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Appendix. The physics of nuclear detection.

What Is to Be Detected?

Detectors must detect complete weapons, which can be quite small. During the Cold War, the United States made 155 mm and 8 inch (diameter) nuclear artillery shells. The United States made even smaller atomic demolition munitions, and there have been reports of Soviet-era "suitcase bombs." A weapon that terrorists fabricated without state assistance would surely be less sophisticated and, as a result, probably much larger. Detectors must also detect the types of uranium and plutonium used in nuclear weapons. The type of uranium used in weapons is harder to detect than plutonium because it emits much less radiation; it is also much easier to fabricate into a weapon component. It is important to detect small quantities of these materials in order to interdict stolen and smuggled materials because small quantities suffice to fuel a bomb. According to a widely quoted report by five nuclear weapon scientists from Los Alamos National Laboratory, it would take 26 kg of uranium, or 5 kg of plutonium (both of types discussed later) to fuel an atomic bomb. (172) These masses would fit into cubes 11.2 cm or 6.3 cm, respectively, on a side. The ability to detect even smaller masses would help thwart nuclear smuggling. How is it possible to find weapons or materials among the vast amount of cargo that reaches the United States each day? Fortunately, there are many clues.

Background

Photons

Nuclear detection makes extensive use of photons. Photons are packets of energy with no rest mass and no electrical charge. Electromagnetic radiation consists of photons, and may be measured as wavelength, frequency, or energy; for consistency, this report uses only energy, expressed in units of electron volts (eV). (173) Levels of energy commonly used in nuclear detection are thousands or millions of electron volts, keV and MeV, respectively. The electromagnetic spectrum ranges from radio waves (some of which have photon energies of [10.sup.-9] eV), through visible light (a few eV), to higher-energy x-rays (10 keV and up) and gamma rays (mostly 100 keV and up). An x-ray photon and a gamma-ray photon of the same energy are identical.

Gamma rays originate in processes in an atom's nucleus. Each chemical element has two or more isotopes. Isotopes of an element have the same number of electrons, and thus in most cases similar chemical properties, but different numbers of neutrons in their nuclei, and thus different nuclear properties. Each radioactive isotope emits gamma rays in a unique spectrum, a plot of energy levels (horizontal axis) and number of gamma rays detected at each energy level (vertical axis). These spectra are a series of spikes at particular energy levels. (174) Figure 1 and Figure 2 show the spectra of uranium-235 and plutonium-239, respectively. Such spectra are the only way to identify an isotope outside a well-equipped laboratory. A detector with a form of "identify" or "spectrum" in its name, such as Advanced Spectroscopic Portal or radioactive isotope identification device, identifies isotopes by their gamma-ray spectra.

X-rays originate in interactions with an atom's electrons. Many detection systems use x-ray beams, which can have higher energies than gamma rays and thus are more penetrating. X-ray beams are often generated through the bremsstrahlung process, German for "braking radiation," which works as follows. An accelerator creates a magnetic field that accelerates charged particles, such as electrons, which slam into a target of heavy metal. When they slow or change direction as a result of interactions with atoms, they release energy as x-rays whose energy levels are distributed from near zero to the energy of the electron beam. They do not exhibit spectral peaks like gamma rays. This difference is important for detection.

Radioactivity

Radioactive atoms are unstable. They decay by emitting radiation, principally alpha particles (a helium nucleus consisting of two neutrons and two protons, thus having a double positive charge), beta particles (electrons or positrons, the latter being electrons with a positive charge), and gamma rays (high-energy photons). These forms of radiation are of differing relevance for detection. Alpha particles, being massive (on a subatomic scale) and electrically charged, are easily stopped, such as by a sheet of paper or an inch or two of air. Beta particles, while much lighter and faster, are also electrically charged and are stopped by a thin layer of material. (175) Gamma rays have no charge and can penetrate much more material than can alpha or beta particles. Depending on their energy, they may travel through several hundred feet of air. When an atom decays by emitting an alpha particle or beta particle, it transforms itself into a different element; it does not do so when it emits a gamma ray. Gamma-ray emission typically follows alpha or beta decay. As discussed in more detail below, each radioactive isotope that emits gamma rays does so in a spectrum of energies unique to that isotope. For example, the spectrum of U-235 has a prominent peak at 186 keV.

In addition to these typical means of radioactive decay, atoms of some heavy elements fission, or split into two smaller atoms. Of the naturally occurring isotopes, only U-238 spontaneously fissions with an appreciable rate (about 7 fissions per second per kg). One by-product of fission is the emission of neutrons (typically 2-3 neutrons per fission). Neutrons have no electrical charge and can penetrate dense materials, as well as many tens of meters of air.

Fissile material

Some isotopes of heavy elements fission spontaneously or when struck by neutrons or high-energy photons, emitting neutrons and gamma rays in the process. U-235 and Pu-239 are unique in that neutrons of any energy can cause them to fission; they are called "fissile." Neutrons of much higher energies are required to cause other isotopes to fission. This characteristic of U-235 and Pu-239 allows them to support a nuclear chain reaction. Fissile material is essential for nuclear weapons; U-235 and Pu-239 are the standard fissile materials used in modern nuclear weapons. The Atomic Energy Act of 1954 designates them as "special nuclear material" (SNM). (176)

Plutonium is not found in nature. It is produced from uranium fuel rods in a nuclear reactor and is separated from uranium and other elements using chemical processes. Weapons-grade plutonium (WGPu) is at least 93% Pu-239. In contrast, uranium in nature consists of 99.3% U-238 and 0.7% U-235, with very small amounts of other isotopes. Enriching it in the isotope 235 for use as nuclear reactor fuel or in nuclear weapons cannot be done through chemical means because isotopes of an element are nearly chemically identical, (177) so other means must be used. For example, uranium may be converted to the gas uranium hexafluoride and placed in centrifuges specially designed to separate U-235 from U-238 based on the very slight differences in the weight of individual molecules. Uranium enriched to 20% in the isotope 235 is termed highly enriched uranium, or HEU; for use in nuclear weapons, uranium is typically enriched to 90% or so, though lower enrichments could be used. For purposes of this report, "HEU" is used to refer to uranium of 90% enrichment. HEU may also be produced from material that has been in a nuclear reactor. HEU produced in this manner contains small amounts of another isotope, U-232, which, as we shall see, is easier to detect than is U-235.

Detection

Nuclear detection uses neutrons and high-energy photons in various ways. Because they can penetrate different materials, they are the main forms of radiation by which most radioactive material can be detected passively, by "listening" for signals coming out of a container without sending signals in. Because of their penetrating properties, they can be used in an active mode to probe a container for dense material. X-rays or gamma rays are used for radiography, that is, creating an opacity map like a medical x-ray. Neutrons of any energy level, and photons above 6 million electron volts (MeV), can be shot into a container to induce fission in SNM. Fission results in the emission of neutrons and gamma rays, which can be detected. Gamma rays can also be used to identify a radioactive source. Neutrons, in contrast, do not have a characteristic energy spectrum by which an isotope can be identified, and it is difficult to measure their energy, though the presence of neutrons in certain situations, as discussed below, can indicate that SNM is present.

Another characteristic of radioactive materials important for detection is the rate at which a material decays. The half-life of an isotope, or the time it takes for half the atoms in a sample to decay, is an indicator of the rate of decay, with shorter half-lives indicating faster decay. The half-lives of cobalt-60, plutonium-239, and uranium-235 are 5.3 years, 24,000 years, and 700 million years, respectively. (178 179) Even if a source emits high-energy gamma rays, it will be difficult to detect if it emits only a few of them. Thus type, energy level, and quantity of radiation are important for detection.

Shielding and background radiation

Different materials attenuate neutrons and gamma rays in different ways. Heavy, dense materials like lead, tungsten, uranium, and plutonium have a high atomic number (the number of protons in the nucleus), or "Z." High-Z materials attenuate gamma rays efficiently. (180) In contrast, neutrons are stopped most efficiently by collisions with the nuclei of light atoms, with hydrogen being the most effective because it has about the same weight as neutrons. (181) The element with the nucleus closest in weight to a neutron is hydrogen, which in its most common isotope consists of one proton and one electron. Thus hydrogen-containing material like water, wood, plastic, or food are particularly efficient at stopping neutrons; other low-Z material is less efficient at stopping neutrons, but nonetheless more effective than high-Z material. Conversely, gamma rays are less attenuated by low-Z material and neutrons are less attenuated by high-Z material.

Different amounts of material are needed to attenuate gamma rays depending on their energy level. Gamma rays from WGPu are sufficiently energetic and plentiful that it is more difficult to shield WGPu than HEU. In contrast, as explained in the footnote, an inch of lead would render gamma rays from U-235 essentially undetectable, though as discussed later other uranium isotopes that may be present in HEU are more readily detectable. (182) Indeed, 186-keV gamma rays from U-235 have so little energy that many are absorbed by the uranium itself, a process known as self-shielding, so that the number of gamma rays emitted by a piece of U-235 depends on surface area, not mass.

Unclassified demonstrations performed at Los Alamos National Laboratory for the author in June 2006 indicate how shielding and self-shielding impair the detection of low-energy gamma rays from HEU. The demonstrations used a top-of-the-line detector that had an excellent ability to identify materials by their gamma-ray spectra. (183) In the first demonstration, the detector picked up gamma rays from a thin sheet of HEU foil at perhaps 30 feet away and quickly identified them as coming from HEU. The foil had a large surface area and little thickness, so there was little self-shielding. In the second, the detector gradually picked up gamma rays from a marble of HEU as it was brought closer to the detector. Because the marble had much more thickness and much less surface area than the HEU foil, there was considerable self-shielding, greatly reducing the gamma-ray output. In the third, the marble of HEU was placed in a capsule of a high-Z material, lead, perhaps 1 cm thick, and the detector picked up nothing even when the capsule was touching the window of the detector.

Sources of radiation other than SNM complicate detection. Background radiation from naturally occurring radioactive material, such as thorium, uranium, and their decay products such as radon, is present everywhere, albeit often in trace amounts. Cosmic rays generate low levels of neutrons. Some legitimate commercial goods contain radioactive material, such as ceramics (which may contain uranium), kitty litter (which may contain thorium and uranium), and gas mantles made of thorium oxide. Other radioactive isotopes are widely used in medicine and industry. Finally, a terrorist group might conceivably place radioactive material in a shipment containing a weapon or SNM chosen so as to mask the unique gamma-ray spectrum of SNM by presenting a spectrum of several known innocuous materials with peaks to interfere with those of SNM or that have an intensity much higher than SNM.

