Radiochemistry experts at Lawrence Livermore National Laboratory (LLNL) recently made the first experimental measurements of nuclear reactions in high-energy-density plasma environments, which are similar to conditions found in stars, as well as thermonuclear explosions.
According to John Despotopulos, an LLNL radiochemist who leads the research team, the ability to collect this experimental data in very hot, dense, star-like plasma will help researchers validate and improve existing models of nuclear reactions that are relevant to astrophysics research, as well as our nation's stockpile modernization efforts.
One of the key factors that enabled Despotopulos and his research team to make these first-ever experimental measurements was access to the world's highest-energy laser system, located at LLNL's National Ignition Facility (NIF). At NIF, researchers can create plasma conditions that are similar to the temperatures and pressures that exist in stars.
In addition, NIF experiments can simulate conditions that exist in a detonating nuclear weapon - a research capability that supports modernization and its focus on maintaining the safety, security and reliability of the nation's nuclear deterrent. During underground nuclear tests, which ended in 1992, scientists analyzed nuclear reaction data, including isotopes observed after each test, to assess the weapon's performance. However, this nuclear reaction data was incomplete. Today, NIF is the only place where researchers can perform some of these measurements for the first time, in an environment that mimics conditions found in underground nuclear tests.
Proof-of-concept experiments
The research team's first task was developing techniques to dope NIF target capsules with radioactive isotopes. NIF's target capsules are about the size of a pencil eraser, and they need to be filled through a very small hole, approximately the size of a grain of salt. Due to this size restriction, the chemical cocktail developed by the scientists needed to be extremely pure, to avoid potentially plugging the fill hole with salts.
Despotopulos and his colleague Kelly Kmak, also with LLNL's Nuclear and Chemical Sciences (NACS) Division, developed the radiochemical techniques to purify target material. They used vacuum or micro-injection techniques to add sub-nanograms of the doped material to the inner surface of each capsule, carefully adding the purified mixture to avoid clogging the hole used to fill capsules. They also collaborated with members of the NIF target fabrication team to ensure that the capsules were ready for the experiments, including efforts to build the delicate target assemblies that incorporate the radioactive capsules.
During experiments at NIF, lasers heated the capsules to extremely high temperatures, causing thermonuclear reactions that produced neutrons. Following the NIF shots, the team analyzed the debris using a range of diagnostic tools, including LLNL's Nuclear Counting Facility, which provides high-sensitivity radiation measurements.
"Our initial proof-of-concept experiments demonstrated that our approach can be used to calculate nuclear cross-sections for long-lived radioisotopes in plasma environments," said Despotopulos. "It's exciting to tackle such a challenging problem and be able to capture this type of experimental data for the first time."
Next steps: New measurements
The team's next steps include testing the new nuclear reaction measurement approach with other radionuclides. In addition, they hope to be able to use the new technique to measure a neutron capture reaction, also known as an (n,gamma) reaction - supporting future research related to stellar nucleosynthesis.
The research team is also exploring ways to obtain radionuclides for their future experiments at NIF. One option may involve isotope harvesting at the Facility for Rare Isotope Beams (FRIB), a U.S. Department of Energy user facility located at Michigan State University. FRIB can produce many of the radionuclides required for these experiments in sufficient quantities.
Teamwork drives development of innovative measurement approach
This research builds on past work by other NACS scientists, including physicist Nick Scielzo, who explored techniques to harvest isotopes from FRIB, and division leader Dawn Shaughnessy, who conducted early research regarding radiochemical diagnostics and started studying nuclear reactions at NIF in 2008.
"Since the earliest days of these NIF experiments, our team recommended that the facility should be used to study nuclear reactions," said Shaughnessy. "It's extremely rewarding to see the years of work we invested in this effort culminate in the results we are now obtaining through this research program."
Project leader John Despotopulos initially joined the Lab in 2013 as a Livermore Graduate Scholar, and he served as a postdoctoral researcher at LLNL from 2015 to 2016 before converting to a staff position in 2017. A portion of the funding for his current research focused on measuring nuclear reactions in plasma environments was provided through his 2022 Early Career Research Program award from the Department of Energy's Office of Science.
-Lisa Valdez
