by 

7.5 cm), but they can be made in many configurations and sizes. NaI detectors have relatively 

poor energy resolutions, at best about 50 keV (7.5%) for the 662 keV gamma rays from a 137Cs 

calibration source. This limits their use in high background situations or for unknown sources 

with many closely spaced peaks. Gamma-ray peaks from a weak source will be difficult to 

observe in a relatively high background environment and peaks that differ by a few percent in 

energy will be unresolved. 

Semiconductor detectors were developed to overcome these limitations of scintillation 

detectors. Germanium semiconductor diodes (Table 11) are the gold standard for gamma-ray 

detectors with resolutions of typically 1.3 keV (0.2%) at 662 keV. This allows precise 

determination of peak energies, separation of close-lying peaks and detection of weak peaks in 

the presence of a strong background. Germanium (Ge) detectors have the disadvantage that they 

must be operated at low temperatures (less than 100 K) to avoid excessive electronic noise. 

Normally, this is achieved by connecting the detector to a dewar containing liquid nitrogen (LN), 

which boils at 77 K. Small hand-held dewars must be refilled daily, while larger dewars may 

last a week or more, but are not easily portable. The detectors require several hours to cool down 

and therefore must be kept cool to be in a ready statea

. The requirement for a steady supply of 

LN can be a logistical problem in remote locations. Mechanical refrigerator coolers are also 

available, but these are more suited to fixed locations, since they are relatively heavy and require 

electric power. In a transportable detector, refrigerators could run off car batteries, but these 

would require frequent recharging. 

 

a

 The earlier lithium drifted detectors, Ge(Li), had to be kept cold at all times. Current detectors made with highly 

purified germanium, known as “Intrinsic” or “High-purity” germanium (HPGe) detectors can be allowed to warm up 

when not in use. 
33 

Large volume germanium detectors are cylindrical with either a 

coaxial or closed-end coaxial diode configuration (Figure 15) and 

can be made with volumes of several hundred cubic centimeters. 

They are often quoted as percent efficiency at 1332 keV relative to a 

3x3 inch NaI detector. Ge detectors are currently available with 

diameters up to 9 cm and relative efficiencies greater than 100%. 

Figure 16 shows calibration curves of peak detection efficiency 

versus energy for typical 32% relative efficiency p-type (GEM) and 

n-type (GMX) coaxial Ge detectors, measured using point gammaray sources 25 cm from the front face of the detectors. The 

maximum efficiency occurs at about 0.1 MeV with a value of 

approximately 0.3%. This is limited by the solid angle of the 

detector face and can be improved by using a larger area detector. 

The efficiency curve for GEM detectors drops off at lower 

energies due to attenuation in the outer dead layer of the detector and 

in the aluminum enclosure. The GMX curve extends the maximum 

efficiency to lower energy by using an n-type detector, which has a thinner dead layer, and by a 

thin beryllium entrance window in the aluminum enclosure. At higher energies, the efficiency 

drops off because the detectors cannot completely stop the high-energy photons. This can be 

improved by going to a thicker detector. Technical descriptions of Ge and other types of 

radiation detectors can be found in [Knoll, 2000]. 

There have been extensive efforts to develop room-temperature semiconductor detectors that 

can be used in place of germanium detectors. Cadmium telluride (CdTe), cadmium zinc telluride 

Figure 15. Cross 

section of a closedend coaxial Ge 

detector [Canberra, 

2004]. 

Figure 16. Calibration curves of peak detection efficiency versus gamma-ray energy 

for 32% relative efficiency coaxial Ge detectors [ORTEC, 2004]. The curve labeled 

GEM is for a standard p-type detector and GMX for an n-type detector with a thin Be 

window. 
34 

(CZT) and mercuric iodide (HgI) semiconductor diode detectors are currently available 

commercially (Table 12). For CZT, best efforts to date have produced detectors with about 13 

keV (2%) resolution at 662 keV. This is significantly better than NaI but still a factor of ten 

poorer than germanium detector resolutions. Commercially available CZT sizes are typically a 

square cm in area by 5 mm depth, which is not thick enough to provide useful full-energy peak 

detection efficiencies in the several hundred keV to several MeV range characteristic of 

emissions from nuclear weapons. CdTe detectors are available with less than 1% resolution at 

662 keV, but only for a small 3x3x1 mm detector. HgI detectors are available in larger sizes up 

to one cubic inch (16 cubic cm), but with poorer resolutions of about 25 keV (4%) at 662 keV. 

B. Gamma-Ray Detector Electronics 

Gamma-ray detectors require specialized electronics to convert the signal from the detector 

into a gamma-ray energy spectrum characteristic of the source. Figure 17 shows the basic 

components required. Traditionally, each of the modules shown represents a separate 

electronics box. Recently, however, several have been combined into compact low-power units 

convenient for surveys in the field. 

 

A DC high-voltage supply is required with output of a hundred to several thousand volts 

depending on the type and size of detector. The pulse from a semiconductor detector goes first to 

a preamplifier (preamp), which is usually attached to or in close proximity to the detector. The 

preamp output then goes to a module that amplifies and shapes the signal for input to the analog 

to digital converter (ADC). The amplifier module usually also contains a low-voltage power 

supply for the preamp. Scintillation detectors such as NaI are coupled to a photomultiplier (PM) 

tube that converts the scintillator light to an electronic pulse and contains up to a dozen 

multiplication stages. The PM output may require no further amplification and can be sent 

directly to the ADC. The output of the ADC is in digital form with a larger number 

corresponding to a higher pulse height, which in turn is proportional to the energy deposited in 

the detector. This number is sent to the multichannel analyzer (MCA), which builds up a 

Detector

Preamp 

or PM Amplifier

Lowvoltage 

power 

supply

Analog to 

digital 

converter 

(ADC)

Multi-channel 

analyzer 

(MCA) 

High-voltage 

power supply

Figure 17. Typical gamma-ray detector electronics. 
35 

spectrum consisting of counts per channel, with higher channels corresponding to higher energy. 

This spectrum is usually available as a real-time display. The MCA is normally computer based. 

It also contains software for applying energy and efficiency calibrations to the spectrum and for 

conducting a peak search and calculating peak energies and intensities. It may also contain a 

library of characteristic gamma-ray energies and intensities, for nuclides of interest, and software 

to identify peaks in the spectrum with the corresponding nuclidea. The combination of ADC and 

MCA is sometimes referred to as a pulse-height analyzer (PHA). 

 

a

 A complete line of nuclear electronic modules is available from ORTEC, Oak Ridge, TN or from Canberra 

Industries, Meriden, CT. (See Table 4.10 for contact information.) 
36 

Table 10. Sodium Iodide Gamma-Ray Detectors 

Illustration Company Size (diameter x length) Dimensions/ Weight/ 

Power/ Cost 

Amptek Inc. 

6 De Angelo Drive 

Bedford, MA. 01730 U.S.A. 

781-275-2242 

http://www.amptek.com

30x30 mm to 152 x 76 mm 

(1.2x1.2” to 6x3”) 

Ruggedized assembly. 

Interfaces with MCA 8000A 

portable analyzer (See 

Table 14) 

Note a. 

3x3 detector: 

3” diam. x 10.5”/ 

2 kg/ 200 mW/ $7800 

including MCA 

(laptop not included) 

1x1, 2x2, 3x3” 

Polycrystalline up to 

4x4x40” 

Bicron/St. Gobain 

Solon, OH 

440-248-7400 

http://www.bicron.com Other scintillators available 

include BaF3, BGO, CsI, 

GSO, LSO, and LaCl3

Note a. 

Berkeley Nucleonics 1.5x2, 2x2, 3x3” Note a. 