Signatures of Plutonium, Highly Enriched Uranium, and Nuclear Weapons

For purposes of this report, a signature is a property by which a substance (in particular, SNM) may be detected or identified. A nuclear weapon or its fissile material may be detected by various signatures, some of which are discussed next.

Atomic number and density

Atomic number, abbreviated "Z," is the number of protons in an atom's nucleus. For example, the Z's of beryllium, iron, and uranium are 4, 26, and 92, respectively. Z is a property of individual atoms. In contrast, density is a bulk property, and is expressed as mass per unit volume, e.g., grams per cubic centimeter. The densities of beryllium, iron, and uranium are 1.848, 7.874, and 19.050 g/cc, respectively. At its most basic, density measures how many neutrons and protons (which constitute almost all of an atom's mass) of a substance are packed into a volume. In general, the densest materials are those of high Z. These properties may be used to detect uranium and plutonium. Uranium is the densest and highest-Z element found in nature (other than in trace quantities); plutonium has a slightly higher Z (94), and its density varies from slightly more to slightly less than uranium, depending on its crystal structure. Some detection methods discussed in Chapter 2, such as effective Z, make use of Z; and some, such as radiography and muon tomography, make use of Z and density combined.

Opacity to photons

An object's opacity to a photon beam depends on its Z and density, the amount of material in the path of the beam, and the energy of the photons. Gamma rays and x-rays can penetrate more matter than can lower-energy photons, but dense, high-Z material absorbs or scatters them. Thus a way to detect an object, such as a bomb, in a container is to beam in x-rays or gamma rays to create a radiograph (an opacity map) like a medical x-ray.

Presence of gamma rays beyond background levels

Background gamma radiation is ubiquitous. Since many materials, including SNM, emit gamma radiation, elevated levels of gamma radiation may or may not indicate the presence of SNM.

Presence of neutrons beyond background levels

Cosmic rays and naturally occurring uranium generate a very low background flux of neutrons. Most materials do not emit neutrons spontaneously, but HEU and plutonium do. The spontaneous emission rates for 1 kg of plutonium and 1 kg of of HEU are 60,000 neutrons per second and 3 neutrons per second, respectively. (184) As a result, neutrons above the cosmic ray background coming from a cargo container would be suspicious. (185) For HEU, however, the rate is not too different from the background and thus is not a strong signature.

Gamma ray spectra

Each isotope has a unique gamma ray spectrum. For example, uranium-235 produces gamma ray peaks at several dozen discrete energy levels. This spectrum of energies is well characterized for each isotope, and is the only way to identify a particular isotope outside a well-equipped laboratory. As a result, any detector with a variant of "spectrum" or "identify" in its name, such as Advanced Spectroscopic Portal or radioactive isotope identification device, relies on identifying isotopes by their gamma-ray spectra.

HEU presents other gamma ray signatures as well. HEU contains some U-238, which produces a gamma-ray peak at an energy of 1.001 MeV. While these gamma rays are energetic, they would be hard to detect unless the detector is very close to the uranium because they are emitted at a very low rate, and could easily be missed because trace amounts of naturally occurring uranium, such as in clay and soil, also generate 1.001 MeV gamma rays. HEU derived from spent nuclear reactor fuel rods also contains small amounts of uranium-232, which is formed when uranium is bombarded with neutrons in a nuclear reactor. Uranium-232 decays through a long decay chain of short-lived isotopes to thallium-208, which has a gamma ray of 2.614 MeV, one of the highest-energy gamma rays produced by radioactive decay, so it is distinctive as well as highly penetrating; it takes 2.041 cm of lead to attenuate half the gamma rays of that energy. Thallium-208 is also a decay product of naturally occurring thorium-232. U-232 decays very much faster than U-235 or U-238 (half-lives of 69 years, 700 million years, and 4.5 billion years, respectively), and thallium-208 decays even faster (half-life of 3 minutes), so even a very small amount of U-232 produces many gamma rays. (186) Similarly, WGPu presents various gamma ray signatures because it is a mix of several isotopes of plutonium and their decay products.

Time pattern of neutrons and gamma rays

SNM is unique in that it can fission when struck by low-energy ("thermal") neutrons. Like some other materials, it also fissions when struck by high-energy gamma rays. In a sufficiently large mass of SNM, the neutrons (usually two or three) released by the fission of one atom cause other atoms to fission, releasing more neutrons in a chain reaction. (187) SNM also fissions spontaneously, and neutrons released by these fissions have a non-negligible probability of causing other SNM atoms to fission. Characteristic products of fission offer indications that SNM is present. These products include neutrons that may be emitted over periods ranging from nanoseconds to many seconds, whether as a result of spontaneous fission or of fission induced by gamma rays or neutrons, and gamma rays emitted within nanoseconds of induced fission.

Prompt gamma rays and neutrons

When U-235 and Pu-239 fission, they release a nearly instantaneous burst of 2 or 3 neutrons and 6 to 10 gamma rays. These prompt neutrons are emitted in a continuum of energies, with an average of about 1 to 2 MeV, and are termed fast or high-energy neutrons. The prompt gamma rays are also emitted in a spectrum of many narrow lines. Only SNM will fission when struck by low-energy neutrons, so a beam of low-energy neutrons that results in a burst of neutrons and gamma rays indicates the presence of SNM. A beam of high-energy gamma rays (with energy greater than 6 MeV) will also cause SNM to fission. However, that beam will also cause other materials to fission, including natural uranium, so emission of a burst of neutrons and gamma rays resulting from interrogation by a high-energy gamma ray beam is a possible, but not a definitive, indicator by itself of the presence of SNM.

Delayed gamma rays and neutrons

When U-235 or Pu-239 atoms fission, they split into two smaller fission fragments in any of approximately 40 ways for each isotope, resulting in "[s]omething like 80 different fission fragments" for U-235 or Pu-239. (188) These fission fragments are unstable and decay radioactively into isotopes of various elements. Fission is a statistical process, so that fissioning of a great many U-235 or Pu-239 atoms produces a complex mixture of some 300 isotopes of 36 elements. (189) These isotopes have a great range of half-lives, from a small fraction of a second to millions of years, but the isotopes with a half-life greater than approximately 30 years emit only very low levels of radiation. This process produces thousands of times more gamma rays than neutrons. Since much cargo consists of low-Z material and since gamma rays penetrate low-Z cargo much more readily than do neutrons, many more gamma rays than neutrons resulting from fission of SNM escape containers holding such cargo. Higher-Z cargo will attenuate the gamma rays more than the neutrons. Some of the gamma rays have energies exceeding those of thallium-208, 2.614 MeV, the highest energy typically observed in natural backgrounds. "Their high energy makes this gamma radiation a characteristic of fission, very distinct from normal radioactive background that typically produces no gamma radiation exceeding an energy of 2.6 MeV." (190) Note that some other isotopes, such as U-238 and Pu-240, are "fissionable," that is, they can undergo fission only when struck by high-energy (fast) neutrons. The high-energy gamma rays resulting from fission are a strong indicator of the presence of SNM. (191) The intensity of the neutron and gamma-ray flux over a short period, caused by rapid decay of many of the fission products, and the prompt response to a probe, are distinctive signatures as well.

There is another time-delay signature. A neutron beam makes atoms of some other elements radioactive, in particular transforming some atoms of stable oxygen-16 to radioactive nitrogen-16. Researchers at Lawrence Livermore National Laboratory conducted experiments in which they bombarded a target of natural uranium (99.3% U-238, 0.7% U-235) inside a cargo container with a neutron beam, and recorded the gamma ray spectrum resulting from radioactive decay. After they turned off the neutron beam, they found that the high-energy portion of the spectrum was dominated by gamma rays from the decay of nitrogen-16 for the first 15 seconds, and after that the dominant signal was from the decay of radioactive fission products, with an average half-life of about 55 seconds. (192) This time difference is an indicator of the presence of SNM.

Differential die-away

Interrogation of SNM with a beam of neutrons or high-energy photons to induce fission produces another unique signature. While the beam may cause neutrons to be emitted immediately through various nuclear reactions (e.g., fission), materials other than SNM will not support a nuclear chain reaction. In contrast, even a subcritical mass of SNM can sustain a chain reaction for a short time. As a result, fission in a multi-kilogram block of SNM will continue to produce neutrons for a short time after the beam has been turned off, with the intensity and duration of the neutron flux depending on the amount of SNM and the cargo loading. This delayed neutron time signature is called differential die-away, is measured on the order of a thousandth of a second after the beam is turned off, and is specific to U-235 and Pu-239 (and, rarely, other fissile isotopes). This technique depends on the detection of the prompt fission signal, but hydrogenous materials such as those found in cargo tend to attenuate this signal, and there may be background neutrons, so that some difficult scans may require more time, possibly two minutes, and some may not be feasible.

Fission chain time signature

A subcritical mass of SNM is too small to support a supercritical chain reaction because too many neutrons escape the SNM for the number of neutrons to increase exponentially. Nonetheless, chain reactions do occur in SNM, triggered by a neutron from spontaneous fission or a background neutron. These chain reactions may last several to dozens of generations, producing a burst of neutrons and gamma rays over some billionths of a second. No other material produces this signature. In contrast, most background neutrons and gamma rays arrive at a detector in a random pattern. The one exception is that neutrons generated as cosmic rays strike matter also tend to be generated in bursts; work is under way to try to differentiate between bursts of neutrons induced by cosmic rays and those generated by fission chains. Detection of this signature is therefore a strong sign of the presence of SNM. Unlike differential die-away or delayed neutrons and gamma rays, this signature can be detected with passive means provided the SNM is not well shielded. This technique places great demands on detector technology but can be done with state-of-the-art electronics.

Chapter 2 discusses in detail two other signatures--deflection of muons and nuclear resonance fluorescence and absorption--and their detection.

Detecting Signatures of a Nuclear Weapon or SNM

Overview: How are signatures gathered, processed, and used?

