New Neutron Irradiation Facility Characterizes NIF's Neutron Time-of-Flight Detectors. UC Berkeley graduate student Joshua Brown—mentored by Lawrence Livermore physicist Darren Bleuel and with NSSC support—published in the Journal of Applied Physics the first complete characterization of a bibenzyl–stilbene mixed single-crystal scintillator. Such crystals have been used for years to measure implosion properties at the National Ignition Facility, such as fusion yield, fuel areal density, ion temperature, and, recently, core bulk velocity and remaining ablator carbon content. However, the fundamental efficiency, light yield, and temporal response of the scintillation material had, until now, not been directly measured. The characterization will improve quantification of these implosion properties on every shot to date and in the future. To perform the measurements, a new neutron irradiation facility and scintillator testing platform was developed at Lawrence Berkeley National Laboratory's 88-Inch Cyclotron, using deuteron breakup reactions to produce a white neutron spectrum. Efficiency measurements were also conducted at the Los Alamos Neutron Science Center. The photo shows NSSC-supported postdoc Brian Daub (top) and Josh Brown (bottom) working on equipment at the new facility. (Photo courtesy of LBNL.)
An Entirely New Approach to Nuclear Forensics. Although the United States ended nuclear testing more than 20 years ago, NSSC-sponsored research is providing exciting new insights into how nuclear fallout forms. Three graduate students—Laurence Lewis and David Weisz (both at UC Berkeley) and Marc Fitzgerald (University of Nevada, Las Vegas)—working with Kim Knight, Ian Hutcheon, and more than a dozen other scientists at Lawrence Livermore and Los Alamos National Laboratories have demonstrated that the conventional view of fallout formation, developed more than 50 years ago, is fundamentally flawed. Rather than consisting of homogeneous, highly refractory objects, fallout from several low-yield, near-surface nuclear tests conducted in the late 1950s was found to contain a variety of millimeter-sized silicate melt droplets with a variety of intriguing features. Perhaps most interesting is the revelation that individual spherules contain uranium varying in isotope composition from near natural to highly enriched—a 12:1 ratio of 235U to 238U. Another important discovery is that these spherules contain krypton and xenon produced by the fission of the device's nuclear fuel. By measuring the isotope composition of the gases, Livermore's Bill Cassata has demonstrated for the first time that the spherules cooled very quickly, from temperatures above 2,000°C to solid objects in only a few seconds. These studies, about to be published in the Journal of Environmental Research and the Journal of Radioanalytical and Nuclear Chemistry, provide an entirely new approach for nuclear forensics and have generated considerable excitement within the National Nuclear Security Administration. Kim's work in nuclear forensics has also been profiled in Science and Technology Review Ian's nuclear forensics work has been supported by the LDRD Program. The figure shows aerodynamic glasses—common to some nuclear events—studied by the researchers.
New Ion Trap Finds “Undetectable” Particles. Two back-to-back publications in the March 1, 2013 issue of Physical Review Letters showcase the potential of an ion trap system pioneered by researchers at Lawrence Livermore National Laboratory (including physicist and NSSC mentor Nicholas Scielzo), Argonne National Laboratory, and universities to perform extremely precise beta decay studies by inferring the energy and direction of neutrons and neutrinos from the nuclear recoil following beta decay. The first publication describes a new technique to perform neutron spectroscopy (the first fundamentally new approach in roughly 30 years) that circumvents the many difficulties associated with direct neutron detection by instead reconstructing the quantity and energies of the emitted neutrons by measuring the time of flight of the recoiling nuclei. The second publication describes tests of the Standard Model of particle physics in which the angular correlation between the emitted beta particle and the neutrino in the beta decay of trapped lithium-8 ions was studied by measuring the beta particle and two alpha particles from the breakup of the daughter nucleus, from which the direction and energy of the antineutrino can be inferred. These novel techniques will have an important impact on improving our understanding of the origin of the elements and fundamental electroweak theory and applications of nuclear science such as nuclear energy and stockpile stewardship. At LLNL, this work was featured in the Laboratory's 2013 Annual Report and in Science and Technology Review.