San Rafael, CA 

800-234-7858 

http://www.berkeleynucleonics.

com

Also available as integrated 

probe (see Table 14) 

Canberra Industries 2x0.5, 2x2, 3x3” 

Meriden, CT 

203-238-2351 

http://www.canberra.com

Other sizes available on 

request. 

Note a. 

ORTEC 1x1, 2x2, 3x3” Note a. 

Oak Ridge, TN 

800-251-9750 

865-483-4411 

http://www.ortec-online.com

Interfaces with portable 

analyzer. Also available as 

integrated probe (see Table 

14) 

3 x 14” overall/ 

3 kg/ 

+12V at 20mA/$731 

(2x2), $937 (3x3) 

Princeton Gamma-Tech 1.5x2, 2x2, 3x3” Note a. 

Rocky Hill, NJ 

800-229-7484 

609-924-7310 

http://www.pgt.com

Semirugged weather 

resistant assembly. 

3x3” assembly: 

4.5x13”/ 7 lb/ 0.5 mA 

@ 1 kV/ $3700 

Standard sizes 0.5 to 5” 

diameter. 

Scionix Note a. 

Utrecht, Netherlands 

In the USA: 

(407) 578-6469 

http://www.scionixusa.com

Available with integrated 

photomultiplier and high 

voltage power supply 

a

 NaI density is 3.67 g/cm3; for a standard 3x3 detector: volume = 347.5 cm3, weight = 1.275 kg (2.8 lb). Typical 

dimensions for a standard 3x3 detector with photomultiplier (PM) tube: 3.2 in diameter x 5.8 in. Typical weight: 

about 8 lb. Typical resolution: 7% at 662 keV, 5% at 1332 keV, typical cost < $1,000 including PM tube and base.
37 

Table 11. Portable Germanium Gamma-Ray Detectors 

Illustration Company Relative 

Efficiencya

Resolution (keV) Peak/ 

Compton 

Ratio 

Dimensions/ 

Weight/ 

Power/ Cost 

10-50% 0.8-1.2@122 keV 

 (0.65-1.0%) 

1.8-2.1@1332 keV 

 (0.135-0.16%) 

38-60 

Canberra Industries 

Meriden, CT 

203-238-2351 

http://www.canberra.

com

55-100% 1.2-1.3@122 keV 

 (1.0-1.1%) 

1.8-2.1@1332 keV 

 (0.135-0.16%) 

60-80 

9x23”/ 16 lbs. 

With full 2.5 l 

dewar/ ~ $29,750.

Compete system 

with portable 

MCA, software 

and laptop 

~ $48,000 

10% See Table14 

The Detective 

Table 14 Table 14 

15-53% 1.8-1.9@1332 keV 

 (0.135-0.14%) 

48-83 

ORTEC 

Oak Ridge, TN 

800-251-9750 

865-483-4411 

http://www.orteconline.com

63-118% 1.8-2.0@1332 keV 

 (0.135-0.15%) 

70-92 

Dim. na/ 31 lbs 

with full 3 liter 

dewar/ $27,662 

for 50% detector 

with dewar 

10-50% 0.8-1.0@122 keV 

 (0.65-0.82%) 

1.75-2.0@1332 keV 

 (0.13-0.15%) 

Princeton Gamma- 37-65 

Tech 

Rocky Hill, NJ 

800-229-7484 

609-924-7310 

http://www.pgt.com 55-100% 1.0-1.4@122 keV 

 (0.8-1.15%) 

1.95-2.3@1332 keV 

 (0.15-0.18%) 

64-75 

6x21” excl. 

handle/18 lbs with 

full 1.5 L dewar/ 

250 mW/ $39,000 

with 50% detector

a

 Efficiency at 1332 keV relative to a 3x3” NaI(Tl) detector with the source at 25 cm from the front face of the 

detector. 
38 

Table 12. Room Temperature Semiconductor Detectors 

Illustration Company Detector Type 

Size (length x 

width x depth) 

Resolution Dimensions/ 

Weight/ Power/ 

Cost 

AMPTEK 

Bedford, MA 

781-275-2242 

www.amptek.com

CdTe, 

3x3x1mm 

1.0% @ 59.5 keV 

0.7% @ 122 keV 

0.9% @ 662 keV 

na 

Constellation Technology 

Largo, FL

727-547-0600 

www.contech.com

HgI, 

up to 25x25x3 

mm 

4% @ 662 keV na 

CdZnTe 

Up to 5x5x2 mm 

4-6.6% @ 122 keV 

CdTe 

Up to 5x5x1mm 

<4% @ 122 keV 

EURORAD 

Marne, France 

+33 (0) 1 56 86 11 49 

http://www.eurorad.com

distributed by Constellation 

Technology 

Largo, FL 

727-547-0600 

http://www.contech.com Integrated 

Pelletier cooler 

available. 

na 

CdZnTe (CZT) 

4x4x2.5 mm up 

to 34x34x5 mm 

Co-Planar Grid 

10x10x5 mm to 

15x15x7.5 mm 

eV Products 

Saxonburg, PA 

724-352-5288 

http://www.evproducts.com

Custom sizes 

available 

3.7% @ 122 keV 

2.5-3.5% @ 662 

keV 

SPEAR system 

(5x5x5 mm with 

preamp): 

12mm diam. x 

89mm/ <2 lb/ 

<50 mW 

CdZnTe 

5x5x5 mm 

Portable detector 

probe, integrated 

with portable 

analyzer. 

ORTEC <3% @ 122 keV 

Oak Ridge, TN 

800-251-9750 

865-483-4411 

http://www.ortec-online.com

CdZnTe, 300 

mm3

<3% @ 662 keV 

na 

na: information not available 
39 

Table 12. (continued) Room Temperature Semiconductor Detectors 

Illustration Company Detector Type 

Size (length x 

width x depth) 

Resolution Dimensions/ 

Weight/ Power/ 

Cost 

Radiant Detector 

Technologies, LLC, 

Northridge, CA 

818-709-2468 

www.radiantdetectors.

com

CdZnTe, 72 mm3

 5.7% @ 122 keV 

1.8% @ 662 keV 

na 

CdTe, 

up to 10x10x2 mm 

Integrated package 

with Pelletier 

cooler. 

Radiant Detector 

Technologies, LLC, 

Northridge, CA 

818-709-2468 

www.radiantdetector

s.com 

 

distributed by

ORTEC, 

Oak Ridge, TN 

800-251-9750 

865-483-4411 

www.ortec.com

High density with 

superior stopping 

power but poorer 

resolution. 

Can operate up to 

55 o

C. 

1.6% @ 122 keV 

0.5% @ 662 keV 

na 

7-0600 ext. 6151

na: information not available 
40 

C. Neutron Detectors 

Neutron detectors (Table 13) generally rely on converting the neutron energy to a charged 

particle that can then be more readily detected. The most common detectors are gas-filled 

proportional counters, which consist of a sealed tube filled with 

boron fluoride (BF3) or 3

He. These come in a variety of sizes 

and configurations (Figure 18). 3

He, a rare isotope of helium, is 

more expensive, but it has a higher detection probability and 

3

He tubes can be filled at high pressure to further increase the 

detection efficiency. BF3 is a highly corrosive gas, so the tubes 

are generally filled at low pressure to decrease the probability of 

leakage. Gas-filled proportional counters are most sensitive to 

low-energy thermal neutrons (about 0.025 eV). The probability 

of detection decreases rapidly at higher energies. For detection 

of fission neutrons, with an average energy of about 1 MeV, 

these detectors need to be surrounded by a moderator, which 

slows the neutrons by multiple scattering reactions. Moderators 

are usually materials, such as polyethylene, with high hydrogen content, since protons are the 

most efficient neutron scatterers. 