Detection involves using detector elements to gain data, converting data to usable information through algorithms, and acting on that information through concept of operations, or CONOPS. Detectors, algorithms, and CONOPS are the eyes and ears, brains, and hands of nuclear detection: effective detection requires all three.

Since photons or neutrons have no electrical charge, their energy is converted to electrical pulses that can be measured. This is the task of detectors, discussed next. The pulses are fed to algorithms. An algorithm, such as a computer program, is a finite set of logical steps for solving a problem. For nuclear detection, an algorithm must process data into usable information fast enough to be of use to an operator. It receives data from a detector's hardware, such as pulses representing the time and energy of each photon arriving at the detector. It converts the pulses to a format that a user can understand, such as displaying a gamma ray spectrum, identifying the material creating the spectrum, or flashing "alarm." Every detector uses one or more algorithms. Improvements to algorithms can contribute as much as hardware improvements to detector capability. Algorithms may be improved in many ways, such as by a better understanding of the physics of a problem, or by improving the computers in detection equipment so they can process more elaborate algorithms.

CONOPS may be divided into two parts. One specifies how a detection unit is to be operated to gain data. How many containers must the unit scan per hour? How close would a detector be to a container? Shall the unit screen cargo in a single pass, or shall it be used for primary screening, with suspicious cargo sent for a more detailed secondary screening? A second part details how the data are to be used. What happens if the equipment detects a possible threat? Which alarms are to be resolved on-site and which are to be referred to off-site experts? Under what circumstances would a port or border crossing be closed? More generally, how is the flow of data managed, in both directions? (193) What types of intelligence information do CBP agents receive, and how do data from detection systems flow to federal, state, and local officials for analysis or action? While this report does not focus on CONOPS because is not a technology, it is an essential part of nuclear detection.

How detectors work

A discussion of how detectors work is essential to understanding the capabilities and limits of current detectors and how detectors may be improved. Detecting each signature of a nuclear weapon or SNM requires a detector that is appropriate for that signature. Further, there is a hierarchy of gamma ray detectors. The simplest can only detect the presence of gamma radiation. The next step up, detectors with low energy resolution, have a modest capability to identify an isotope by its gamma ray spectrum. Next, detectors with high energy resolution have very accurate isotope identification capabilities. More sophisticated detector systems can also identify the presence of SNM by the time pattern of gamma rays released when such material fissions. The most sophisticated detector systems can produce an image showing where each gamma ray came from.

Detectors require a signal-to-noise ratio sufficient to permit detection. That is, they must be able to extract the true signal (such as a gamma-ray spectrum) from noise (spurious signals caused, for example, by background radioactive material or by imperfect detectors or data-processing algorithms). Two concepts are central to gamma-ray detector sensitivity: detection efficiency and spectral resolution. The former refers to the amount of signal a detector records. One aspect of detection efficiency is the fraction of the total emitted radiation that the detector receives. Radiation diminishes according to an inverse square law; that is, the intensity of radiation (e.g., number of photons per unit of area) from a source is inversely proportional to the square of the distance from the source. (194) Since a lump of SNM emits radiation in all directions, moving a detector closer to SNM, or increasing its size, increases efficiency. Reducing the cost of the active material in a detector may increase efficiency by making a larger sensor area affordable. Another aspect is the fraction of the radiation striking the detector that creates a detectable signal. For example, a detector that can absorb 90% of the energy of photons striking it is more efficient than one that can absorb 10%. A more efficient detector will collect information faster, reducing the time it takes to screen a cargo container.

Spectral resolution refers to the sharpness with which a detector presents energy peaks in a radiation spectrum. A graph of the gamma-ray spectrum of a radioactive isotope plots energy levels along the horizontal axis of the graph and the number of counts per unit time at each energy level along the vertical axis. A perfect device would record the energy levels of a gamma-ray spectrum as a graph with vertical "needles" of zero width because each radioactive isotope releases gamma rays only at specific energy levels. In practice, however, detectors are not perfect, and 186-keV gamma rays will be recorded as a bell curve centered on 186 keV. The narrower the spread of the bell curve, (195) the more useful the information is. Polyvinyl toluene (PVT), a plastic that is widely used in radiation detectors because it can be fielded in large sheets at low cost, is sensitive but has poor resolution, i.e., extremely wide bell curves for each gamma-ray energy level. As a result, while PVT can detect radiation, the peaks from gamma rays of different energy levels blur together, making it impossible to identify an isotope. Figure 3 makes this point; it shows the spectra of 90% U-235 and background radiation as recorded by a PVT detector. At the other extreme, high-purity germanium (HPGe) produces very sharp peaks, permitting clear identification of specific isotopes. These detectors are expensive, heavy, have a small detector area, and must be cooled to extremely low temperatures with liquid nitrogen or a mechanical system, making them less than ideal for use in the field. However, mechanically cooled HPGe detectors weighing some 2.5 kg are being developed for field use. (196) Figure 4 shows the spectrum of Pu-239 as recorded by various types of detectors with better resolution than PVT.

Various means can improve detector sensitivity. (197) One type of semiconductor detector crystal is cadmium-zinc-telluride, or CZT. The peak on the far right of each spectrum (198) in Figure 5 shows improvement in the resolution of the gamma-ray spectrum for cesium-137 (a radioactive isotope) taken with different CZT detectors that, for the years indicated, were at the high end of sensitivity. The top line shows a spectrum taken with a CAPture device developed by eV Products (19951998); the middle line shows a spectrum taken with a coplanar-grid device developed by Lawrence Berkeley National Laboratory (2000-2003); and the bottom line shows a spectrum taken with a 3-D device developed by the University of Michigan (2008). Better CZT crystals and better ways to overcome limitations of these crystals have both improved sensitivity in various ways:

* Researchers have been able to grow larger crystals. CZT crystal volume for the three devices was 1.00 cc in 1995-1998, 2.25 cc in 2000-2003, and 6.00 cc at present. Larger crystals are more efficient, i.e., they can capture more photons, and more of the energy of individual photons, permitting more counts per unit time (i.e., more data).

* Crystal quality has improved. A more uniform crystal structure and fewer impurities allow for better transport of the photon-induced electrical charge through the crystal and thus more accurate determination of the energy of each photon.

* University of Michigan researchers have constructed three-dimensional arrays of CZT crystals, permitting their detector to determine the 3-D coordinates of each individual gamma ray photon as it interacts with the CZT crystal, in turn permitting location as well as identification of gamma-ray sources.

* Electronics have improved. Researchers have made significant progress in reducing the noise inherent in electronic circuits (application-specific integrated circuits) that translate signals from the interaction of photons with CZT into a form in which algorithms can process them. Reducing the noise in these circuits permits more accurate measurement of gamma-ray energy. For example, a circuit developed in 2007 by Brookhaven National Laboratory has improved energy resolution substantially, and other advances in detector electronics in the last few years enable electronic components to compensate for defects in the crystals (analogous to adaptive optics in astronomy).

* Algorithms to reconstruct the signal from gamma rays have improved, also permitting more accurate measurement of gamma ray energy.

Another factor that affects the ability to detect SNM is the time available for a detector to scan a container or other object, often called "integration time. " Detectors build up radiography or tomography images, or gamma-ray spectra, over time. More time enables a detector to have more photons per pixel (in the case of radiography) or per bin (in the case of gamma-ray spectra), or more muons per voxel. More time also enables a neutron detector to detect more neutrons and measure their times of arrival, as discussed below, helping to determine if the neutrons are generated by SNM or by background materials. More time thus provides better data, which provides for better separation of signal from noise, better separation of different sources of radiation, fewer false alarms, and a better chance of detecting and identifying shielded threat material. Figure 16 illustrates how one system builds up an image over time. From a physics perspective, then, increasing integration time improves the accuracy of the result, but from a port operator's perspective, longer integration time impedes the flow of commerce, which costs money, so a balance must be struck between these two opposed goals. This balance may be stated formally in a concept of operations (discussed in more detail below), which specifies how, among other things, a detection system will be operated; detection equipment must be designed to operate within the time required, and port operations must allow that amount of time for scans.

Still another means of improving the ability to detect SNM is to increase the spatial resolution of a detector. According to DTRA,
   This is easily demonstrated in the example of a shielded versus
   unshielded radiation detector. Unshielded detectors are sensitive
   to radiation impinging on it in all directions, which is
   characteristic of the nature of naturally-occurring background
   radiation. By adding shielding, a detector's field-of-view can be
   controlled, and background radiation levels reduced, increasing the
   signal-to-noise ratio for the detector in the direction the
   detector is aimed. (199)


Detecting gamma rays

Gamma rays do not have an electrical charge, but an electrical signal is needed to measure them. There are two main ways by which a gamma ray can be turned into electrical energy. One is with a scintillator material, such as PVT or sodium iodide. When a single higher energy photon, such as a gamma ray, strikes the scintillator and interacts with it, the scintillator emits a large number of photons of lower energy, usually visible light ("optical photons"). A photomultiplier tube (PMT) converts the optical photons to electrons, then multiplies the electrons to generate a measurable pulse of electricity whose voltage is proportional to the number of optical photons, which is in turn proportional to the energy deposited by the gamma ray. An electronic device called a multi-channel analyzer sorts the pulse into a "bin" depending on its energy and increases the number of counts in that bin by one. A software package then draws a histogram with energy level on the horizontal axis and number of counts on the vertical axis. The histogram is the gamma ray spectrum for that isotope.

In contrast, a semiconductor material, such as HPGe, turns gamma rays directly into an electrical signal proportional to the gamma-ray energy deposited. A voltage is applied across the semiconductor material, with one side of the material being the positive electrode and the other being the negative electrode. When a gamma ray strikes the material, it knocks some electrons loose from the semiconductor's crystal lattice. The voltage sweeps these electrons to the positive electrode. Their motion produces an electric current whose voltage is proportional to the energy of each gamma ray. Each pulse of current is then sorted into a bin depending on its voltage and the spectrum is computed as described above. (200)

This approach, with either type of detector, is used to detect the various gamma-ray signatures described earlier. However, the requirements for detecting time signatures varies somewhat. Because prompt gamma rays are emitted so quickly, identifying them requires the ability to record time of arrival to several billionths of a second. Delayed gamma rays of interest are generated over a period of tens of seconds, so the ability to record precise time of arrival is less important. Detecting fission chain time signature requires a high-efficiency detector because long fission chains are relatively rare. Thus to detect SNM rapidly, the detector must have a high efficiency for detecting every fission chain. While the delayed emissions from fission chains are too weak to detect passively, fission chain time signature focuses on detecting the prompt emissions from fission, which are stronger. High efficiency is also important for neutron and gamma-ray interrogation, but the emphasis is less stringent because far more fissions are induced (i.e., the signal is stronger). Detecting nuclear resonance fluorescence requires high-resolution detectors in order to differentiate between the various materials being analyzed.

