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Neutrons: It Is All in the Timing—The Physics of Nuclear Fission Chains and Their Detection

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Neutrons: It Is All in the Timing—The Physics of Nuclear Fission Chains and Their Detection Neutrons: It Is All in the Timing—The Physics of Nuclear Fission Chains and Their Detection William A. Noonan hould a nuclear response team locate a potential threat object, it will be imperative for the team to quickly determine whether or not the object is an actual nuclear device. Only a nuclear device or a large amount of special nuclear material will sustain a significant number of fission chains. Therefore, the detection of fission chains constitutes a “smoking gun.” This article is a technical introduction to the physics of fission chains and their detection using neu- tron time-correlation methods; it also touches on some of the Johns Hopkins Univer- sity Applied Physics Laboratory’s efforts to bring this important detection capability to the field. INTRODUCTION Since the 1970s the U.S. government has been con- A third troubling trend has been the steadily grow- cerned about the “loose nukes” scenario, in which a ing number of nuclear weapon-owning states that are state with nuclear weapons loses control of one and it not signatories of the Treaty on the Non-Proliferation of is used for a terrorist attack. In recent decades, several Nuclear Weapons and that might act to further weaken trends have heightened this fear. There has been wide- the non-proliferation regime. Specifically, Pakistan has spread proliferation of nuclear weapons technology, a growing stockpile of weapons but questionable politi- most famously by the A. Q. Khan network.1 Special cal stability, and the government is having trouble con- nuclear material (SNM), the key component of nuclear tending with local terrorist organizations. Here, the weapons, has also proliferated. In the aftermath of the loose nuke scenario is particularly worrisome. In addi- Soviet Union’s dissolution, materials protection, con- tion, the Pakistani government has been complicit in trol, and accountability for SNM in the former Soviet the proliferation of nuclear weapon technology via the Union was poor. In the years since, materials protec- now-defunct smuggling network set up by A. Q. Khan, tion, control, and accountability has been strength- the founder of its nuclear weapons program. North ened but not entirely fixed,2 and there have been Korea has also demonstrated a nuclear capability while several documented cases of SNM disappearing from behaving as a seemingly irrational international actor, Russian stockpiles. heedless of international norms and unmoored by inter- 762 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 32, NUMBER 5 (2014) national commitments. Finally, Iran, a known sponsor the response team would be whether or not the object of terrorism, is arguably working toward a nuclear capa- was an actual nuclear device. Because the presence of bility, although recent diplomatic developments might nuclear chain reactions is the sine qua non of a nuclear ultimately limit the Iranian program. device, their detection would provide the smoking gun Although it is impossible to estimate the likelihood that escalates the national response. that a terrorist organization could actually acquire a By their nature, these small rapid response teams nec- functioning nuclear device, the possibility exists via at essarily have a limited load-out of equipment. The Johns least two different pathways.3, 4 Terrorists could steal Hopkins University Applied Physics Laboratory (APL) a state weapon or they could acquire SNM and pro- is developing technology that would give these teams liferated nuclear technology and then build their own the ability to detect fission chains without increasing improvised nuclear device. their load-out. Thus, the U.S. government maintains an assortment of assets and mechanisms for interdicting the movement of illicit SNM or an actual nuclear device. The U.S. THE PHYSICS OF FISSION CHAINS Customs and Border Patrol operates radiation portal To understand how fission chains can be detected, it monitors at ports of entry throughout the United States. is first necessary to delve into some aspects of the phys- Radiation portal monitors are also operated in ports ics of fission chains. As is widely known, certain iso- around the world, in collaboration with host nations as topes of the heaviest elements can fission into smaller part of Customs and Border Patrol’s Container Security nuclei. This process can happen spontaneously, or it Initiative and the Department of Energy’s Megaports can be induced by a free neutron colliding with the project.5 In addition, a string of sensors was emplaced nucleus. When a nucleus fissions, it emits zero, one, or along the borders around Russia as part of the Depart- more fission neutrons with probabilities that depend on ment of Energy’s Second Line