Other types of neutron detectors consist of an isotope with high thermal-neutron reaction 

probability, such as 6

Li, combined with a plastic or glass fiber scintillator [Nucsafe, 2004.] 

These have the advantages that they can be made in any size and shape and they have relatively 

high thermal-neutron detection efficiency. 

A unique neutron detection technology is that of bubble detectors [Bubbletech, 2004.] These 

detectors consist of small droplets of superheated liquid inside a pressurized gel-like polymer 

matrix. Neutron interactions cause the droplets to expand into small gas bubbles, which remain 

trapped in the gel. Cumulative neutron exposure can be obtained simply by counting the 

bubbles. They can be erased and the detectors recycled by increasing the pressure. Real-time 

readout can be achieved by observing a change in the light transmission of the gel or by using an 

acoustic sensor to detect the bubble formation. 

Figure 18. Gas-filled 

neutron detectors 

[Canberra, 2004A]. 
41 

Table 13. Neutron Detectors 

Illustration Company Detector Types Efficiencies Dimensions/ 

Weight/ Power/ 

Cost 

Bubble Technologies 

Chalk River, Ontario, 

Canada 

 613-589-2456 

http://www.bubbletech.ca

Defender and 

Defender-XL 

passive bubble 

detectors: 

superheated liquid 

in gel matrix. 

Defender: 100 

bubbles/μSv. 

Defender XL: 

10,000 

bubbles/μSv. 

Defender: 

7.9x0.75” diam./ 

2.6 oz./ $500 

Defender XL: 

13.8x1.6” diam./ 

1.5 lb./ in 

development 

Canberra Industries 

Meriden, CT 

203-238-2351 

http://www.canberra.com

3

He filled tubes, 

cylindrical up to 5 

cm diameter, or 

rectangular up to 

15 mm thick. 

75% for thermal 

neutrons, up to 500 

cps/nva

up to 1 m long/ 

Model 256109 

 SS 3He filled 1267 

cps/nva

 

72” effect. length 

x 2”diam./ 

1.359 kg/ 1000V/ 

$2,800 

LND, Inc. 

Oceanside, 

New York 

516-678-6141 

http://www.lndinc.com

3

He and BF3 filled 

tubes, cylindrical 

up to 2” diameter 

or spherical, 4” 

diameter. Model 25291 

SS 3

He filled 

10 cps/nva

6.1” overall 

length, 2.75” 

effect. x 1”diam./ 

150 g/ 1300V/ 

$380 

NucSafe LLC 

Oak Ridge, TN 

865-220-5050 

http://www.nucsafe.com

6

Li filled glass 

fiber panels. 

15 to 50% for 

thermal neutrons 

15x10x20”/20 lb/ 

100-240 VAC 1.5 

amp / $6,628 

 up to 

50x48x11.5”/ 450 

lb/ 120 VAC 1.5 

amp / $21,469 

a

 nv = neutrons/cm2/s 
42 

D. Survey Meters and Portable MCAs 

Small hand-held survey meters (Figure 19) are available to detect 

gamma rays and neutrons separately or in combination (Table 14). These 

generally are much less sensitive than the individual detectors discussed 

above and are useful only for strong sources at close range. Gamma-ray 

survey meters usually have small NaI detectors. Some may contain other 

sensors such as CZT, but these are useful only for detecting low-energy 

gamma rays and x-rays from unshielded sources. Neutron survey meters 

usually contain gas-filled proportional counters inside a moderator. 

Portable MCAs (Table 14) generally combine the functions of an 

amplifier, ADC, MCA and power supply in a small integrated package, 

which interfaces with a handheld detector. Some recent models offer a complete package with a 

built in NaI, CZT or Ge detector and nuclide identification software. 

Figure 19. 

Hand-held 

survey meter 

[Ludlum, 2004]. 
43 

Table 14. Portable MCAs and Survey Instruments 

Illustration Company Model, Features Detector Types Dimensions/ 

Weight/ Power/ 

Lifetime/ Cost 

Amptek Inc. 

Bedford, MA. 

781-275-2242 

http://www.amptek.com

MCA8000A 

up to 16K data 

channels, 32K 

channel memory, 

high speed PC 

interface 

Interfaces to 

CdZnTe (CZT) 

or scintillator 

detectors (NaI, 

CsI or BGO) 

6.5x2.8x0./ 

<300g (0.7 lb)/ 

2 AA batteries/ 

24 hours/ $3500 

Amptek Inc. 

Bedford, MA. 

781-275-2242 

http://www.amptek.com

GammaRad probe 

with DP4 digital 

pulse processor, up to 

4K MCA channels, 

USB interface to PC. 

Integrated 3x3” 

NaI detector, in 

ruggedized water 

and gas tight case. 

na / USB Powered 

Berkeley Nucleonics 

Corp., San Rafael, CA 

800-234-7858 

http://www.berkeleynucleo

nics.com

SAM 935, up to 1024 

channel MCA, LCD 

display, RS232 

interface, isotope ID 

Internal 1.5x2” 

NaI, optional 

external 2x2” or 

3x3”, internal 3

He 

neutron counter 

12x8.5x2”/ 5 lb/ 

rechargeable NiMH 

battery/ 

8 hours/ $9,695 to 

$24,950 depending 

on detector 

Berkeley Nucleonics 

Corp., San Rafael, CA 

800-234-7858 

http://www.berkeleynucleo

nics.com

PalmRAD 904, 

handheld meter with 

LCD digital readout 

Internal GM tube 5.9x3.2x1.2”/ 

8 oz/ 9-volt alkaline 

battery/ 

24-200 hours (count 

rate dependent) 

Bubble Technologies 

Chalk River, Ontario, 

Canada 

 613-589-2456 

http://www.bubbletech.ca

Microspec-2, 

handheld 256 

channel MCA, 

nuclide ID, 

Microspec-3 includes 

GPS dose rate 

mapping 

External gamma 

probe, neutron 

probe optional, 

interfaces to HP 

palmtop 

10.3x6.3x2.7”/ 

1.7 kg/ rechargeable 

batteries/ 

<14 hrs (M-2)/ 

9 hrs (M-3)/ $9910 

(M-2), $12,995 (M3) 

Canberra Industries 

Meriden, CT 

203-238-2351 

http://www.canberra.com

InSpector 1000, up to 

4096 channel MCA, 

512K channel 

memory, real-time 

nuclide ID, LCD 

display, USB 

interface 

Up to 3x3” NaI 

probe, other sizes 

available 

7.5x6.5x2.5”/ 4 lb/ 

rechargeable 

Li-ion battery/ 

12 hours/ $10,200 
44 

Table 14. (continued) Portable MCAs and Survey Instruments 

Illustration Company Instrument, 

Features 

Detector Types Dimensions/ 

Weight/ Power/ 

Lifetime/ Cost 

D-tect Systems 

Santa Barbara, CA 

http://www.dtectsystems.