Detecting neutrons

Neutrons, like photons, do not have an electrical charge, but the two interact with matter differently. Photons interact chiefly with electrons, while neutrons interact with atomic nuclei. As a result, neutrons are counted by a different process. A common neutron detector is a tube of helium-3 gas connected to a high-voltage power supply, with positively and negatively charged plates or wires in the tube. In its rest state, current cannot pass through the helium because it acts as an insulator. When a low-energy neutron passes through the tube, it is absorbed by a helium-3 atom, producing a triton (1 proton and 2 neutrons) and a proton. These particles are highly energetic and lose their energy by knocking electrons off other helium-3 atoms. Positively charged ions of helium-3 move to the negative plate, while electrons move to the positive plate. Since electric current is the movement of charged particles, these particles generate a tiny electric current that is measured and counted. Neutrons are emitted as a continuum of energies. While the mean energy of each neutron spectrum varies somewhat from one isotope to the next, neutron energy spectra do not have lines representing discrete energies as with gamma rays. Moreover, neutrons lose energy as they collide with low-Z material, further blurring their spectra. Thus neutron spectra are of little value for identifying isotopes. Instead, the total neutron count is an important means of identifying SNM because only SNM gives off neutrons spontaneously in significant numbers, though some neutron background is generated mainly when cosmic rays knock neutrons off atoms. Several other methods of detecting SNM by neutron emission, discussed above, rely on the time pattern in which a group of neutrons arrives.

Several systems detect neutrons with tubes filled with helium-3 (He-3), a standard method. DOE obtains He-3 as a byproduct from the decay of tritium used in nuclear warheads. With the decades-long decline in numbers of warheads and a hiatus in tritium production for many years, there is little new supply of He-3. DOE plans to supply customers with 10,000 liters of He-3 a year, with a starting bid price expected to be around $72 per liter, and states, "This appears short of what customers are requesting." (201) (Russia sells He-3 to U.S. companies, but quantities are proprietary and not available.) Deploying He-3 neutron detection systems in large numbers would require a considerable amount of He-3. Customs and Border Protection (CBP) states that "based on our RPM [radiation portal monitor] deployments CBP would need approximately 2500 [detector] units to cover sea and land borders." (202) (Data for number of units needed for air cargo are not available.) Given the shortage and cost of He-3, deployment of neutron detectors using large amounts of He-3, or large numbers of units requiring small amounts of He-3, does not appear feasible. (203)

Alternative neutron detection systems are possible. They include tubes coated with boron-10 or lithium-6, tubes filled with boron-10 trifluoride (a toxic gas), nanocomposite scintillators, and "neutron straws," thin tubes being developed commercially under sponsorship of the Defense Threat Reduction Agency. (204) Substituting any of these technologies for He-3 in a system would necessitate re-engineering the system's neutron detectors, revising algorithms, conducting tests, perhaps modifying the resulting system for operational conditions, and so on. Those changes have the potential to add delay and affect system performance (for better or worse), though given the high cost of He-3 (about $2.7 million for 38,000 liters) they might well reduce cost.

Detecting absorption or scattering of high-energy photons

Photons of sufficiently high energy can penetrate solid objects. Denser, higher-Z material within a solid object absorbs photons of lower energy and scatters photons of higher energy. For cargo scanning, a fan-shaped planar beam of photons is sent through a cargo container as the container passes through the beam, and a detector array on the other side consisting of semiconductor or scintillator material records the opacity of each pixel to the photons. An algorithm then creates a two-dimensional opacity map of the contents of the container and displays it as an image on a computer screen.

Increasing the energy of photons allows them to penetrate more material. Radiography is used to search cargo containers for terrorist nuclear weapons, among other things. (205) A radiograph would reveal clearly a large dense object, such as a nuclear weapon encased in lead shielding. Two limitations of radiography are noteworthy. First, radiographs do not detect radiation and thus do not specifically detect SNM, just high-density, high-Z material. Second, if a terrorist bomb is placed in a shipment of dense or mixed objects, the image of the bomb might be hidden or a radiographic equipment operator might not notice it. It would be much harder to detect a small piece of SNM using radiography than to detect a bomb.

Evasion of Detection Technologies

In order to understand the capabilities of detection systems, it is important to know their weaknesses as well as their strengths. However, detailed discussions of means of evasion tend to become classified. Some references are made throughout this report, but some are withheld to keep the report unclassified. In general, an enemy could use various means in an effort to defeat these technologies. For example, high-Z material absorbs and deflects gamma rays, low-Z material deflects neutrons, radiography might miss a small piece of SNM (especially if mixed in with other dense material), and reducing the apparent density and Z of SNM by mixing it with a low-Z substance reduces the deflection of muons.

Further, enemy attempts to defeat one type of detection system may complicate plans or make a plot more vulnerable to detection by other means, as several examples illustrate. (1) The use of multiple detection systems that detect different phenomena are harder to defeat than those detecting one phenomenon only. Placing a lead shield around a bomb in order to attenuate gamma rays from plutonium would create a large, opaque image that would be quite obvious on a radiograph. It is for this reason that Congress mandated, "A container that was loaded on a vessel in a foreign port shall not enter the United States (either directly or via a foreign port) unless the container was scanned by nonintrusive imaging equipment and radiation detection equipment at a foreign port before it was loaded on a vessel." This restriction is to apply by July 1, 2012. (206) (2) An enemy could attempt salvage fuzing, which would detonate a weapon if the weapon sensed attempts to detect it, such as with photon beams, or if it was tampered with. However, salvage fuzing has various shortcomings. It could result in a weapon detonating by accident or if it is scanned overseas. It could detonate a weapon in a U.S. port, where it would do much less damage than in a city center. (3) Enemy attempts to smuggle HEU into the United States in order to avoid detection of a complete bomb would require fabricating the weapon inside this nation, which in turn could require such activities as smuggling other weapon components and purchase of specialized equipment, and could run the risk of accidents (such as with explosives), any of which could provide clues to law enforcement personnel. For these reasons, it is important to view technology development not only as advances in capabilities per se but also in the context of an offense-defense competition.

Author Contact Information

Jonathan Medalia

Specialist in Nuclear Weapons Policy

jmedalia@crs.loc.gov, 7-7632

(1) Comparison would require a detailed review of hundreds of technology projects to determine which are most worthy of further examination; creating metrics to compare the selected projects; and obtaining accurate data for use in the metrics. Each of these tasks would require the work of many experts over many months.

(2) For further information, see CRS Report RL34574, The Global Nuclear Detection Architecture: Issues for Congress, by Dana A. Shea.

(3) See Charles Ferguson and William Potter, The Four Faces of Nuclear Terrorism, Monterey, CA, Center for Nonproliferation Studies, 2004; Michael Levi, On Nuclear Terrorism, Cambridge, MA, Harvard University Press, 2007; and Carson Mark et al., "Can Terrorists Build Nuclear Weapons?," in Paul Leventhal and Yohan Alexander, Preventing Nuclear Terrorism: The Report and Papers of the International Task Force on Prevention of Nuclear Terrorism, a Nuclear Control Institute book, Lexington, MA, Lexington Books, 1987, pp. 55-65.

(4) Some materials can fission only when struck by fast neutrons; fissile materials are the only materials that can fission when struck by slow as well as fast neutrons.

(5) The Atomic Energy Act of 1954, 42 U.S.C. 2014, defines SNM as uranium enriched in the isotopes 233 or 235 or plutonium. The Nuclear Regulatory Commission has not declared any other material to be SNM even though the Act permits it to do so. U.S. Nuclear Regulatory Commission. "Special Nuclear Material," http://www.nrc.gov/materials/sp-nucmaterials.html.

(6) Mark et al., "Can Terrorists Build Nuclear Weapons?"

(7) An electron volt is a very small unit of energy, "a unit of energy equal to the work done by an electron accelerated through a potential difference of 1 volt." http://wordnetweb.princeton.edu/perl/webwn?s=electron%20volt.

(8) For further information on gamma rays generated by uranium, as well as shielding and detection of uranium, see Bernard Phlips et al., "Comparison of Shielded Uranium Passive Gamma-Ray Detection Methods," Proceedings of SPIE, vol. 6213, 62130H (2006); doi:10.1117/12.666342, online publication date May 24, 2006; abstract available at http://spiedl.aip.org/getabs/servlet/GetabsServlet?prog=normal& id=PSISDG00621300000162130H000001&idtype=cvips&gifs=yes&ref=no; and J.R. Lemley et al., "Confirmatory Measurements for Uranium in Nuclear Weapons by High-Resolution Gamma-Ray Spectrometry (HRGS)," Brookhaven National Laboratory, BNL-66293, July 25, 1999, http://www.osti.gov/bridge/product.biblio.jsp?osti_id=750764.

(9) Roger Byrd et al., "Nuclear Detection to Prevent or Defeat Clandestine Nuclear Attack," IEEE Sensors Journal, August 2005, p. 594.

(10) Ibid.

(11) For further analysis of this topic, see CRS Report RL34070, Fusion Centers: Issues and Options for Congress, by John Rollins.

(12) Personal communication, Defense Threat Reduction Agency, August 8, 2008.

(13) The American Association for the Advancement of Science held a workshop on the helium-3 shortage on April 6, 2010. Briefing slides are available at http://cstsp.aaas.org/agenda_meeting.html.

(14) See, for example, Joby Warrick, "Iran's New Centrifuge Raises Concerns about Nuclear Aims," Washington Post, May 2, 2010, p. 11.

(15) P.L. 110-53, Implementing Recommendations of the 9/11 Commission Act of 2007, Section 1701, 121 Stat. 489.

