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Radiochemical diagnostics have been used since the 1945 Trinity test to understand the nature of the reactions involved in nuclear explosions. Following the Atmospheric Test Ban Treaty in the early 1960s, the United States conducted more than 800 underground tests (UGTs) of nuclear weapons. Analysis of the debris produced by UGTs allowed scientists to continue to improve their understanding of nuclear weapon performance and refine their physics models. After the enactment of the 1996 Comprehensive Nuclear Test Ban Treaty that eliminated UGTs, collecting and studying detonation debris became a more substantial challenge as radiochemical diagnostic capabilities at the time were limited to analysis of fresh radioactive material.
Radiochemical data are obtained from the post-detonation analysis of debris samples that contain residual bomb materials. As a radioactive material decays, the atoms emit ionizing radiation until the material’s radioactivity has greatly subsided. Much of the radioactivity in debris from UGTs has since decayed and with it the possibility of acquiring certain types of information from bygone tests. “Gathering data from old debris using traditional methods that were developed during the testing program has become increasingly difficult, which is a consequence of radioactive decay,” says Timothy Rose, who led the Lawrence Livermore effort to characterize previously radioactive test debris until his retirement in 2024. Hugh Selby, one of the scientists leading the research collaboration from Los Alamos National Laboratory and leader of Los Alamos’s radiochemistry assessment team, adds, “Performance information was thought to be lost, and samples were not considered to be particularly useful.”
Among radiochemistry experts, many were concerned that the end of UGTs and the extinction of remaining radioactive products meant that the field of radiochemistry might become obsolete, as previous analytical methods could no longer obtain new information. “I had thought if we’re not generating new materials, all of our existing samples and the work we’ve been doing is going to disappear,” says Selby. Lawrence Livermore and Los Alamos national laboratories have combined expertise and resources in recent years to develop new methods of finding information from materials thought to be too old to use.
Reviving the Extinct
Not only has the field of radiochemistry continued into the present, but the team at Los Alamos has developed a new technique that allows radiochemists to study aged, formerly radioactive debris even after it is radioactively extinct. Rather than tracking radioactive emissions, the new technique uses mass spectrometry, which measures the mass-to-charge ratio of different isotopes in a sample.
Atoms of a given element with different numbers of neutrons are known as isotopes of that element. Different isotopes of an element each have distinct masses, while their charge remains consistent, which changes their mass-to-charge ratio measured by mass spectrometry. In nature, isotopes come in specific ratios, and scientists understand these natural isotope ratios for all natural elements, so unnatural relative abundances of an element’s different isotopes carry information about the strength of a detonation. By monitoring how much the ratio differs, mass spectrometry can accurately determine the number of atoms in a sample that were born from the now “extinct” fission product.
Fission products are formed when neutron bombardments from detonations split heavy elements such as uranium. The fission products used for radiochemical diagnostics are short-lived and are characterized using traditional radiometric methods at the time of the test. Over time, the fission product decays into a naturally occurring stable isotope, perturbing—or changing—the abundance of that isotope compared to its natural isotope abundance. The Los Alamos team’s method relies on comparing the isotope ratios in test debris to the natural ratios of the same element. Working backward from this comparison, the team then calculates how many isotopes are products of fission to better understand the parameters of the test from which they originated.
Detection limits for mass spectrometry are very low. Resolution has improved significantly over the past decade such that mass spectrometry can detect previously undetectable fine-scale changes, serving as a feasible method for studying isotope abundances. “We can now take something that is 30 to 50 years old and regenerate prior results and collect new information from it,” says scientist William Kerlin, who leads Livermore on the project following Rose’s retirement.
For example, molybdenum, which has many natural, stable isotopes, is the stable end product following radioactive decay of several fission mass chains. As fission products decay, stable zirconium absorbs some of the decay products, acting as a shield for certain molybdenum isotopes and changing the abundance of the different isotopes’ presence relative to their natural ratio. Molybdenum-96, which has 54 neutrons, is shielded by zirconium-96, while molybdenum-97, with 55 neutrons, can include a fissiogenic component, or a product of fission. When mass spectrometry is used to analyze the sample, the ratio of molybdenum-97 to molybdenum-96 is higher than expected due to the increase of fissiogenic molybdenum-97 while molybdenum-96 remains shielded and unchanged. “For natural samples, the variations are small, but when we add in fission, we have distinct isotopic compositions that are very different from natural materials,” says John Rolison, who leads project analysis efforts at Livermore.