com

Rad-ID, 4096 channel 

MCA, nuclide ID, 3.6 

inch color LCD display 

Internal 4 cm2

CZT array, 24.2 

cm3 NaI, 83 cm3 

3He neutron tube, 

GM tube 

Dim. na/ 5.7 lb/ 

3 D cell batteries/ 

36 hours 

Ludlum Measurements 

Sweetwater, TX 

800-622-0828 

325-235-5494 

http://www.ludlums.com

Model 3-97 with 

Model 44-38 GM probe, 

meter readout, 

many other 

configurations available 

Internal 1x1” NaI, 

external GM 

probe 

9.5x3.5x8.5”/ 4.4 

lbs/ 

2 D cell batteries/ 

typically 600 hr/ 

ORTEC 

Oak Ridge, TN 

800-251-9750 

865-483-4411 

http://www.orteconline.com

The Detective 

internal mechanical 

cooler, nuclide ID 

software, LCD display, 

8K channel digital 

MCA, stores up to 40 

spectra 

Internal Ge, 5 cm 

diam. x 3 cm 

(~ 10% relative 

efficiency), 

optional neutron 

detector 

12.5x6.3x12.6”/ 

23.3 lb/ 10-17 V 

DC, 30 W/ 

 >3 hours on 

internal batteries/ 

$55,000, $65,000 

with neutron 

detector 

ORTEC 

Oak Ridge, TN 

800-251-9750 

865-483-4411 

http://www.orteconline.com

MicroNOMAD 

up to 2K data channels, 

32K memory, high 

speed PC interface 

Interfaces to CZT 

or NaI detectors 

2.8x2.8x8.4”/ 

700 g (1.5 lb)/ 

2 AA batteries/ 

8 hours/ 

ORTEC 

Oak Ridge, TN 

800-251-9750 

865-483-4411 

http://www.orteconline.com

DigiBase, PM tube base 

with integral Bias 

supply, amplifier, 

digital 1024 channel 

MCA and analysis 

software, USB interface

Integrated PM 

tube base for NaI 

detectors 

6.3 cm diam. x 8 

cm/10 oz/ <500 

mA via USB/ na/ 

$3995 (base only)

ORTEC 

Oak Ridge, TN 

800-251-9750 

865-483-4411 

http://www.orteconline.com

DigiDart, MCA for 

portable HPGe systems, 

preamp and HV power 

supplies, spectroscopy 

amplifier, digital 16K 

MCA, nuclide ID, 23 

16K spectra memory, 

LCD display 

Interfaces to 

portable HPGe 

detectors 

8x5x3”/ 1.9 lb/ 

rechargeable Li 

battery/ >9 hr/ 

$9738 
45 

Table 14. (2nd continuation) Portable MCAs and Survey Instruments 

Illustration Company Instrument, 

Features 

Detector Types Dimensions/ 

Weight/ Power/ 

Lifetime 

Quantrad Sensor 

Madison, WI 

608-821-8360 

http://www.quantradse

nsor.com

Ranger, Ranger Plus, 

built-in MCA and 

gamma reference 

library, LCD display, 

RS232 interface, 

submersible to10m 

1.125 x 2” NaI 

(Ranger), 

0.5 x 4” 3

He 

neutron counter 

(Ranger Plus) 

6.5x13.25x2.75”/ 

5.9 lb/ 

rechargeable 

nickel battery, 

150-200 mA/ 

12 hours/ 

TSA Systems, Ltd. 

Longmont, CO 

970-535-9949 

http://www.tsasystems.

com

mMCA 430, 

256 data channels, 

stores 159 gamma 

spectra and neutron 

counts, 128x32 pixel 

display 

Internal 1x2” NaI, 

optional internal 

1x0.2” LiI(Eu) 

neutron detector 

10x4.75x4”/ 

5 lb/ six AA 

batteries/8 hours 

(or rechargeable 

batteries/12 

hours)/ 

TSA Systems, Ltd. 

Longmont, CO 

970-535-9949 

http://www.tsasystems.

com

MCA 465, 

256 data channels, 14 

spectra storage, 

256x128 pixel display 

Internal NaI or 

external probes 

6.2x9.4x4.9”/ 

7.5 lb/ 

rechargeable 

battery/12 hours/ 

Exploranium 

Mississauga, Ontario 

Canada 

905-670-7071 

http://www.exploraniu

m.com

The Identifier, GR-135, 

multiple detectors, 

nuclide ID 

NaI, GM, optional: 

CZT, neutron 

6.75x9x4”/ 

4.5 lb/ 2 D cells/ 

8-12 hours/ 

Target Instruments Inc.

Oak Ridge, TN 

865 2208700 

http://www.targetsystems-gmbh.de

 

identiFinder, 1024 

channel MCA, nuclide 

ID 

NaI, optional: GM, 

CZT, neutron 

23x9x7 cm/ 

1 kg/ 4 AA cells/ 

8 hours with 

rechargeable 

NIMH batteries/ 

XRF Corporation 

Somerville, MA 

617-623-7300 

http://www.xrfcorp.com

ICS-4000, 4000 channel 

MCA, nuclide ID 

CZT: 10x10x2 

mm, 1.6% @ 662 

keV, built in 

recharger 

10x3.4x1.2”/ 

1.5 lb/ 6.6V Li/ up 

to 7 hr/ 
46 

E. Portals and Search Systems 

Pedestrian and vehicle portals for detecting nuclear materials combine large plastic 

scintillators or NaI gamma-ray detectors with gas-filled neutron detectors. These are contained 

in pillars similar in configuration to airport metal detectors (Table 15). Nuclear search systems 

typically contain large NaI detectors and 3

He neutron tubes mounted in a van or cartop container. 

They have been in use for some time by the US Department of Energy and have recently become 

commercially available (Table 15). 

8. Imaging Technologies 

Imaging detectors (Table 16) can be either passive, 

looking at the natural emissions from the target 

material, or active, using high-energy x-rays or gamma 

rays to image the target. Imaging can improve the 

signal to background ratio, since the target generally 

covers a small field of view while the background 

tends to come from all directions. Astronomers have 

led the development of passive imaging techniques for 

gamma-ray astronomy. Coded-aperture imaging uses 

a computer designed mask and software reconstruction 

to produce an image [Ziock, 2003]. It works best in 

the energy range up to 300 keV, above which it is 

difficult to stop gamma rays with a mask. Compton 

imaging, which works without a mask and uses the 

physics of gamma-ray scattering in the detector to 

reconstruct an image, has been used to detect gamma 

rays up to 30 MeV [COMPTEL, 2003]. The GammaCamTM imaging system (Figure 20) 

employs the coded aperture technique. It has been used by DOE to image relatively high-activity 

sources at the Hanford, WA facility and at Argonne National Laboratory, Argonne, IL [EMDOE, 1998]. This technology is particularly good for imaging hot spots in a high radiation 

background, but would be less capable for imaging the relatively weak attenuated signal from a 

shielded weapon. Current passive imaging systems are relatively complex and limited in 

efficiency. 

Active imaging systems, using a high-energy x-ray, gamma-ray or neutron source, can 

penetrate low atomic number materials to obtain a transmission image of embedded uranium or 

plutonium, which are very dense materials. They may also be used to image stimulated 

emissions of neutrons and gamma rays, which are induced in nuclear materials by the 

interrogation source radiation. These systems typically employ a scanning procedure in which 

the source and detectors move together along either side of the vehicle being inspected. The 

EAGLE® cargo inspection system, which employs active imaging using transmission x-ray 

technology, is used by the U.S. Customs service for inspecting trucks or cargo containers (Figure 

21). This system uses high-energy X- rays of 3 or 6 MeV, which can penetrate up to 30 cm of 

Figure 20. GammaCamTM image of 

a gamma-ray source inside a truck 

[EDO, 2004]. 
47 

steel and image a highly shielded weapon. The manufacturer has signed a CRADAa with two 

DOE laboratories to use this system for imaging photo-fission induced neutrons and gamma rays 

from nuclear materials. The VACISTM transmission gamma-ray imaging system uses highenergy gamma rays from a 137Cs or 60Co source to image a vehicle (Figure 22). The ShapedEnergyTM x-ray system uses a 220 or 450 keV x-ray beam and detects both transmission and 

backscatter x-rays from the cargo to form dual sided images. This system can simultaneously 

detect gamma rays and neutrons emitted from the cargo during the scan. 