(16) The principal investigator for this project, Edward McKigney, Senior Project Leader, Safeguards Science and Technology Group, Nuclear Nonproliferation Division, Los Alamos National Laboratory, provided detailed information for this section, personal communications, April-August 2008. Others have commented as well to provide alternative perspectives.

(17) Specifically, the number of gamma rays depositing all their energy increases as Z to the 4.5th power.

(18) Information provided by Edward McKigney, Los Alamos National Laboratory, e-mail, July 21, 2009.

(19) These properties include probability of a full energy deposition interaction; efficiency of converting deposited energy to light, the signal that is measured; transparency to photons generated in this way so the detector volume responds uniformly; and physical robustness.

(20) For example, dense packing of nanocrystals may change the material's mechanical flow. That would make it harder to process the material by the lowest-cost method, extrusion, but it could still be cast and machined to shape. PVT scintillator is cast and can be machined, suggesting that that approach offers low cost while producing large sheets of material.

(21) For example, the project needs an understanding of the structural features of the materials being synthesized that an instrument called a resonant Raman spectrometer could provide. McKigney states that this instrument would speed up development of the process to synthesize the scintillator material by providing direct information about what material has been synthesized. Currently, the project infers such information from measurements that McKigney states are harder to interpret.

(22) The principal developer of this algorithm, Dean Mitchell, Distinguished Member of the Technical Staff, Contraband Detection Organization, Sandia National Laboratories, provided detailed information for this section, personal communications, April-August 2008. Others have commented as well to provide alternative perspectives.

(23) For a technical discussion of GADRAS, see Dean J. Mitchell, "Variance Estimation for Analysis of Radiation Measurements," Sandia Report SAND2008-2302, April 2008.

(24) Dean J. Mitchell, "Analysis of Chernobyl Fallout Measured with a RAMP Detector," SAND87-0743-UC-32, Sandia National Laboratories, 1987.

(25) Dean J. Mitchell, "Analysis of Low-Resolution Gamma-Ray Spectra by Using the Unscattered Flux Estimate to Search an Isotope Database," Systems Research Report, Sandia National Laboratories, 1997.

(26) Information provided by Dean Mitchell, e-mail, July 7, 2009, and April 1, 2010.

(27) Information provided by Dean Mitchell, e-mail, April 1, 2010. A neutron multiplicity counter detects SNM by detecting the time pattern of neutron generation. A subcritical mass of highly enriched uranium or weapons-grade plutonium can support a fission chain reaction producing an increasing number of neutrons. (Since it cannot support enough fissions to create a nuclear explosion, such chain reactions die out.) These neutrons are generated in a closely spaced pattern over a brief time. In contrast, background neutrons occur in a random time pattern.

(28) Information provided by Dean Mitchell, e-mail, April 1, 2010. A neutron multiplicity counter detects SNM by detecting the time pattern of neutron generation. A subcritical mass of highly enriched uranium or weapons-grade plutonium can support a fission chain reaction producing an increasing number of neutrons, though it cannot support enough fissions to create a nuclear explosion, so such chain reactions die out. These neutrons are generated in a closely spaced pattern over a brief time. In contrast, background neutrons occur in a random time pattern.

(29) For further information, see U.S. Department of Homeland Security. Customs and Border Protection. "Laboratories and Scientific Services." Available at http://www.cbp.gov/xp/cgov/trade/automated/labs_scientific_svcs/.

(30) Personal communication, August 5, 2008

(31) A compiler is a computer program that translates an application (such as GADRAS) into instructions that a computer can process.

(32) As another example, because of incompatibilities between VB6 and current generations of Fortran, components of GADRAS that are written in Fortran must be compiled using Compaq Fortran, another discontinued and unsupported programming language.

(33) Personal communication, August 5, 2008.

(34) Richard Wheeler, Lead for Homeland Security Analysis, Global Security Directorate, Lawrence Livermore National Laboratory, provided detailed information for this section, personal communications, April-August 2008. Others have commented as well to provide alternative perspectives.

(35) For a helpful interactive tutorial on ROC curves, see Anaesthetist.com, "The Magnificent ROC," at http://www.anaesthetist.com/mnm/stats/roc/Findex.htm.

(36) Karl Nelson, Thomas Gosnell, and David Knapp, The Effect of Gamma-Ray Detector Energy Resolution on the Ability to Identify Radioactive Sources, Lawrence Livermore National Laboratory, Radiological and Nuclear Countermeasures Program, LLNL-TR-411374, February 2009, p. i, https://e-reports-ext.llnl.gov/pdf/370769.pdf.

(37) Lawrence Livermore National Laboratory, Radiological and Nuclear Countermeasures Program, "Radiation Detection Modeling and Operational Analysis: Benchmarks, Nuisance Source, Algorithm and Resolution Studies," LLNL-TR-401531, December 31, 2007, by Padmini Sokkappa et al., pp. 21-22 This source is a report marked by Lawrence Livermore National Laboratory, the originating organization, "For Official Use Only." That laboratory has approved CRS use of the information cited by this footnote.

(39) Radiological and Nuclear Countermeasures Program, "Radiation Detection Modeling and Operational Analysis," p. 17. This source is a report marked by Lawrence Livermore National Laboratory, the originating organization, "For Official Use Only." That laboratory has approved CRS use of the information cited by this footnote.

(40) Ibid., pp. 20-21

(41) Joel Rynes, Program Manager, CAARS Program, Domestic Nuclear Detection Office, Department of Homeland Security, provided detailed information for this section, personal communications, April-August 2008. Others, including L-3 Communications Corporation, have commented as well to provide alternative perspectives.

(42) "Prior to 9/11, not a single radiation portal monitor [RPM] and only 64 large-scale non-intrusive inspection [i.e., radiography] systems were deployed to our nation's borders. By October of 2002, CBP had deployed the first RPM at the Ambassador Bridge in Detroit." Testimony of Thomas Winkowski, Assistant Commissioner, Office of Field Operations, U.S. Customs and Border Protection, before the Senate Homeland Security and Governmental Affairs Committee, September 25, 2008. CBP has 1,145 RPMs and 209 large-scale radiography systems deployed as of October 23, 2008. Personal communication, Patrick Simmons, Director, Non-Intrusive Inspection, Customs and Border Protection, October 23, 2008. Radiography has been used for industrial applications (e.g., inspecting metal parts for cracks and voids) for decades.

(43) For information on these systems, see Rapiscan Systems, "Rapiscan Eagle P6000," at http://www.rapiscan.com/eagle-P6000.html, and Science Applications International Corporation (SAIC), "Safety & Security: VACIS Cargo, Vehicle, and Contraband Inspection Systems," at http://www.saic.com/products/security/.

(44) Jonathan Katz et al. argue that it would be more effective to have the beam interrogate a cargo container vertically rather than horizontally. "In innocent cargo long slender dense objects are packed with their longest axes horizontal, and dense cargoes are spread on the floor of the container. Therefore, near-vertical irradiation will only rarely show regions of intense absorption [of photons] in innocent cargo. In contrast, horizontal irradiation would often find this 'false positive' result, requiring manual unloading and inspection. Another advantage of downward near-vertical illumination is that the Earth is an effective beam-stop, combined with a thin lead ground plane, its albedo [the fraction of the beam that is reflected] is negligible and additional shielding would not be required." J.I. Katz et al., "XRadiography of Cargo Containers," Science and Global Security, Vol, 5, No. 1, January 2007, pp. 49-56.

(45) Katz states, "The real issue in radiographing thick targets is not the source strength, but scattering in the target. If you don't have a narrow Bucky collimator matched to the geometry, the signal (increase of attenuation of unscattered photons showing the high-Z object ...) is swamped by forward-scattered photons from the mass of solid material. The small feature disappears from the image (rather like exposing undeveloped camera film to the light)." Personal communication, August 8, 2008. For cargo screening using radiography, a "Bucky collimator" is a piece of high-Z metal (such as tungsten) placed just in front of a photon detector element. The metal has a small hole precisely aligned with the direction of the interrogation beam in order to eliminate most photons that have been scattered by the cargo and that could obliterate small features of the image, such as a piece of SNM. The resulting increase in signal-to- noise ratio greatly improves the detector's ability to pick out suspicious cargo.

(46) The scattering mechanisms are much more complicated than can be described here. For details, see Glenn Knoll, Radiation Detection and Measurement, third edition (New York, John Wiley & Sons, 2000), pp. 48-53, 308-312.

(47) The threshold value of Z>72 is used because elements between Z=57 (lanthanum) and Z=72 (hafnium), inclusive, are very rare in commerce, making 72 a reasonable boundary between high Z and lower Z elements for the purposes of CAARS.

(48) For a periodic table of the elements, see "WebElements" at http://www.webelements.com/.

(49) For a technical discussion of dual-energy radiography, see S. Ogorodnikov and V. Petrunin, "Processing of Interlaced Images in 4B10 MeV Dual Energy Customs System for Material Recognition," Physical Review Special Topics--Accelerators and Beams, Volume 5, 104701 (2002).

(50) DNDO dropped its original goal of scanning 120 containers per hour. Testimony of Vayl Oxford, Director, Domestic Nuclear Detection Office, before the Senate Homeland Security and Governmental Affairs Committee, September 25, 2008.

(51) U.S. Department of Homeland Security. Domestic Nuclear Detection Office. "Cargo Advanced Automated Radiography (CAARS) Program Update Brief," presented to the Senate Committee on Homeland Security and Government [sic] Affairs, February 25, 2008, by William Hagan, Assistant Director, DNDO, Transformational and Applied Research, slide 4. This source is a briefing marked by the Domestic Nuclear Detection Office, the originating organization, "For Official Use Only." That office has approved CRS use of the information cited by this footnote.

(52) Personal communication, Joel Rynes, Program Manager, CAARS Program, DNDO, July 8, 2009.

(53) "Opening Statement of Mr. Vayl S. Oxford, Director, Domestic Nuclear Detection Office, Department of Homeland Security, before the Senate Homeland Security and Governmental Affairs Committee," September 25, 2008, p. 4.

(54) Information in this paragraph was provided by Joel Rynes, Program Manager, CAARS Program, DNDO, February 9, 2009.

