Radiochemistry Nuclear Test Experience and Unique Capabilities to New National Security and Renaissance Scientific Challenges
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S&TR March 2014 Radiochemists apply Radiochemistry nuclear test experience and unique capabilities to new national security and Renaissance scientific challenges. 4 Lawrence Livermore National Laboratory S&TR March 2014 Advances in Radiochemistry BOUT a century ago, the enduring A dream of alchemists was realized when physicist Ernest Rutherford and his collaborators demonstrated that one element can change into other elements through the process of radioactive decay. Rather than turning lead into Livermore scientists have engaged in radiochemistry, nuclear chemistry, and heavy-element research gold, Rutherford’s discovery led to the since the Laboratory’s founding in 1952. In the photo above, Wes Hayes works in hot cells on debris founding of radiochemistry—the study collected from the Hutch Event, a 1969 underground nuclear experiment that produced large quantities of radioactive isotopes—and nuclear of rare, heavy isotopes for debris analysis. At left, Roger Henderson prepares a uranium sample for chemistry, which focuses on the properties electrodeposition onto a stub for testing a resonance ionization mass spectrometry technique. of atomic nuclei and the processes involved in element transformation. Together, these fields form a pillar that Nuclear chemist Dawn Shaughnessy, research and isotope production facilities, supports Lawrence Livermore’s nuclear who leads the experimental and nuclear including the Center for Accelerator Mass security mission, a role they have held radiochemistry group in the Physical Spectrometry (CAMS) and the National since the institution was founded in 1952. and Life Sciences Directorate, notes Ignition Facility (NIF). Nuclear chemist (See S&TR, June 2002, pp. 24–30.) that many potential collaborators and Narek Gharibyan observes, “We have Radiochemists provided crucial project sponsors are surprised by the places to safely perform irradiation, contributions to the Laboratory’s nuclear breadth of radiochemistry capabilities chemical separation, and nuclear counting, weapons test program. They created tracer at the Laboratory. Says Shaughnessy, all on-site. CAMS also produces radioactive components for test devices by using “In the past, Livermore radiochemistry isotopes for a variety of chemistry actinides—radioactive elements with atomic to most people really meant nuclear test experiments. It’s unusual to have all of numbers from 89 through 103, the most analysis, although even in the heyday of these facilities in one place.” common of which is uranium. They also underground testing, that was only part Shaughnessy adds, “As radiochemists, assessed a weapon design’s performance of the endeavor.” we see NIF as a giant neutron source, one by studying the radioactive debris and Today, Lawrence Livermore offers a of the brightest in the world. For instance, gases produced in an experiment. Since the unique recipe for radiochemistry research, it can help us make samples for nuclear moratorium on underground nuclear testing combining nuclear testing expertise and forensics experiments.” But for everything began in 1992, radiochemistry research resources, such as rare radioisotopes Livermore offers, the recipe would be at Livermore has grown to encompass collected from past underground incomplete without Shaughnessy’s team of a wider range of national security and experiments, with robust materials a dozen or so innovative chemists, working scientific missions. handling capabilities and an array of in such diverse areas as fission, nuclear Lawrence Livermore National Laboratory 5 Advances in Radiochemistry S&TR March 2014 forensics, radioactive target fabrication, 10 and 15 megaelectronvolts. The foils are Gharibyan, 40 to 50 spectra are collected transactinide chemistry, and rapid thin enough for protons to pass through and analyzed for each sample. radiochemical separations. without losing much energy, yet thick The more quickly samples arrive at enough to generate an adequate number of Livermore’s nuclear counting facility, the Mapping Fission’s Outcomes fission reactions for study. Yttrium foils more radioactive products researchers are During fission, an excited nucleus splits act as a proton flux monitor, allowing the likely to detect. For many of the team’s into two (or occasionally more) lighter team to calculate the likelihood of a fission early experiments, samples received only fragments and in the process releases reaction occurring in the uranium foils. 