 

a

 CRADA: Cooperative Research and Development Agreement. 

Figure 22. Car image using the VACISTM

vehicle inspection system [SAIC, 2004]. 

Figure 21. Truck image using the Eagle®

cargo inspection system [Aracor, 2004]. 
48 

Table 15. Vehicle Portals and Search Systems 

Illustration Company Instrument, Features Detector Types Dimensions/ 

Weight/ Power/ 

Cost/ Lifetime 

Canberra Industries 

Meriden, CT 

203-238-2351 

http://www.canberra.com

RADsentry, for 

vehicles up to 2 m or 

cargo containers up 

to 4 m. Also rail and 

pedestrian 

configurations. 

plastic scintillator, 

975 in2. 3He 

optional neutron 

detectors, 59x2” 

diam., operating 

temp. -40 to 

+50C. 

73x28.5x9.5” ea./ 

559 lb./ 24VDC, 

120/220 VAC/ 

$23,000 to 

$90,000 

ORTEC 

Oak Ridge, TN 

800-251-9750 

865-483-4411 

http://www.orteconline.com

Car-top search 

system, built-in 

power supply, MCA, 

GPS mapping, USB 

interface to laptop 

computer, NaI 

spectral analysis, 

count rate vs. time or 

position 

4x4x16” NaI 

(10% resolution 

@ 662 keV) two 

3

He neutron tubes 

50x2.5 cm diam. 

with 4” 

polyethylene 

moderator. 

12V battery power

Safety-GuardTM II 

vehicle portal 

inspection system 

Up to 8 gamma or 

4 gamma and 4 

neutron, optional 

speed and 

occupancy 

sensors, ethernet 

interface 

72x18x12”/ 

154 kg/ 

$21,700 to 65,800 

Thermo-Electron 

Corporation 

Waltham, MA 

781-622-1000 

http://www.thermo.com

Safety-GuardTM I 

conveyor system for 

package monitoring 

gamma (two 

plastic 

scintillators,

optional two 

neutron (3

He gas 

filled), optional 

speed and 

occupancy 

sensors, ethernet 

interface 

30x17x8”/ 

91 kg/ 

$17,450 to 

$23,250 

TSA Systems, Ltd. 

Longmont, CO 

970-535-9949 

http://www.tsasystems.com

Model VM250/ 

VM250AGN, based 

on Los Alamos 

design, optional 

heaters and insulation

gamma (two 

plastic 

scintillators, 

6x30x1.5”, per 

column), optional 

neutron ( four 

2x36” 3

He) 

VM250: 

10x10x96 or 120” 

ea./ 136 kg 

VM250AGN, 

8x26x96 or 120”/ 

273 kg/ battery 

powered/ >24 

half-life 

na: information not available
49 

Table 16. X-Ray and Gamma-Ray Imagers 

Illustration Company Instrument, Features Detector 

Types 

Dimensions/ 

Weight/ Power/ 

Cost 

ARACOR 

Sunnyvale, CA 

408-733-7780 

http://www.aracor.com

EAGLE® mobile cargo 

inspection system uses 

high-energy x-rays (3 or 

6 MeV) and produces a 

transmission image 

na na 

AS&E 

Billerica, MA 

978-262-8700 

www.as-e.com

Shaped-EnergyTM

mobile cargo inspection 

system uses 220 or 450 

keV x-rays and 

produces both 

transmission and 

backscatter images 

na na 

EDO Corporation 

New York, NY 

212-716-2050 

http://www.nycedo.com

GammaCamTM portable 

gamma-ray imager, 25 

or 50 degree field-ofview, 1.2 or 2.6 degree 

resolution, 1 μR dose 

sensitivity, interfaces to 

PC 

High-density 

Terbiumactivated 

scintillating 

glass 

19x10x15”/ 

60 lb/ 100-240 

VAC/ $180,000 

SAIC 

San Diego, CA 

858-826-9738 

www.saic.com

VACISTM mobile cargo 

inspection system uses 

high-energy gamma-ray 

sources and produces a 

transmission image. 

na na 

na: information not available 
50 

9. New Detector Technologies 

A. Integrated Detection Systems 

Most current and developmental nuclear detection systems display the information they 

capture on or near the units. However, wireless connectivity to remote locations, such as 

command centers, is increasingly desired and possible. One example of a new gamma ray 

detection system is shown in Figure 23. It contains a NaI detector and associated electronics, 

plus a radio that can transmit information on the output of the detector, all within a small case. 

This system was employed at the Athens Olympics in 2004. It can also be outfitted with sensors 

for chemical and biological detection. The integration of multiple sensors into one detection 

system permits sharing of the power supply, computer and communications sub-systems. It also 

reduces the number of systems that must be bought, maintained and used by field personnel. 

AC Power

Connector

Battery

Radio

Detector

Processor

Detector

Electronics

Power

Supply

AC Power

Connector

Battery

Radio

Detector

Processor

Detector

Electronics

Power

Supply

Figure 23. Drawing of the Mobile Defender wireless gamma ray 

detection system used in the 2004 Olympics [Soflinx, 2004].
51 

A unique portable system, the PIRATE, shown in Figure 24, provides a compact, secure 

wireless communication platform for hazardous material analysis. It can be equipped with 

sensors for nuclear, chemical or biological materials, GPS, a graphical interface and image 

recognition software. 

B. Detector Technologies in Development 

There is much ongoing research on room temperature semiconductor detectors for gamma 

rays, which is the subject of a biannual workshop [RTSD, 2001]. This research focuses on two 

main approaches, a) to grow larger single crystals in order to be able to make useful detectors for 

gamma rays in the several hundred keV to several MeV range and b) to obtain uniform charge 

collection efficiency in order to improve the full-energy peak resolution. Much of the effort has 

been focused on CZT, but progress has been slow. 

Another technology that shows promise for gamma-ray detection uses high-pressure xenon 

detectors [Mirmar, 2003]. A new scintillator, LaCl3(Ce), is available with properties similar to 

NaI, but with superior resolution of less than 4% at 662 keV for a 1” diameter x 1” crystal [St. 

Gobain, 2004]. Other new scintillators, such as LaBr3, are under development with resolutions 

better than 3%, which is less than half that of NaI detectors [Shaw, 2003]. 

A very compact, low-power mechanically-cooled Ge detector system is under development 

using a miniature Stirling-cycle cooler. This instrument is designed for long shelf-life in the 

field and contains peak analysis software and nuclide identification using a stored gamma-ray 

library. The prototype can operate up to nine hours using a rechargeable Li ion battery [Madden, 

2003]. 

Ongoing research in neutron detectors is focused on solid-state detectors to replace gas-filled 

tubes [McGregor, 2003; Phillips, 2002]. The approach here is to make use of neutron-capture 

reactions to convert the neutron into an energetic charged particle, which can be then be detected 

 

Figure 24. The PIRATE portable WMD analysis platform. The sensors 

and communication transmitter are installed in the suitcase-like container 

[KC, 2004]. 
52 

by solid-state detectors. A converter material is used such as 10B, which has a high thermalneutron capture cross-section and emits energetic charged particles following capture. 

Efficiencies as high as 30-50% can be achieved by the deposition of 10B in grooves or dimples in 

the detector surface [McGregor, 2003]. 