(55) "Developmental Test and Evaluation" (DT&E) and "Technology Demonstration and Characterization" (TD&C) are both tests of systems conducted by the government (as opposed to contractors). DT&E is a term used in the acquisition process; DT&E tests systems before beginning procurement. As noted earlier, DNDO undertook a "course correction" in April 2008, moving CAARS from an acquisition program to an R&D program. Some felt it was confusing to use the term DT&E for a non-acquisition program, so DNDO decided to call the government tests of CAARS candidates TD&C.

(56) Information provided by Joel Rynes, April 16, 2010.

(57) Information by Joel Rynes, July 8, 2009

(58) Personal communication, e-mail, May 12, 2010

(59) Information provided by Ira Reese, Executive Director, Laboratories and Scientific Services, and Patrick Simmons, Director, Non-Intrusive Interrogation, both of Customs and Border Protection, personal communication, October 23, 2008.

(60) Information provided by Joel Rynes, personal communication, October 28, 2008.

(61) Domestic Nuclear Detection Office. "Cargo Advanced Automated Radiography (CAARS) Program Update Brief," slides 3, 18. This source is a briefing marked by the Domestic Nuclear Detection Office, the originating organization, "For Official Use Only." That office has approved CRS use of the information cited by this footnote.

(62) Ira Reese, Executive Director, Laboratories and Scientific Services, and Patrick Simmons, Director, Non-Intrusive Interrogation, both of Customs and Border Protection, provided information in this paragraph, personal communication, October 23, 2008.

(63) Testimony of Thomas Winkowski, Assistant Commissioner, Office of Field Operations, U.S. Customs and Border Protection, before the Senate Homeland Security and Governmental Affairs Committee, September 25, 2008.

(64) Information provided by Joel Rynes, personal communication, October 28, 2008.

(65) Personal communication, July 11, 2008.

(66) U.S. Department of Homeland Security. "Advanced Technology Demonstration for Shielded Nuclear Alarm Resolution." Broad Agency Announcement 08-102 for the Domestic Nuclear Detection Office, Transformational and Applied Research Directorate, March 2008, p. 22, available at https://www.fbo.gov/files/6f6/6f6f82cc03fe6fcbf298a7e3903a15 b7.doc?i=640d9818cdf76fa884b8c064b8f19376.

(67) As of April 2010, all Shielded Nuclear Alarm Resolution contractors had completed their preliminary design reviews by September 2009 and are working toward their critical design reviews, in which DNDO would approve the design. The first unit is scheduled to begin testing early in CY2011. Information provided by Joel Rynes, April 16, 2010.

(68) Rex Richardson, Vice President and Principal Scientist, Science Applications International Corporation, provided detailed information for this section, personal communications, June-August 2008. Others have commented as well to provide alternative perspectives.

(69) See http://www.accuray.com

(70) Electron beams of the levels discussed here generate bremsstrahlung photons of a wide spectrum of energies. Most of the photons are of relatively low energy, well below 1 MeV. They contribute almost nothing to the radiographic image but can represent a major portion of the radiation exposure to personnel. To remove or "filter" them, a piece of copper is placed in front of the photon beam. The material is thick enough to stop lower-energy photons but not higher-energy ones.

(71) The beam flux is further reduced by filtering. Electron beams of the levels discussed here generate bremsstrahlung photons of a wide spectrum of energies. Most of the photons are of relatively low energy, well below 1 MeV. They contribute almost nothing to the radiographic image but can represent a major portion of the radiation exposure to personnel. To remove or "filter" them, a piece of copper is placed in front of the photon beam. The material is thick enough to stop lower-energy photons but not higher-energy ones. After filtering, 6- and 9-MeVbeams generate the greatest number of photons at about 2 and 3 MeV, respectively.

(72) Beam stability refers to uniformity in both the duration of an x-ray pulse and the number of x-rays in a pulse.

(73) Personal communication, February 9, 2009.

(74) E-mail from Rex Richardson, May 11, 2009.

(75) E-mail from Rex Richardson, February 9, 2009.

(76) E-mail from Rex Richardson, SAIC, February 17, 2009.

(77) U.S. Department of Homeland Security. Science & Technology Directorate. "Non-Intrusive Inspection and Automated Target Recognition Technologies," Broad Agency Announcement BAA-10, CanScan, December 16, 2009, http://www.aqd.nbc.gov/Business/uploads/BAA10-CanScan.pdf.

(78) Jeffrey Illig, Program Manager, AS&E CAARS Program, and Stephen Korbly, Director of Science, Passport Systems, provided detailed information for this section, October 2008. Others have commented as well to provide alternative perspectives.

(79) U.S. Securities and Exchange Commission, Form 8-K, American Science and Engineering, Inc., Washington, DC, March 10, 2009, p. 2, http://www.sec.gov/Archives/edgar/data/5768/000110465909017589/ a09-7688_18k.htm.

(80) Information provided by Joel Rynes, personal communications, July 8, 2009

(81) The EZ-3D unit can operate without a radiography system. However, the AS&E CAARS system would use an EZ3D system and a radiography system together.

(82) A Rhodotron is a circular electron accelerator manufactured by Ion Beam Applications. Unlike most linear accelerators, it generates electron beams in continuous waves rather than in pulses.

(83) Sodium iodide detectors are used instead of detectors with a higher resolution, such as high-purity germanium. Since the EZ-3D system requires only that the detectors count photons, as opposed to identifying isotopes by their gamma-ray spectra, detectors with medium energy resolution suffice, and are much less costly than germanium detectors. Since the Rhodotron generates an electron beam in a continuous wave, and thus generates an x-ray beam in a continuous wave with no large spikes in the number of x-rays, the sodium iodide detector is able to record all the time. In contrast, a beam that turned on and off several thousand times a second would produce large spikes in the number of photons, overloading the detector.

(84) Personal communication, October 27, 2008

(85) Personal communication, October 8, 2008.

(86) Personal communication, October 22, 2008.

(87) Personal communication, February 9, 2009.

(88) Information provided by Ion Beam Applications, October 24, 2008. These figures are for a Rhodotron for use in xray mode, i.e., with a target for the electron beam to strike to create bremsstrahlung x-rays. As of October 30, 2008, 1 [euro] = $1.2923.

(89) Personal communication, October 22, 2008. The home page for the Bates Linear Accelerator Center is http://mitbates.lns.mit.edu/bates/control/main.

(90) Michael Sossong, Director of Nuclear Research, Decision Sciences Corporation, and Guest Scientist, Los Alamos National Laboratory, and Mell Stephenson, Executive Director of Government Programs, Decision Sciences Corporation, provided detailed information for this section, personal communications, April 2008-February 2009. Others have commented as well to provide alternative perspectives.

(91) A lump of plutonium, whether shielded or not, seems an implausible threat because it would be very difficult for terrorists, by themselves, to fabricate a bomb using plutonium. It would be even harder for them to fabricate such a bomb inside the United States using plutonium they had smuggled in because they would need to take added measures to avoid detection.

(92) Personal communication from Michael Sossong, April 23, 2008.

(93) Jonathan Katz, Karol Lang, and Roy Schwitters, "Muon Tomography--The Future of Vehicle and Cargo Inspection," report prepared for Decision Sciences Corporation, July 19, 2007, p. 5.

(94) Ibid.

(95) Ibid., p. 4.

(96) Ibid., p. 9-10.

(97) The mixture currently includes a small fraction of helium-3, which is of use for detecting neutrons. Recognizing the scarcity of that gas, DSIC would replace helim-3 with helium-4 (the gas used to fill balloons, for example) to maintain the same proportion of gases, and would use boron-10 to detect neutrons.

(98) A voxel is a volume element, analogous to a two-dimensional pixel, or picture element.

(99) Deflection is influenced both by Z and density. A muon is more likely to interact with a larger atom (higher Z) than with a smaller one. A muon is also more likely to interact with atoms the closer they are packed together (density). Scattering density combines Z and density into one unit.

(100) Information provided by Leon Feinstein, DNDO, e-mail, May 17, 2010.

(101) Information provided by Michael Sossong, DSIC, e-mail, May 4, 2010.

(102) Information provided by Michael Sossong, DSIC, e-mail, May 9, 2010.

(103) Information in this paragraph provided by Leon Feinstein, DNDO, e-mails, May 17 and 19, 2010.

(104) Information provided by DSIC, e-mail, April 28, 2010.

(105) Personal communication, July 15, 2008.

(106) According to DSC, "Thermal [low-energy] neutrons undergo reaction with the boron-10 nuclei, forming a compound nucleus (excited boron-11) which then promptly disintegrates to lithium-7 and an alpha particle. Both the alpha particle and the lithium ion produce closely spaced ionizations in the tube gas, permitting the system to count neutrons." Personal communication, Michael Sossong, June 30, 2009. The initial plan was to use helium-3, but as of April 2010 the plan calls for the scanner to use boron-10. Because of its scarcity, helium-3 may well be unavailable for neutron detection; see Steve Fetter, "Overview of Helium-3 Supply and Demand," presentation at American Association for the Advancement of Science workshop on helium-3, April 6, 2010, http://cstsp.aaas.org/files/he3_fetter.pdf.

(107) Richard Kouzes et al., "3He Alternatives for National Security," presentation to American Association for the Advancement of Science workshop on helium-3, April 6, 2010, slide 6, http://cstsp.aaas.org/files/he3_kouzes.pdf.

(108) According to DSC, "In another SNM detection technique, the time correlation of a muon stopping in the volume can be correlated with a burst of neutrons from muon-induced-fission in the SNM. This would provide a positive signal of the presence of SNM and when fused with other signals from the system, could provide faster, more accurate scanning." Personal communication, Michael Sossong, June 30, 2009.

(109) Decision Sciences Corporation, "Decision Sciences Corporation Announces Agreement with Los Alamos National Laboratory to Collaborate on Homeland Security," press release, May 3, 2007, p. 1.

(110) Information provided by R. Leon Feinstein, Transformational and Applied Research Directorate, DNDO, program manager for near-term testing of the DSIC muon tomography prototype, personal communication, July 29, 2009.

(111) Information provided by R. Leon Feinstein, personal communication, May 10, 2010.

(112) U.S. Department of Energy. Office of Declassification. "Drawing Back the Curtain of Secrecy: Restricted Data Declassification Policy, 1946 to the Present, RDD-1." June 1, 1994. Item V (C) (2) (s), at https://www.osti.gov/opennet/forms.jsp?formurl=document/ rdd-1/drwcrtf3.html#ZZ1.