10 minutes of proton irradiation, to limit by-products such as neutrons or photons Most fission products are unstable and radiation levels and obviate the need for a in the form of gamma rays. Although will decay by emission of a beta particle cooldown period before sample removal the results of an individual fission event and characteristic gamma rays. High-purity and analysis. The fast turnaround enabled are random, the distribution pattern for germanium radiation detectors record the researchers to detect products with half- the particles emitted for a given parent signatures of gamma rays emitted after lives as short as 5 minutes. isotope and initiating energy is statistically decay, and automated analysis software Longer-lived and lower-yield products predictable. Precisely determining these processes the data to determine the identity experience fewer decay events and patterns benefits important national and quantity of the gamma-emitting thus can be more difficult to detect. To security efforts, from nuclear forensics nuclides in the source. According to create these elusive particles in greater investigations of postexplosion debris to nuclear reactor safety. Gharibyan and his colleagues are refining techniques for measuring the products emitted during proton- and neutron-induced fission of several isotopes, including uranium-238. Researchers have extensively studied neutron-induced fission of uranium-238 because of its importance 10 to nuclear power, but the proton-induced reactions are not as well understood. To examine these events, researchers stack uranium, aluminum, and yttrium foils at the end of an accelerator chamber at CAMS and bombard the target with protons accelerated to energies between A team of nuclear chemists at Livermore is Cross section, millibarnsCross 1 refining techniques for precisely determining the unique patterns of isobars—nuclides of different elements with the same mass number—produced by the fissioning of various parent isotopes at different energies. This graph shows the likelihood 14.5 ± 0.4 MeV 12.8 ± 0.3 MeV of various isobars being produced during fission 11.9 ± 0.4 MeV of uranium-238 when the proton inducing the 10.6 ± 0.4 MeV reaction is moving at different speeds. Colored symbols indicate proton energy measured in megaelectronvolts (MeV); 1 millibarn is equivalent to 10–27 square centimeters. 10–1 80 100 120 140 160 Mass 6 Lawrence Livermore National Laboratory S&TR March 2014 Advances in Radiochemistry numbers, the Livermore team increased Nuclear forensics readiness exercises use samples of synthetic the irradiation period to a full day Actinides and the nuclear counting period to nuclear debris to test participants’ two months. After days or weeks, the skills at isotope identification. interfering signatures produced by many The current method for preparing of the shorter-lived fission products samples is to irradiate a single Fission products are eliminated, so that longer-lived and fuel in a reactor and collect the lower-yield products are somewhat easier Matrix material fuel and its fission products. A truly to distinguish. realistic sample would include all the pieces of the puzzle—fuels Chemically isolating and removing Activation selected fission products from some products plus fission and activation products samples before nuclear counting also mixed in the right proportions helps the researchers fill in data gaps. and embedded in a matrix of dirt “Radiochemistry allows us to detect and debris. emissions produced in small quantities or those that have interferences,” says Gharibyan. In one uranium-238 experiment, the researchers demonstrated that, by chemically separating samples, are prepared at one facility, typically in 2012 and two round-robin exercises they could measure fission yields for by irradiating uranium in a reactor and with Los Alamos, Pacific Northwest, and about 50 percent more isobars—nuclides splitting it into chunks. The samples are the United Kingdom’s Atomic Weapons of different elements with the same then sent to the participating laboratories Establishment in 2013. mass number—than they could identify for analysis. Nearby facilities such as CAMS and the by nuclear detection alone. In future About three years ago, Livermore cyclotron at Davis (about 135 kilometers experiments, the team will measure nuclear chemists, led by Kevin Roberts, or 85 miles north of Livermore) are fission product distributions for other began designing more realistic debris to preferred for sample preparation because actinide isotopes. further challenge their fellow chemists many of the fission and activation and modelers. A fully realistic sample products are short-lived. Samples must Mixing a Radionuclide Cocktail would include a carefully curated selection be created and shipped promptly so that If an illicit nuclear explosive were of fuels, fission products, and activation exercise participants can measure the more detonated on U.S. soil, nuclear forensics products embedded in a matrix of dirt ephemeral products. In fact, logistics and experts would be tasked with examining