Compact Compton and coded aperture gamma-ray imaging systems are being developed 

using segmented Germanium detectors to obtain good sensitivity, high energy resolution, and 

good angular resolution [Vetter, 2003], [Hull, 2003]. An innovative imaging technology under 

development uses scattering of cosmic-ray muons to image high atomic number materials, such 

as uranium and plutonium [Borodzin, 2003], [Physics Today, 2003]. The developers estimate 

that a border detection system could be built to detect a 10 cm cube of uranium in one minute. 

Active interrogation, using a pulsed neutron source, is being investigated as a means of 

detecting highly shielded HEU. Delayed high-energy gamma rays in the range of 2.5 to 4.0 

MeV are the result of the decay of induced fission products. These are highly penetrating, and 

occur at energies where the interference from natural background is low. Large detector arrays 

can be used to locate the source inside a cargo container [Slaughter, 2003]. A similar device is 

being developed using interrogation by high-energy photons from a linear accelerator [Jones, 

2003]. Another pulsed-neutron interrogation device under investigation uses delayed epithermal 

(eV energy) induced neutrons to detect shielded nuclear materials [Melton, 2003]. 

10. Summary and Conclusions

A small nuclear weapon, such as that dropped on Hiroshima during World War II, could 

deliver a lethal dose of radiation and level unreinforced buildings out to a km or more. A 

radiological weapon used to disperse radioactive material in a densely populated area could 

result in hazardous doses, if the population is not promptly evacuated. The extent of the affected 

area would be hard to predict due to variable wind patterns and the influence of large buildings 

on particle dispersion patterns. The resulting contamination could render affected areas 

temporarily uninhabitable and require costly cleanup operations. 

The principle fissionable materials used in nuclear weapons are highly enriched uranium and 

weapons grade plutonium. Radiological weapons may contain a variety of industrial or medical 

radioactive sources along with chemical explosives or other dispersion methods, such as 

aerosols. Even a small amount of radioactive material in the debris after a chemical explosion 

could complicate cleanup of the site. 

The primary observables from nuclear weapons, nuclear materials and radiological materials 

are gamma rays and neutrons. Both have a mean free path in air of the order of a hundred 

meters, which means they are not observable by high-flying aircraft or by satellite. There is also 

a substantial natural background of both gamma rays and neutrons, which limits the detectability 

of contraband nuclear or radiological materials. The signal from a point source of gamma-rays or 

neutrons decreases with the inverse square of the range while the background remains constant. 

This limits the distance at which the source can be detected above the statistical noise in the 

background. Small hand-held detectors are useful out to a few meters at best. With large area 

efficient detectors, unshielded nuclear weapons or nuclear materials are detectable out to ranges 

of a few tens of meters. 

Gamma-ray imaging detectors can be used to locate a source and reduce the interference of 

the background, which is usually non-local. However, current passive imaging systems are 

complex and relatively inefficient. Active imaging systems can efficiently image large 
53 

containers at close range using high-energy x-rays or neutrons to obtain a transmission image or 

to stimulate gamma-ray and neutron emissions from nuclear materials. 

Plutonium weapons are generally detectable by their neutron emissions. These can be 

shielded by large amounts of neutron moderating materials combined with thermal neutron 

capture materials. The most efficient moderators contain significant amounts of hydrogen in 

their molecular structure, such as water, paraffin or polyethylene. Highly enriched uranium 

weapons, in principle, emit few neutrons and only low-energy gamma-rays, which are easily 

blocked by thin lead shields. However, other isotopes of uranium, which emit high-energy 

penetrating gamma-rays that require many cm of lead to shield, are typically present in these 

weapons. Active interrogation by beams of high-energy x-rays or neutrons can best be used to 

detect highly enriched uranium or plutonium that is well shielded from passive detectors. 

Radiological weapons materials are highly radioactive and relatively easy to detect using passive 

gamma-ray detectors. 

For nuclear detectors, size still matters. The physics of the detection process requires 

detector volumes of a few hundred cubic centimeters for useful detection of gamma rays and 

neutrons from nuclear weapons. It takes a detector several centimeters thick to stop the 

characteristic gamma rays from nuclear materials. Fission energy neutrons must be reduced to 

thermal or near-thermal energies for efficient detection, which requires several centimeters of 

moderator in or around the neutron detector. 

Pulse-height analyzers combined with specialized nuclear electronics are used to display an 

energy spectrum from a gamma-ray detector. Characteristic peaks in the spectrum can be 

recognized visually or by use of analysis software to identify the gamma-ray emitting material. 

Germanium semiconductor diodes are still the “Gold Standard” for detection and 

identification of gamma-rays from nuclear material. They are available with volumes of several 

hundred cm3

 and have excellent peak resolutions of about 0.2%. This allows the characteristic 

gamma rays from these materials to be detected above the environmental background, thus 

providing positive identification of the source. However, Ge detectors must be cooled to 

temperatures below 100 K, which requires liquid nitrogen cooling or electric power to operate a 

mechanical cooler. Room temperature semiconductor detectors such as CZT are currently 

limited to a few cm3

 in size and 2-3% in resolution. Large efficient NaI scintillator detectors are 

available, which operate at room temperature, but with limited gamma-ray resolution of about 

7.5%. However, newer scintillators with improved resolution are under development. 

Neutrons must be moderated to thermal energies for efficient detection. Efficient neutron 

detectors combine a moderator with a material that captures the neutron and emits charged 

particles for detection. He3

 or BF3 gas-filled proportional counters are still the standard for 

neutron detection, but other technologies such as 6Li glass fibers and bubble detectors may be 

useful in some applications. Detectors under development combine a neutron capture material 

with a semiconductor detector for compact and efficient neutron detection. 

 Protection against nuclear and radiological weapons requires a suite of detectors including 

active interrogation for cargo containers, stationary active or passive detectors for scanning 

smaller containers, large gamma-ray and neutron detectors at ports of entry, and handheld 

detectors for inspecting suspicious packages. Small gamma-ray monitors in heavily populated 

areas could provide valuable warning of a radiological attack, giving time to evacuate the area 

before significant doses are acquired. First responders should be equipped with handheld 

detectors and trained to use them and to interpret the results. 
54 

There are many possible sources of false alarms in the environment due to terrestrial, 

atmospheric and cosmic-ray induced backgrounds, as well as man-made sources for medical and 

industrial uses. Responding to false alarms can be time consuming and expensive. If a detector 

system is subject to excessive false alarms it will eventually tend to be ignored. The only way to 

reduce these to an acceptable minimum is to have the right equipment in the hands of trained and 

experienced inspectors. Calibration curves and receiver operator characteristics (Appendix A) 

should be developed for all fielded nuclear detectors in the types of scenarios that their users are 

most likely to encounter. 
55 

Appendix A. Calibration and Operating Curves 

Regardless of the signature being detected, there are some fundamental aspects of analytical 

procedures that remain unchanged for all detection technologies. Prime among them is the need 

for and use of a calibration curve that quantitatively relates the signal from the detector in the 

system to the quantity of interest, usually the intensity of radiation or the number of molecules in 

an analytical situation. A generic linear calibration curve is sketched in Figure A.1. 

Nonlinear calibration curves are not uncommon. The detector output, possibly a voltage V or 

a frequency f, is quantitatively related to the measurement goal, with the intensity I of the i‘th 

radiation or the concentration C of the j’th chemical, indicated in the figure. Implicit in this 

calibration curve is the specificity of the analytical system. That is, the instrument uses some 

spectral or other feature that is uniquely related to the material of interest. Interferences from 

other materials are a common problem. Sensitivity is a different concept from specificity and is 

defined as the rate at which the signal changes relative to changes in the analyte, namely ∆O/∆M, 

the slope of the calibration curve. Sensitivity is commonly used to mean the lowest concentration 

of a material that an instrument can detect. However, the phrase "minimum detectable limit" 

(MDL) should be used for the threshold value. It depends on the combination of a specific 

instrument, the material of interest and the employment scenario. The dynamic range of the 

system is the range of the measurand over which an analytical instrument operates, before some 

factor limits performance. High sensitivities tend to go with narrow dynamic ranges, and vice 

versa. 