(113) The Royal Society, "Detecting Nuclear and Radiological Materials," RS policy document 07/08, March 2008, p. 6, at http://royalsociety.org/displaypagedoc.asp?id=29187.

(114) Personal communication, DSIC, e-mail, April 28, 2010.

(115) Katz et al., "Muon Tomography--The Future of Vehicle and Cargo Inspection," pp. 19-20. DSIC stated in April 2010 that the number of muons required for this purpose is 31; personal communication, April 28, 2010.

(116) Information provided by Dr. R. Leon Feinstein, Transformational and Applied Research Directorate, DNDO, program manager for near-term testing of the DSIC muon tomography prototype, e-mail, May 18, 2010.

(117) Personal communication, July 31, 2008.

(118) Information provided by DSIC, e-mail, May 21, 2010.

(119) Richard Kouzes, Pacific Northwest National Laboratory, lists the following: "agricultural products like fertilizer, kitty litter, ceramic glazed materials, aircraft parts and counter weights, propane tanks, road salt, welding rods, ore and rock, smoke detectors, camera lenses, televisions, medical radioisotopes" (but not bananas, contrary to public perception). Richard Kouzes et al., "3He Alternatives for National Security," presentation to American Association for the Advancement of Science workshop on helium-3, April 6, 2010, slide 6, http://cstsp.aaas.org/files/he3_kouzes.pdf.

(120) Information provided by Michael Sossong, e-mail, July 7, 2009.

(121) Personal communication, May 10, 2010.

(122) Personal communication, July 15, 2008.

(123) Personal communication, Michael Sossong, DSIC, e-mail, May 5, 2010.

(124) For further information, see State University of New York at Buffalo, Department of Nuclear Medicine, Center For Positron Emission Tomography, "Positron Emission Tomography," at http://www.nucmed.buffalo.edu/prevweb/petdef.htm.

(125) Personal communication, Michael Sossong, July 12, 2008.

(126) Information provided by Lawrence Delaney, Senior Vice President for System Development, Decision Sciences Corporation, e-mail, June 30, 2009.

(127) See "Feds Fight Threat of Small-Boat Terror Strikes," CNN, April 27, 2008, at http://www.cnn.com/2008/US/04/27/small.boat.terror.ap/ index.html#cnnSTCText.

(128) Stephen Korbly, Director of Science, Passport Systems, Dennis McNabb, Deputy Division Leader, N Division, Lawrence Livermore National Laboratory, and Glen Warren, Senior Research Scientist, Pacific Northwest National Laboratory, provided detailed information for this section, April-August 2008. Others have commented as well to provide alternative perspectives.

(129) For an image of minerals fluorescing under ultraviolet light, see Glenbow Museum, Calgary, Alberta, "Fluorescent Minerals," at http://www.glenbow.org/collections/museum/minerals/flourescent.cfm.

(130) For a description of this process, see Passport Systems, Inc., "Technology," at http://www.passportsystems.com/tech.htm.

(131) Personal communication, Dr. R. Leon Feinstein, DNDO, August 8, 2008. This technology would accelerate particles to high energies over much shorter distances than are possible at present. For example, existing accelerators can increase the energy of particles (e.g., electrons) by 5 to 10 MeV per meter; the new technology might increase that to over 150 MeV per meter, making for a much more compact accelerator.

(132) Collimation filters many forms of electromagnetic radiation so that only photons traveling in a certain direction are allowed through. In the case of x-rays or gamma rays, a collimator is typically a plate of lead or tungsten with many small parallel holes drilled through it.

(133) William Bertozzi and Robert Ledoux, "Nuclear Resonance Fluorescence Imaging in Non-Intrusive Cargo Inspection," Nuclear Instruments and Methods, B241, 820 (2005), p. 7.

(134) Personal communication, Stephen Korbly, Passport Systems, June 9, 2008.

(135) Personal communication, Stephen Korby, Passport Systems, August 8, 2008.

(136) In order to identify the composition of a voxel with high confidence, the germanium detector must receive enough photons. If the material is dense, a longer scan time may be needed to accumulate the required number of photons.

(137) Personal communication, Stephen Korbly, Passport Systems, Inc., August 23, 2008.

(138) Personal communication, August 9, 2008.

(139) Personal communications, August 9 and 21, 2008.

(140) See U.S. Department of Homeland Security. "Advanced Technology Demonstration for Shielded Nuclear Alarm Resolution," Broad Agency Announcement 08-102 for the Domestic Nuclear Detection Office, Transformational and Applied Research Directorate, March 2008, HSHQDC-08-R-00020, https://www.fbo.gov/download/6f6/6f6f82cc03fe6fcbf298a7e3903a15b7/ HSHQDC-08-R-00020(3-28-08).doc.

(141) David Jordan and Glen Warren, "Simulation of Nuclear Resonance Fluorescence in Geant4," in Institute of Electrical and Electronics Engineers, Nuclear Science Symposium Conference Record, 2007, vol. 2, pp. 1185-1190.

(142) This model uses the Monte Carlo N-Particle Transport Code, developed by Los Alamos National Laboratory; see http://mcnp-green.lanl.gov/.

(143) The Rhodotron is made by Ion Beam Applications, a Belgian company. The company states, "The Rhodotron[R] is a recirculating accelerator where electrons gain energy by crossing a coaxial-shaped accelerating cavity several times. This original design makes it possible to operate the machine in continuous mode for maximum efficiency and throughput." http://www.iba.be/industrial/rhodo-files/rhodo.php.

(144) Information provided by Ion Beam Applications, October 24, 2008.

(145) "Duty cycle" refers to the fraction of time that an accelerator's beam is on. Most accelerators operate at duty cycles of about 0.1 percent, i.e., the beam is on only 1/1000 of the time. Passport Systems states that its accelerators use a beam with duty cycles in the range of 5 percent to 100 percent. Information provided by Stephen Korbly, Passport Systems, e-mail, March 1, 2010.

(146) Information provided by Stephen Korbly, Passport Systems, e-mail, March 1, 2010.

(147) Dr. G. Peter Nanos, Jr., Associate Director for Research and Development, Defense Threat Reduction Agency (DTRA), Major Brad Beatty, USAF, Branch Chief, Standoff Detection Branch, Nuclear Detection Technology Division, DTRA, and Dr. Luc Murphy, Research Scientist, Locate and ID Branch, Nuclear Detection Technology Division, DTRA, and others at DTRA provided detailed information for this section, personal communications, AprilJuly 2008. Others have commented as well to provide alternative perspectives.

(148) Personal communication, August 5, 2008.

(149) The Royal Society, "Detecting Nuclear and Radiological Materials," RS policy document 07/08, March 2008, p. 5, available at http://royalsociety.org/displaypagedoc.asp?id=29187.

(50) Personal communication, August 5, 2008

(151) "Photonuclear" refers to a nuclear reaction caused by a photon, in this case fission of SNM induced by high- energy photons.

(152) Information provided by DTRA, e-mail, April 22, 2010.

(153) "Raytheon Awarded Contract for Integrated Standoff Inspection System," Raytheon news release, April 26, 2010, http://raytheon.mediaroom.com/index.php?s=43&item=1546& pagetemplate=release.

(154) Information provided by DTRA, e-mail, April 30, 2010.

(155) U.S. Department of Defense. Defense Threat Reduction Agency. Broad Agency Announcement HDTRA1-09-NTDBAA, "Advanced Detector Development (ADD) and Nuclear Forensics Research and Development Programs," November 2008, pp. 43-44, https://www.fbo.gov/download/4f8/ 4f898fe516d2145e703eaf8bbd33ff12/NTD-09BAA.pdf.

(156) Information provided by DTRA, e-mail, May 10, 2010.

(157) Data cannot be gathered when the beam is on, for there would be far more photons from the beam than photons from fission of SNM, making it difficult or impossible for the detectors to identify the latter.

(158) Jonathan Katz states: "In principle, a sufficiently long and narrow collimator could produce an arbitrarily narrow xray beam. The price paid is that there is very little energy in the beam. For example, if the initial divergence is 30 degrees (plausible for an x-ray beam), then a beam collimated to 10 cm in diameter at 200 m (5 X 10^-4 radian, or 2.5 X 10^-7 steradian) will contain about 2.5 X 10^-7 of the source's energy and power. The rest is absorbed in the collimator or scattered into a diffuse flux; those photons are of no use for detection. The result would be a tiny amount of energy on the target. Any signal from fission of SNM generated by that energy would be emitted roughly equally in all directions, so a 100 cm^2 detector collocated with an x-ray source 200 m away would only pick up about 2 X 10^-8 of the signal. If that same detector were 5 m from the target but the accelerator were 200 m from the target, the detector would pick up only 3 X 10^-5 of the signal. Of course, background radiation isn't reduced at all, and would be several orders of magnitude stronger than the signal from fission. These estimates also ignored attenuation in the air. For a 10MeV photon (a typical product of a 30-MeV accelerator), the beam is reduced by a factor of about 3 every 250 meters due to attenuation in the air, and the attenuation of 1-MeV fission gammas is about 5 times as great (a factor of 3 every 50 m). The attenuation of fission neutrons is about a factor of 3 every 150 m." Personal communications, September 29-October 8, 2008.

(159) The surface area of a sphere is 4[][pi][][r.sup.2], where r is the radius. The surface area of a sphere with a 1-meter radius is 12.6 square meters. The surface area of a sphere with a 1,000-meter radius is 12.6 million square meters. Thus a detector one meter square would receive one-millionth as many photons at a distance of 1 km as compared to a distance of 1 m.

(160) DTRA states, "It is recognized that perhaps the single greatest challenge in successful implementation of this technology is the placement of high sensitivity (and specificity) detectors at ranges that may have to be significantly closer than the interrogation to be effective. These studies, to include modeling and experimentation are proceeding as part of the research program." Personal communication, August 5, 2008.

(161) Information provided by DTRA, e-mail, April 22, 2010.

(162) For example, the American Association for the Advancement of Science held a workshop on helium-3, focusing on the shortage, on April 6, 2010; see http://cstsp.aaas.org/agenda_meeting.html for presentations made at the workshop.

