National Academies Press: OpenBook

Nuclear Physics: Exploring the Heart of Matter (2013)

Chapter: Highlight: Nuclear Crime Scene Forensics

« Previous: 5 Nuclear Science Going Forward
Suggested Citation:"Highlight: Nuclear Crime Scene Forensics." National Research Council. 2013. Nuclear Physics: Exploring the Heart of Matter. Washington, DC: The National Academies Press. doi: 10.17226/13438.
×

Highlight: Nuclear Crime Scene Forensics

An attack using a nuclear device on one of our cities would be both catastrophic and world changing. President Obama describes it as the single biggest threat to U.S. security. In the event of such an attack, a set of urgent and crucial questions would have to be answered: What was exploded? Who was responsible? Do they have more? Was the device improvised or sophisticated? Did they steal it or have help making it? Is the material reactor-grade or weapons-grade fuel? How old is it? Detonation of a radiological device, a “dirty bomb,” could also result in widespread contamination and public concerns. Nuclear forensics is the technical means and set of scientific capabilities that, in the event of such attacks, would be used to answer these questions (see Figure FOR 1).

Nuclear forensics involves the analysis and evaluation of postdetonation debris following a nuclear explosion. It is also essential in the analysis of unexploded devices or material that have been seized. The basic idea behind nuclear forensics is the same as that behind the

image

FIGURE FOR 1 Lead container and the glass ampoule containing highly enriched 235-uranium seized in Bulgaria in 1999 and analyzed at Lawrence Livermore National Laboratory for forensic purposes. SOURCE: “Forensic Analysis of a Smuggled HEU Sample Interdicted in Bulgaria,” UCRL-ID-145216, August 2001.

Suggested Citation:"Highlight: Nuclear Crime Scene Forensics." National Research Council. 2013. Nuclear Physics: Exploring the Heart of Matter. Washington, DC: The National Academies Press. doi: 10.17226/13438.
×

analyses of stellar nucleosynthesis. In stellar astrophysics, the debris from nuclear reactions inside a star is used to infer detailed information about the nature of the star and the reactions, including the mass, density, compositional layers, and temperature of the star. Very similar analyses are used in nuclear forensics to answer the pertinent questions for an exploded nuclear device here on Earth.

During the nuclear testing era, the United States and other countries used radiochemistry techniques to characterize the explosions from collected debris. The application of the radio-chemistry experience and techniques developed over those decades has proved invaluable in generating the key concepts underlying nuclear forensics. The current forensic capabilities have been used in postdetonation exercises in which the national laboratories have demonstrated that they can characterize nuclear debris and other forensic data and can infer the key design features for a variety of hypothesized nuclear explosive devices.

Several key nuclear physics concepts apply in forensics. One is the use of the natural radioactive decay of a nuclear material to determine its age. Plutonium, which does not occur naturally on Earth, would originally have been produced in a reactor with some unavoidable plutonium-240 and plutonium-241 content. The decay of plutonium-241 into americium-241 with a 14.4-year half-life indicates the time lapsed since production. This is analogous to the way in which carbon dating is used to determine the age of some material. Key information regarding the origin of plutonium material can also be obtained from the ratio of the different plutonium isotopes—for example, whether it is weapons grade or reactor grade and the total reactor neutron fluence (or burnup) to which it was originally exposed.

The design of a detonated device from the explosion debris can be inferred from the shape of the neutron flux spectrum, which serves as a fingerprint for the design. An analogous fingerprint for nuclear reactors is the average energy of the neutron spectrum, from which one can deduce whether the reactor is a thermal light or heavy water reactor or a fast reactor, which will point to whoever designed the reactor. To extract the shape of the neutron flux spectrum for a detonated nuclear device, forensics takes advantage of the very different energy dependencies of nuclear cross sections. Useful nuclear reactions fall into three categories: (1) neutron capture, which characterizes the low-energy part of the spectrum, (2) inelastic neutron scattering to nuclear isomers, which characterizes the fission neutron component of the spectrum, and (3) threshold (n, 2n) reactions, which characterize the high-energy component of the spectrum from fusion neutrons.

As an example, let us consider how we could use the americium-241 present in the fuel of a plutonium-based nuclear device to extract information about the shape of the neutron spectrum. During the explosion, some of the americium-241 will be transmuted to americium-240 through the (n,2n) reaction; this americium-240 could not have been present before detonation because it lives for only 2.1 days. Producing americium-240 from americium-241 requires the presence of neutrons with energies of at least 6.67 MeV, because the nuclear reaction involved is a so-called threshold reaction. Thus, the relative abundance of americium-240 provides unique information about the high-energy component of the neutron spectrum (see Figure FOR 2).

