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S&TR Staff

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Wayne Shotts
Associate Director
for Nonproliferation, Arms Control, and International Security

Radiation Detection at the Leading Edge of Scientific Discovery

MANY science historians have relegated nuclear science to the 20th century, as if the field were fully mature with nothing left to discover. However, as we address the threats of the 21st century, we are finding that significant advances must be made in our understanding of nuclear phenomena if we are to devise effective countermeasures against nuclear proliferation and terrorism.
Even though Lawrence Livermore has been a leader in nuclear science for more than 50 years, today’s technical challenges are markedly different from those posed by earlier nuclear weapon development programs. Instead of needing to rapidly collect and interpret data on large events with strong signals (nuclear tests, for example), we now need to be able to detect and characterize small events with weak signals (smuggled nuclear materials, for example). This challenge is compounded by the fact that the world is filled with radioactivity, and small or distant signals can be swamped by the naturally occurring background.
However, the very nature of radioactive materials makes them detectable at a distance, giving us a technological approach to locating and interdicting such materials. Indeed, different materials and isotopes give off unique emission spectra, making it possible not only to detect them but also to characterize and identify the detected material. This capability in turn allows us to discriminate fissile and other threat materials from the large background of naturally occurring radiation and legitimate sources, such as medical isotopes.
In addition, by understanding the nature of nuclear proliferation and terrorism, we can identify points in the “threat timeline” where we can effectively apply detection and interdiction measures. Clearly, responding after the fact—after a country or terrorist group has obtained or used nuclear weapons—is the least satisfactory approach. Our greatest leverage lies at the front end of the problem—in locking up weapons and nuclear materials, in forestalling proliferators from developing nuclear weapon capabilities, in detecting threat materials and weapons far from their intended target, and in preventing the detonation of any nuclear explosive or radiological dispersal device. At each of these points, novel approaches to radiation detection can make significant contributions. However, we are finding that, in many cases, new scientific understanding is needed to turn detection concepts into real technologies.
For example, the International Atomic Energy Agency has a need for technology that can monitor nuclear power reactors to ensure that nuclear fuel is not diverted for illicit purposes. To this end, Livermore scientists are investigating the feasibility of using scintillator-based antineutrino detectors to provide a near-real-time, nonintrusive way to measure changes in fissile content and total fission rates (that is, power levels) at nuclear reactors and thereby detect the diversion of a significant quantity of plutonium or uranium-235.
New science is also playing an essential role in efforts to detect fissile materials inside cargo containers. Detecting highly enriched uranium (HEU) is particularly challenging, because its quiescent emissions are easily shielded. Livermore researchers recently discovered a delayed gamma-radiation signature for HEU that is two to three orders of magnitude more intense than its delayed neutron signals (the signature most often used in active interrogation). Use of this gamma signature should permit the detection of HEU even when shielded by thick cargo. (See S&TR, May 2004, Screening Cargo Containers to Remove a Terrorist Threat.)
Other Laboratory scientists are working to develop aluminum antimonide (AlSb) as a promising new material for achieving excellent energy resolution without the cumbersome cooling systems required for the germanium-based detectors currently in use. Although this project is in the research stage, Livermore has successfully overcome a number of technical challenges related to crystal purity and stoichiometry that have discouraged other researchers from pursuing AlSb for this application.
As the article entitled Radiation Detection on the Front Lines describes, the Radiation Detection Center is leading the effort to reinvigorate Laboratory research in nuclear science, with a focus on nonproliferation and counterterrorism. The center is also supporting outreach activities with universities across the country to revive nuclear science as a vital field of study. Both efforts leverage current interest among scientists and students to work on research topics with real-world relevance and, in so doing, help to build the pool of technical talent that is essential to ensuring Livermore’s continued preeminence in nuclear science and technology and its ability to fulfill its national security mission today and into the future.

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UCRL-52000-04-9 | September 3, 2004