(163) Information provided by DTRA, e-mail, April 22, 2010. Scientific Committee (SC) 1-18 of the National Council on Radiation Protection and Measurements is Use of Ionizing Radiation Screening Systems for Detection of Radioactive Materials That Could Represent a Threat to Homeland Security; SC 1-19 is Health Protection Issues Associated with Use of Active Detection Technology Security Systems for Detection of Radioactive Threat Materials. See National Council on Radiation Protection and Measurements, "Current Program," http://www.ncrponline.org/Current_Prog/Current_Program.html.

(164) Information provided by DTRA, e-mail, May 10, 2010.

(165) U.S. Department of Homeland Security. Domestic Nuclear Detection Office. "Advanced Radiation Monitoring Devices (ARMD): Near Term Research Project," Broad Agency Announcement (BAA) BAA10-DNDO-01, March 1, 2010, https://www.fbo.gov/download/ccf/ccf3ddfc085144aa0e5cc2cfbfa2cb65/ ARMD_BAA_Final_2010.doc.

(166) The Royal Society, "Detecting Nuclear and Radiological Materials," RS policy document 07/08, March 2008, p. 1, available at http://royalsociety.org/displaypagedoc.asp?id=29187.

(167) Personal communication, July 19, 2008.

(168) Personal communication, August 4 and 5, 2008; emphasis added.

(169) Personal communication, August 1 and 8, 2008.

(170) Herbert York, Race to Oblivion: A Participant's View of the Arms Race, New York, Simon and Schuster, 1970, p. 211.

(171) John Valentine, Lawrence Livermore National Laboratory, provided invaluable assistance in explaining the science presented in this section, April-July 2008. Others reviewed and commented on this section as well.

(172) Mark et al., "Can Terrorists Build Nuclear Weapons?"

(173) An electron volt is a unit of energy used for measuring atomic and nuclear processes. One electron volt (eV) is equal to the amount of energy gained by a single unbound electron (one not part of an atom) when it accelerates through an electrostatic potential difference of one volt. It is equal to 1.6x10-19 Joules. For comparison, the energy release in the fission of one uranium atom is 200 million electron-volts, and the energy required to remove an electron from a hydrogen atom is 13.6 eV. Information provided by Defense Threat Reduction Agency, personal communication, August 5, 2008.

(174) For the gamma ray spectra of various isotopes, see "Gamma-Ray Spectra of Isotopes," within the Radiochemistry Society website at http://www.radiochemistry.org/periodictable/gamma_spectra/. For the percentage distribution of the dozens of gamma rays from uranium-235, see "TORI Data" in "WWW Table of Radioactive Isotopes," available at http://ie.lbl. gov/toi/nuclide. asp?iZA= 920235.

(175) A half-centimeter of air will stop a beta particle emitted by tritium, while 0.04 cm of water or 2 mm of aluminum will stop a beta particle emitted by iodine-131. Eckhardt, "Ionizing Radiation--It's Everywhere," pp. 18, 19.

(176) Under the Atomic Energy Act of 1954, P.L. 83-703, 42 U.S.C. 2014, SNM is uranium enriched in the isotopes 233 or 235 or plutonium. The Nuclear Regulatory Commission has not declared any other material to be SNM even though the Act permits it to do so. U.S. Nuclear Regulatory Commission. "Special Nuclear Material." Available at http://www.nrc.gov/materials/sp-nucmaterials.html.

(177) The degree to which chemical properties of isotopes are similar "depends on the element. For hydrogen/deuterium the chemical differences are substantial; you cannot survive on heavy water. For other light elements they are small but produce measurable effects (the whole field of paleoclimatology is based on this). For uranium they are infinitesimal." Personal communication, Jonathan Katz, August 7, 2008.

(178) U.S. Department of Energy. Office of Environmental Management. Integrated Data Base Report--1996: U.S. Spent Nuclear Fuel and Radioactive Waste Inventories, Projections, and Characteristics, revision 13, December 1997; table B.1, "Characteristics of important radionuclides," http://web.em.doe.gov/idb97/tabb1.html.

(179) A more precise indicator of decay is specific activity, the number of curies per gram of material, where 1 curie = 3.7 x 10A10 disintegrations per second. Plutonium-241, for example, has a specific activity of 102 curies/gram, and its rapid radioactive decay makes it so hot that pieces of it glow red. Plutonium-239 has a specific activity of .062 curies/gram, while the corresponding figure for uranium-235 is .000002.

(180) "The attenuation of gamma rays depends on the energy of the gamma ray (generally more energetic gamma rays penetrate better, though there are some exceptions), the density of electrons (generally nearly proportional to the mass density or specific gravity) and how tightly the electrons are bound to the nuclei (much more strongly for high-Z elements). The last factor is the most important, and is why lead is used in shielding." Personal communication, Professor Jonathan Katz, Department of Physics, Washington University in St. Louis, August 7, 2008.

(181) As an analogy, when one billiard ball strikes another squarely, the first transfers its energy to the second and stops, while the second moves with about the same speed and direction as the first. By contrast, if a billiard ball strikes a bowling ball squarely, the bowling ball will move forward slightly and the billiard ball will bounce back with nearly the same velocity with which it struck the bowling ball.

(182) It takes .074 cm of lead to block half the gamma rays with an energy of 186 keV. Thus, 1 inch (2.54 cm) of lead has 2.54/.074 = 32.3 such thicknesses for 186-keV gamma rays, so (54) to the 32.3 power, or 1.9 x 10-10, of these gamma rays will penetrate 1 inch of lead. One kg of HEU emits 4 x 107 gamma rays per second at 186 keV, ignoring absorption of the gamma rays by the uranium. (Source: Roger Byrd et al., "Nuclear Detection to Prevent or Defeat Clandestine Nuclear Attack," IEEE Sensors Journal, August 2005, p. 594.) Accordingly, one 186-keV gamma ray photon could be expected to escape 1 kg of HEU surrounded by an inch of lead every 500 seconds or so. Absorption of gamma rays by uranium would reduce this number considerably. Increasing the amount of U-235 in a bomb-usable shape would not affect this calculation much because most gamma rays would be absorbed by the uranium and the amount of lead would increase as the surface area of the uranium lump increased. Further, gamma rays radiate in all directions. Since most detectors do not surround the object to be inspected, such as a cargo container, it would capture only a part of these gamma rays, further reducing the probability of detection.

(183) The detector used high purity germanium was cooled with liquid nitrogen

(184) Roger Byrd et al., "Nuclear Detection to Prevent or Defeat Clandestine Nuclear Attack," IEEE Sensors Journal, August 2005, p. 594.

(185) "There is an important caveat in this statement, and that is cargo containers at sea. The so-called "ship-effect" that results from higher neutron levels aboard ships due to cosmic ray interactions with iron and other ship contents can result in spurious neutron readings from cargo containers at sea." Information provided by Defense Threat Reduction Agency, personal communication, August 5, 2008. Another caveat is that while innocent neutron sources other than background are rare, there are some, such as californium-252, which is produced in nuclear reactors and is used as a laboratory neutron source.

(188) U.S. Department of Defense and Department of Energy. The Effects of Nuclear Weapons, Third Edition, compiled and edited by Samuel Glasstone and Philip Dolan, Washington, U.S. Govt. Print. Off., 1977, p. 633.

(189) Ibid.

(190) D.R. Slaughter et al., "The 'Nuclear Car Wash': A Scanner to Detect Illicit Special Nuclear Material in Cargo Containers," UCRL-JRNL-202106, January 30, 2004, p. 4.

(191) They are not a definitive indicator, however, because there could be other sources of fission, such as californium252, and cosmic rays could induce fission.

(192) D.R. Slaughter et al., "The 'Nuclear Car Wash': A Scanner to Detect Illicit Special Nuclear Material in Cargo Containers," UCRL-JRNL-202106, January 30, 2004, pp. 6-7.

(193) For further analysis of this topic, see CRS Report RL34070, Fusion Centers: Issues and Options for Congress, by John Rollins.

(194) See, for example, "Inverse Square Law, General," at http://hyperphysics.phy-astr.gsu.edu/Hbase/Forces/isq.html.

(195) The spread is measured as "full width at half maximum," that is, the width of the curve measured halfway from the top to the bottom of the curve.

(196) Personal communication, Defense Threat Reduction Agency, August 8, 2008.

(197) This paragraph was prepared with the assistance of Aleksey Bolotnikov, Physicist, Brookhaven National Laboratory, Professor Zhong He, Department of Nuclear Engineering and Radiological Sciences, University of Michigan, and Ralph James, Senior Physicist, Brookhaven National Laboratory, July 2008.

(198) The peak in the energy spectrum corresponds to the total energy of an incoming photon completely stopped inside the detector. The continuum on the left side of the peak is caused when an incoming photon scatters in the detector, depositing an unpredictable fraction of its total energy. For such events, the information about the photon's energy is lost. Such events contribute to the background that may affect detector performance.

(199) Personal communication, August 8, 2008.

(200) For a simple discussion of how semiconductors work, see Marshall Brain, ""How Semiconductors Work," at http://www.howstuffworks.com/diode.htm. For further detail, see Knoll, Radiation Detection and Measurement, Chapter 11, "Semiconductor Diode Detectors," pp. 353-403.

(201) Information provided by Isotope Program, U.S. Department of Energy, personal communication, June 30, 2008.

(202) Information provided by Customs and Border Protection, Department of Homeland Security, personal communication, July 25, 2008.

(203) The American Association for the Advancement of Science held a workshop on the helium-3 shortage on April 6, 2010. Briefing slides are available at http://cstsp.aaas.org/agenda_meeting.html.

(204) See Proportional Technologies, Inc., "Neutron Straws," at http://www.proportionaltech.com/neutron.htm.

(205) See, for example, Katz, J. I., Blanpeid, G. S., Borozdin, K. N. and Morris, C., "X-Radiography of Cargo Containers," Science and Global Security, Vol. 15, 1: 49-56.

(206) P.L. 110-53, Implementing Recommendations of the 9/11 Commission Act of 2007, Section 1701, 121 Stat. 489.
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Title Annotation:Detection of Nuclear Weapons and Materials: Science, Technologies, Observations
Author:Medalia, Jonathan
Publication:Congressional Research Service (CRS) Reports and Issue Briefs
Article Type:Technical report
Geographic Code:1USA
Date:Jun 1, 2010
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