Making Data Useful
Conducting radiochemistry analysis using mass spectrometry measurements required additional data inputs—specifically, researchers needed to determine the number of atoms in the sample that came from fissions. To determine the number of atoms to which the offset isotope ratio corresponds, the Los Alamos and Livermore researchers first identified how many total atoms were in a sample by adding a known amount of the shielded isotope to the sample to dilute the fissiogenic isotope of interest. Next, they used mass spectrometry to measure changes to the isotope ratio. From these measurements, the team determined the number of atoms in their original sample, as well as the number of fissiogenic atoms. “Even in the absence of knowing about the initial conditions of the test, we could compare the relationship between the fission products’ abundance to what we expected from nuclear data and extract information about the main fissioning isotope,” explains Rolison.
The data in this technique is imperfect as the inclusion of some natural background is inevitable. “Debris is fractionated, meaning there’s always separation of materials. Nothing is perfectly mixed,” says Rose. “The more we can interrogate different parts of the debris field, the better picture we can develop.” To address the natural background, the researchers dissolved the debris samples in acid to remove the unnecessary materials and homogenize the samples being studied, increasing the quality of the overall data by reducing the background noise and distortion. The sample itself can be a source of fractionation—for example, larger samples would lead to more homogeneous data—but the researchers are limited to the debris obtained from test sites decades ago.
Nonetheless, Livermore and Los Alamos’s mass spectrometry technique enables improvements in data otherwise impossible to obtain and expands scientists’ understanding of UGT performance by increasing the amount of data available. “Demonstrating that we can get new measurements at the same levels of precision as we used to do through different methodologies was a real win,” says Rose.
A Collective Effort
While the samples Livermore and Los Alamos used came from archival debris obtained from UGTs, Selby’s team at Los Alamos first used debris from the Trinity test to develop the technique. “We have an archive of samples from many tests—some of which are undersampled or underanalyzed—and we’re starting to show that we can apply these novel techniques to study these unique materials that exist nowhere else in the world except for at Lawrence Livermore and Los Alamos,” says Rolison. Both laboratories’ last UGTs were in 1992, just before the Comprehensive Nuclear Test Ban Treaty, but the samples the teams studied are much older.
The two laboratories worked closely together to calibrate and corroborate the data. Each laboratory selected four samples to share with the other and provided the partner institution with solid debris and dissolved solution to test for each sample. They conducted the same high-precision isotope ratio and concentration measurements, and combined them for radioanalysis, ensuring the fission product calculations were consistent between the two groups.
The collaborative research effort required a combination of unique skillsets, ranging from radiochemistry to geology to mass spectrometry. “It took a village to show that this was a dependable approach to getting new data without any standard reference material for comparison. We needed the two laboratories to work together to identify where biases in the data are,” says Rose. Selby adds, “This is a very large group of people—some of the best chemists in the country and the world. A whole bunch of folks on these teams are critical for doing this work for national security purposes.”
The Future of Past Tests
Rose says the ability to drill for additional debris, rather than relying on samples collected in the past, can lead to better sampling for the intended studies and thus, better data. In the absence of UGTs, the initial exploration of this research was mostly a scientific inquiry, but its applications are vast. “Our motivations are now quite different. We built all these wonderful tools such as advanced scientific computing, the National Ignition Facility, and other state-of-the-art scientific platforms that allow us to ask questions about our test history that are unresolved,” says Rose. The new measurement capability is one more tool that can now be used to inform questions from test history.
Livermore and Los Alamos plan to continue their partnership and focus on applying basic science method development to programmatic missions. Such tools can be used to understand nuclear detonations by measuring their radioisotope products and to answer questions informing nuclear forensics to stockpile stewardship. Says Rose, “We will make sure we can start using this directly for program benefit. We have some basic tools that are developed and calibrated, and now the next step is to demonstrate them for program questions.”
—Anashe Bandari
For further information contact William Kerlin (925) 424-3105 (kerlin3 [at] llnl.gov (kerlin3[at]llnl[dot]gov)).