 The minimum detectable limit (MDL) for the combination of a specific instrument and 

material of interest, indicated in Figure A.1, is the least amount of the material that can be 

detected. The calibration curve clearly indicates how an increase in the noise will give an 

apparent signal that falsely indicates a value for the measurand that is above the MDL. In such a 

case, the increased noise leads to a false positive report of the presence of the measurand. The 

noise floor of an analytical instrument determines the MDL for the particular material. There are 

Figure A.1. A generic calibration curve relating the output of a sensor or detector to the 

value of the quantity of interest (the measurand). 
56 

many sources of noise, which can enter at each step in the detection process of an analytical 

instrument. Figure A.2 indicates the sources of noise, which must be considered by both the 

designers and the users of analytical devices. Noise can arrive with the signal or sample and, 

thus, may be unavoidable. However, noise is sometimes introduced from the outside somewhere 

else in the chain of events in an instrument, or by the instrument itself, for example, electrical or 

mechanical noise. 

Interferants produce unwanted signals that can cause two problems. First, they can serve to 

provide an enhanced background noise that limits the ability to detect low levels of something of 

interest. Second, they can interfere with the signals from the material of interest. Interferants may 

or may not be related to the materials that are being sought. Radioactivity from nature, including 

isotopes in rocks, pollution from man-made sources, and cosmic rays, provide backgrounds 

above which the gamma rays or neutrons from suspect objects must be measured. 

 The calibration curve is essentially a detailed performance specification of a detection 

technology. It is generally obtained by measurements free of interferants and extraneous sources 

of background. The resulting calibration is usually stored in the analysis system computer, which 

uses it to convert the received data signal to the desired information. Determination of a 

calibration curve is challenging since it requires establishing and independently ascertaining the 

measurand, for example, 2 grams of enriched uranium inside an aluminum can of certain 

characteristics. Very often, standard samples are used for the calibration of detection 

technologies. Many of these are available from the National Institute for Standards and 

Technology (NIST) in Gaithersburg MD, or from companies that sell secondary standards 

traceable to NIST standards. 

 Ideally, the calibration curve is scenario independent. However, substances that interfere with 

the measurement of interest and variable backgrounds in some locales where a detection 

technology is used tend to modify the calibration curve. In many cases, the calibration curve is 

sensitive to temperature or some other ambient factor. Then, the analysis system computer also 

has to perform calculations to compensate for extraneous effects that degrade the accuracy of the 

analytical determination. These are measured by other sensors in the system, such as temperature 

and humidity monitors. 

This general calibration curve applies to various instruments for detection of nuclear and 

other materials. In the case of radiation from nuclear materials, be it natural radioactivity or 

stimulated neutron, gamma ray or x-ray signals from a nuclear device, the measurand is the 

intensity and the detector output is generally numbers of counts or an energy spectrum. The 

efficiency of the detector system, which depends in turn on many material and geometrical 

factors, will determine how well the system “uses” the information in the available radiation. The 

Figure A.2. The multi-step conversion of incident energy into information, with the 

potential injection of noise at and within all stages. 
57 

minimum detectable limit for nuclear detectors is often taken to be three times the standard 

deviation of the noise. That standard deviation is the reciprocal of the square root of the number 

of counts or, for spectra, the number of counts per energy region of interest. 

 Detection technologies and associated instruments are like any system. They will not operate 

properly for long times without appropriate attention. Testing of the operation of an instrument 

needs to be done after it is manufactured, at the beginning of use and during its service lifetime. 

The most fundamental test of an instrument is to determine if it will simply turn on. Indicator 

lights are usually built into an instrument to give such notice. The next level is determination of 

whether or not an instrument operates. Again, it is sometimes possible to provide indications of 

such a function. It is more difficult to ascertain if an instrument works properly. That is, will the 

system give the correct answer? To learn this generally requires introduction of a known value of 

the measurand and the observation of a correct readout. Small sealed radiation sources 

containing a known amount of a particular radionuclide are commonly used to check nuclear 

detection devices. Very few instruments have built-in test capabilities that are a good facsimile 

of the analyte of interest. 

 It should be noted that most commercial analytical devices and systems are subject to 

procedural standards that serve to insure their proper operation. Standards published by the 

International Organization for Standardization (ISO) and other organizations must be satisfied by 

analytical services in manufacturing, medical, environmental and other industries. The 

technologies used to detect NRWMD can be subject to the same kind of constraints. Such 

standards should not be confused with the standard samples that are often employed to calibrate 

analytical techniques. 

 It is important to appreciate the difference between detection of a material for WMD and the 

quantification of that material. Both are operationally important. The detection question is 

binary: is the material present within the range of the instrument being used? However, making 

that decision, based on the output of the instrument, can be both complex and uncertain. That is 

usually the case for the detection of NRWMD, because either there is very little material in the 

first place or else a larger amount is more distant from the instrument. Non-subjective and, 

ideally, operator independent criteria should be applied to make a yes-no decision about a 

detection. 

 There are two situations regarding the presence or absence of the material of interest within 

the measurement range of a particular instrument. The first has to do with whether the material is 

actually present or not. The second has to do with whether the instrument says it is there or not. 

Hence, there is a two by two matrix of possibilities, as illustrated in Figure A.3 
58 

 The results for each of the four cases are stated in terms of fractions normalized to unity. If 

the sample is actually present at the input of the instrument, the case called “positive”, then the 

instrument output may say it is there (the True Positive Fraction or TPF) or not there (the False 

Negative Fraction or FNF). TPF and FNF sum to one, because there are only two potential 

outcomes of the measurement. If the material is absent from the input, the “negative” case, then 

again, the instrument may indicate either, that it is there (the False Positive Fraction or FPF) or 

not (the True Negative Fraction or TNF). Again, FPF and TNF add to one. In an ideal situation, 

there would be no False Negatives or False Positives, and True Positives and True negatives 

would each occur 100% of the time when the sample is present or absent, respectively. However 

the presence of noise in the output of the detection instrument blurs the situation. 

 

 In Figure A.4, there is a curve to the right indicating the output distribution for repeated 

measurements with a sample present and another to the left for a second group of measurements 

with no sample. Each has a width dependent on the noise in the system. The part of each 

distribution above the chosen threshold used to declare a positive detection event is critical to the 

Figure A.3. The “truth table” for the signal output of an instrument in response to 

input from samples that either have or do not have the material of interest. 

Figure A.4. Distributions of the signals from a detection technology for two cases: 

without and without the material of interest being present. 
59 

utility of the system. In the case shown, the True Positive Fraction is large (97.8%). This means 

that the system will miss an actual sample only about 2.2% of the time. The False Positive 

Fraction is intermediate in size (64.6%). This means that about two-thirds of the time when no 

sample is there, the system will say that there is a sample present. This is highly undesirable, 

since in an airport screening situation, a nuclear sensor might falsely indicate that it is necessary 

to detain the majority of passengers who have no nuclear materials. 

 Figure A.4 makes it clear why reduction in the noise in a detection system is highly desirable. 

Less noise gives narrower distributions, which will yield higher TPF because less of the low 

output tail of the sample-present distribution will be found to the left of the threshold. 