Detailed studies of the implication of the production of americium-240 or other isotopes of interest require that all of the significant paths for producing and destroying these isotopes during a nuclear explosion be known. To address this requirement, a number of multiinstitutional programs are performing a series of measurements. One of the collaborations is attempting the first measurement of the fission of americium-240, a very challenging undertaking for several reasons. The first two challenges are the production of a significant quantity of americium-240 and its subsequent chemical separation to form a target. These are being carried out at LBNL using the 88-in. cyclotron and radiochemistry facilities. Stewardship Science fellow Paul El-

Suggested Citation:"Highlight: Nuclear Crime Scene Forensics." National Research Council. 2013. Nuclear Physics: Exploring the Heart of Matter. Washington, DC: The National Academies Press. doi: 10.17226/13438.
×

ison, who is featured in the Highlight “Future Leaders” between Chapters 5 and 6, is part of this team. Only very small targets of americium-240 will be possible, making the fission cross section measurement another challenge. For the fission measurement, the very high neutron intensity capability of the Lead Slowing-Down Spectrometer (LSDS) at the Los Alamos Neutron Science Center will be used. The LSDS will also be used to measure another fission reaction of interest for uranium devices—in particular, the fission of uranium-237. Like americium-240, uranium-237 is very radioactive and difficult to produce and chemically separate. In these experiments, uranium-237 is produced by irradiation of uranium-236 in the high-flux reactor at Oak Ridge National Laboratory. Uranium-237 has a half-life of 6.75 days, and handling the irradiated sample to chemically separate uranium-237 requires use of Oak Ridge’s hot cell facilities. Once the targets are fabricated, the fission measurements will be carried out at Los Alamos using the LSDS.

The committee hopes that the validity of our nation’s nuclear forensics schemes will never need to be tested directly. Confidence in the program capability is greatly enhanced by the tight coupling between the fundamental nuclear data community, radiochemists, and the weapons design community. It hopes as well that broadcasting the capabilities of nuclear forensics to identify the source of a nuclear device and its fuel will deter advocates of such unthinkable acts.

image

FIGURE FOR 2 Plutonium (Pu) and americium (Am) reaction chains. One of the pieces in a nuclear forensics puzzle is the amount of plutonium-241 in a nuclear device, which can be determined after an explosion. The figure illustrates all of the neutron-induced reactions that need to be understood to deduce the original amount of plutonium-241 in the device and the fluence of high-energy neutrons in the subsequent explosion. These include reactions on americium-241, the daughter from radioactive decay of plutonium-241. The relative abundance of americium-240 in the explosion debris would provide key information for forensic analyses, because americium-240 can only be produced by high-energy (En > 6.67 MeV) neutrons. SOURCE: Courtesy of A. Haynes, Los Alamos National Laboratory.

Suggested Citation:"Highlight: Nuclear Crime Scene Forensics." National Research Council. 2013. Nuclear Physics: Exploring the Heart of Matter. Washington, DC: The National Academies Press. doi: 10.17226/13438.
×
Page 227
Suggested Citation:"Highlight: Nuclear Crime Scene Forensics." National Research Council. 2013. Nuclear Physics: Exploring the Heart of Matter. Washington, DC: The National Academies Press. doi: 10.17226/13438.
×
Page 228
Suggested Citation:"Highlight: Nuclear Crime Scene Forensics." National Research Council. 2013. Nuclear Physics: Exploring the Heart of Matter. Washington, DC: The National Academies Press. doi: 10.17226/13438.
×
Page 229
Next: 6 Recommendations »
Nuclear Physics: Exploring the Heart of Matter Get This Book
×
Buy Paperback | $64.00 Buy Ebook | $49.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

The principal goals of the study were to articulate the scientific rationale and objectives of the field and then to take a long-term strategic view of U.S. nuclear science in the global context for setting future directions for the field. Nuclear Physics: Exploring the Heart of Matter provides a long-term assessment of an outlook for nuclear physics.

The first phase of the report articulates the scientific rationale and objectives of the field, while the second phase provides a global context for the field and its long-term priorities and proposes a framework for progress through 2020 and beyond. In the second phase of the study, also developing a framework for progress through 2020 and beyond, the committee carefully considered the balance between universities and government facilities in terms of research and workforce development and the role of international collaborations in leveraging future investments.

Nuclear physics today is a diverse field, encompassing research that spans dimensions from a tiny fraction of the volume of the individual particles (neutrons and protons) in the atomic nucleus to the enormous scales of astrophysical objects in the cosmos. Nuclear Physics: Exploring the Heart of Matter explains the research objectives, which include the desire not only to better understand the nature of matter interacting at the nuclear level, but also to describe the state of the universe that existed at the big bang. This report explains how the universe can now be studied in the most advanced colliding-beam accelerators, where strong forces are the dominant interactions, as well as the nature of neutrinos.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!