of Defense program.6 This the particular isotope involved; on whether the fission is patchwork of detection capabilities is coordinated under spontaneous or induced; and in the case of induced fis- the Global Nuclear Detection Architecture managed by sion, on the kinetic energy of the incident neutron. The the Domestic Nuclear Detection Office in the Depart- number of neutrons emitted in a given fission event is ment of Homeland Security.7 called its “multiplicity,” and the probability distribution Should any of these portals or sensors detect the pas- of different multiplicities is called a “multiplicity distri- sage of suspicious radiological material, a special rapid bution.” The multiplicity distributions for the spontane- response team could be deployed to assess the threat. ous and induced fission of various important isotopes are Alternatively, there might be an intelligence cue for the plotted in Fig. 1. movement of SNM or a nuclear device, in which case It is possible for the neutrons emitted by the fission of the U.S. government would deploy national assets to one nucleus to collide with and induce fissions in other search for, locate, and asses the threat. nuclei, which in turn emit more neutrons. But in order Once the threat object is located, the response team for this phenomenon to create a self-sustaining nuclear would use diagnostic instrumentation to determine the chain reaction, the probability that fission neutrons level of threat it poses. The most pressing question for successfully induce additional fissions needs to be suf- (a) (b) 0.45 0.35 vs vi 0.40 U-235 U-238 U-238 2.21 0.30 U-235 2.52 Pu-240 Pu-240 2.16 U-238 U-238 2.43 0.35 Cf-252 Cf-252 3.76 Pu-239 Pu-239 3.01 0.25 0.30 0.25 0.20 0.20 0.15 Probability Probability 0.15 0.10 0.10 0.05 0.05 0.00 0.00 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 Multiplicity, vs Multiplicity, vi Figure 1. Multiplicity distributions and average multiplicity for various important isotopes for (a) spontaneous fission and (b) fission induced by 1-MeV neutrons.8, 9 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 32, NUMBER 5 (2014) 763 W. A. NOONAN ficiently high. Although there are many isotopes that the average number of fission neutrons in the first gen- are fissionable, there are relatively few isotopes for which eration of a fission chain is then pvi (see Fig. 2). Because this probability is high enough to sustain chain reac- each of these first-generation neutrons will induce their tions, and these select few are called “fissile.” own fission with probability p, there are pp$ vi second- 2 Consider the case of uranium. The largest constituent generation fissions and pvi second-generation fission ^ h n of naturally abundant uranium is uranium-238 (U-238), neutrons. Thus, there are pvi neutrons in the nth gen- which is fissionable but not fissile. The kinetic energies eration, and the total number^ h of neutrons produced by of most fission neutrons are not high enough to induce the fission chain is fission in U-238. Furthermore, U-238 has a strong pre- dilection for absorbing intermediate energy neutrons, 23 1 1 Np=+1 vvii+++ppvi f ==, (1) preventing them from inducing further fissions. (Inci- ^^hh 1 – pvi 1 – k dentally, plutonium-239, or Pu-239, used in both power reactors and nuclear weapons, is produced using this where kp= vi is the multiplication factor. If k ≥ 1, then absorption reaction: U-238 is placed inside a nuclear the series in Eq. 1 diverges, and the assembly is said to be reactor, where it absorbs neutrons and coverts to U-239. either critical or supercritical. U-239 is radioactively unstable and quickly decays into Each of these N fission neutrons has probability 1 – p Pu-239.) On the other hand, a minor constituent, U-235, of escaping the nuclear assembly, so the average number can be induced to fission by neutrons with any kinetic of neutrons from a fission chain that manage to escape is energy, and so it can sustain a chain reaction if there is 1 – p enough of it present. Because naturally abundant ura- 1 – pN==ML, (2) ^ h 1 – pvi nium consists of 99.3% U-238, there is too little U-235 and too much neutron-absorbing U-238 to sustain chain where ML is called the leakage multiplication. reactions. Therefore, uranium needs to be enriched in its Note that this model does not take geometry com- U-235 content before it can be used in a nuclear reactor pletely into account. For instance, a neutron born near or weapon. Weapons require highly enriched uranium the surface of a nuclear assembly will have a greater prob- (HEU), whereas reactors require lower enrichments and ability of escaping than a neutron born at the center, run on low enriched uranium. and so p is not really a constant. This effect is often We can construct a simple model for the average taken into account approximately by using a semiem- number of neutrons released by a single chain reaction pirical effective value for k (keff k) in Eq.
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