Alternatively, the threshold could be raised, maintaining the same True Positive Fraction, but 

reducing the False Positive Fraction. If there is actually no material of interest present in the 

sample, then the two distributions for no-sample and sample are coincident, and the TPF and 

FPF are equal for any setting of the threshold. At the other extreme, if there is enough of the 

material in the sample, an output signal large enough to entirely separate the two distributions 

will result. That is, the distribution with a sample present will move to the right, away from that 

with no sample. Then, setting the threshold between the two distributions will give a True 

Positive Fraction of unity and a False Positive Fraction of zero, the ideal situation. 

 Referring back to the calibration curve in Figure A.1, when the measurand is well below the 

MDL, the sample and no-sample distributions are coincident. When the measurand is near the 

MDL, then the two distributions are distinct but strongly overlapping. They are separate, that is, 

non-overlapping, when the value of the measurand is sufficiently above the MDL, depending on 

the size of the noise. These three cases lead to three distinct curves if the True Positive and False 

Positive Fractions are plotted against each other. Such a curve is called, for historical reasons, a 

“Receiver Operating Characteristic”, or ROC curve. Possible ROC curves are shown in Figure 

A.5 for the ideal case (separate distributions), the worst case (coincident distributions) and the 

common intermediate case (overlapping but distinct distributions). The area under the ROC 

curve is a measure of the “goodness” of the detector system. A perfect detector system has an 

area of one. A detector system with an area of one half is useless for the yes-no question of the 

presence of material. All detector systems fall between these two limits. The essential point of 

Figure A.5. ROC curves for the two extremes and the common intermediate cases when 

an instrument is being used near its minimum detection limit to make a present or notpresent decision.
60 

the ROC curve is that the goodness of a detector system can only be evaluated by extensive 

measurements in the environment in which that system will be used. 

 ROC curves grew out of attempts to detect weak returns in radar systems, but they are quite 

general and no less applicable to detection technologies for WMD. It is very rare to find ROC 

curves in the ordinary literature on analytical instruments or devices for detecting WMD. 

However, they provide a systematic, non-subjective means to assess the performance and risk in 

using a detection technology in a binary fashion. It would be helpful for evaluation of WMD 

detection technologies if ROC curves for these systems were to become a standard by which to 

judge detector system utility. 

 There are three types of information desired from detection technologies: (a) whether or not a 

material of interest is present, (b) determination of the amount and (c) its identification. The yesno detection scenario for the presence of NRWMD materials is important both before and after 

the use of a device. If such materials are detected before an event, then appropriate actions might 

be taken to prevent the use of the dangerous materials. If residues from nuclear or explosive 

materials are detected after an event, information on the kinds and amounts of materials must be 

sought. This has uses that range from deciding what to do next for public safety to forensic 

determination of who is responsible for the attack. 

 If the amount of the material of interest in a sample is well above the MDL, the output signal 

from the sensors and the system will be well above the noise floor. Then, it is possible to use the 

calibration curve to quantify the amount of material. In this case, the detection is certain, and the 

question is how much of the material is present. Quantifying the amount of WMD material is 

very useful because it can dictate immediate actions. If a large amount of nuclear material or 

explosive is found, it would indicate a course of action different from the detection of trace or 

small amounts of the same material. Quantification of nuclear materials is useful well after their 

dispersion because they can be difficult to cleanup, they are slow to move in the environment 

and they can have very long half lives. 

 Knowing specifically what is being dealt with will determine the actions of emergency and 

medical personnel. This knowledge is critical both to patients and care givers in the aftermath of 

an attack. Determination of what material is present may require equipment different from that 

used for detection and quantification. For example, a Geiger counter will give a useful counting 

rate, but no indication of the type of material emitting the measured radiation. In that case, it is 

necessary to use some spectral instrument, such as a germanium detector with a multi-channel 

analyzer, to see from the peaks in the spectrum what is the source of the radiation. 

Quantification and identification are both necessary to determine what actions are needed before 

the affected facilities can be returned to use. 
61 

Appendix B. Practical Factors 

Many factors have to be considered by managers responsible for buying detection 

technologies for WMD. Most of them are applicable to most instruments for detecting and 

quantifying materials and devices for WMD. Below is a list of important factors in the form of 

questions that should be answered in preparation for a purchase of detection instruments: 

 1. What has to be measured and over what range of values? 

 2. What are the options for making the needed measurements? 

 3. What are the advantages and disadvantages of each option? 

 4. How long has each technology been on the market? 

 5. How many of each type of instrument have been sold? 

 6. Where and when will the measurements be needed? 

 7. What MDL and accuracy are needed? 

 8. How does the MDL compare with thresholds for damage to people and things? 

 9. Who will be making the measurements? 

 10. Are the instruments portable? 

 11. What kind of power is needed, alternating current or batteries? 

 12. If batteries are used, how long will the instrument operate fully on one set? 

 13. What is the size, weight, required power, ruggedness, etc. of candidate instruments? 

 14. What are the ranges of temperature, humidity, etc. over which an instrument will work? 

 15. How long does it take to set up or warm up the potential instruments? 

 16. How fast must the measurements be made? 

 17. How fast can the measurements be made, analyzed and communicated? 

 18. How often might the measurements be made? 

 19. What interferences might be encountered? 

 20. How much do the instruments cost? 

 21. How many instruments will be required? 

 22. How long does it take to get replacements? 

 23. What supplies are needed for the instruments? 

 24. Who makes replacement parts and sells supplies? 

 25. How much do the supplies cost? 

 26. How long does it take to get the supplies? 

 27. What is the shelf life of the technologies and the supplies? 

 28. Is recalibration in a central facility required, and how often? 

 29. What education level is needed for operators? 

 30. How much operator training is needed? 

 31. Is recertification of operators needed, and how often? 

 33. What is the reputation of the detection technology? 

 34. What is the reputation of the sellers of instruments and associated parts and supplies? 

 35. What is the reputation of the companies that provide training for operators? 

 36. What are the case histories of the actual use of specific instruments in the field? 

 

 Many other questions can be asked. The set of relevant questions, and their relative 

importance, is highly dependent on the organization that will be using the instruments and the 
62 

scenarios it expects to encounter. Managers responsible for procurement decisions for WMD 

detection technologies are faced with complex choices. 

 The gathering of information and the assessment of each alternative solution, or mixes of 

solutions, are difficult. Getting the information upon which overall assessments and procurement 

decisions will be based requires contacting a large number of companies and others. Performing 

the assessments, given the available information, is not easy because of the range of options and 

potential scenarios for use. Training is needed for the hands-on users of detection technologies, 

for analysts who might examine and integrate the information from detection technologies, and 

for the ultimate users of the information. Failure to properly employ an instrument will lead to 

problems that range from obtaining no information to getting wrong answers, possibly without 

realizing that they are in error. Analysts must know the limitations of, and uncertainties in the 

information that comes from various instruments in order to properly assess its importance. The 

managers who make decisions based on the output of detection technologies will usually not 

have time to deal with questions of calibration and other details. However, they should have 

some idea of the basics of a detection technology in order to avoid ascribing undue importance to 

information from such sources. 

 Despite the often high cost of advanced instruments, even hand held devices, people are 

usually the most costly part of an overall capability. The costs of hiring, training, support, 

salaries, benefits and even the retirement of people are commonly not considered when dealing 

with detection technologies. However, they are usually significant. 

 The logistics needed to keep detection technologies in proper operating condition can be as 

“simple” as timely replacement of batteries. They can be as complicated as repairing a complex 

instrument with the proper components. Recalibration of instruments, either in the field or after 

return to a central facility, is another aspect of their proper use. The costs of the acquisition of the 

needed supplies, their shipment and storage, and their timely delivery to the operators in the field 

must be taken into account. 
63 

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