Taking the Temperature of Plutonium

Understanding how plutonium reacts to extreme pressures is a key aspect of the science-based effort to maintain the safety, security, and effectiveness of the nation’s nuclear stockpile without underground testing. And while much is already known about plutonium’s properties in these conditions, scientists have yet to measure its internal energy, or temperature.

In an ambitious five-year project requiring the creation of new x-ray sources, targets, spectrometers, and other diagnostic equipment, Lawrence Livermore National Laboratory (LLNL) researchers and their colleagues have developed an experimental platform designed to determine the internal energy of plutonium under pressures of 10 million Earth atmospheres or more.

The platform was described in an article in the October 2024 issue of Review of Scientific Instruments.

“We want to study materials under extreme conditions,” said LLNL physicist Yuan Ping, who leads the Lab’s Extended X-ray Absorption Fine Structure (EXAFS) campaign. “We have different methods to measure the structure of the material, how it responds to pressure, and the strength it maintains when compressed. But we don’t know the temperature.”

“This is the missing piece” in determining plutonium’s equation of state, added LLNL materials scientist Jim McNaney. “We don’t have a measure for the thermal state, and it is a very important measurement.

“We are trying to generate information that allows us to describe the way plutonium behaves,” he said, “so that when we do modeling of the behavior of a nuclear weapon, we have confidence in the models that we're using at a level of detail that currently isn't available to us.”

EXAFS “allows us to establish a comprehensive set of information in a particular material state,” McNaney added, “and that's really where its power comes in. Because it is able to look at these thermal states, it also can give us information about how these states are changing in time.”

How It Works

EXAFS is a technique for determining a material’s properties by knocking electrons out of their orbits, or “shells,” around the atomic nucleus – a process called ionization – and observing their interactions with nearby atoms.

The negatively charged electrons in an atom are linked to the positively charged protons in the nucleus by their binding energy – the amount of energy needed to remove an electron from its shell. The binding energy is different for every element and depends on the number of protons in the nucleus and how close the electrons’ shells are to the nucleus. The K-shell, or innermost, electrons are the most tightly bound.

When a beam of x rays with energy matching the electron’s binding energy, known as its “absorption edge,” is fired into the material, the x-ray energy is absorbed and the electron is ejected. The resulting photoelectron waves scatter off nearby atoms and are sensitive to the local atomic structure and thermal disorder in the material’s lattice structure. High-resolution crystal spectrometers record tiny intensity variations, or “wiggles,” in the x-ray absorption spectrum, allowing researchers to infer the material’s structure, density, and internal energy.

Starting with Copper

EXAFS researchers use the lasers of LLNL’s National Ignition Facility (NIF), the world’s highest-energy laser system, to compress a sample of material to ultra-high densities and to generate the x rays that probe the sample.

The campaign began by studying copper, with an atomic number, or Z, of 29 (having 29 protons in the nucleus), under pressures of up to about 9.8 million Earth atmospheres. Copper’s K-shell electrons have an absorption edge, or K-edge, of almost nine kiloelectronvolts (keV) and thus require a minimum energy range of 8.9 to 9.8 keV of x-ray energy to probe the absorption structure.

From copper the campaign moved up to higher-Z materials – tantalum (Z=73), requiring about 10 keV of x-ray energy for L-edge experiments, and lead (Z=82) at about 13 keV for L-edge shots. The researchers recognized, however, that to study even higher-Z elements such as uranium, zirconium, and ultimately plutonium (Z=94, the highest atomic number to occur in nature) they would need as much as 18 keV of energy and a new generation of equipment.

“Five years ago we made a plan to go up in energy,” Ping said, “and this gets more and more difficult” because fewer x-ray photons are produced at higher energies, and spectrographic resolution becomes problematic. To increase the x-ray flux, the team developed a titanium foil backlighter that can produce 30 times as many photons in the x-ray spectral range of interest  as implosion capsule backlighters and two to four times as many as a standard silver or gold backlighter.

They also worked closely with collaborators at Princeton Plasma Physics Laboratory (PPPL) to develop a custom absorption x-ray spectrometer called HiRAXS. The spectrometer features a novel spiral crystal design with varying radii to achieve good resolution despite the drop-off in x-ray flux at higher  energies. A second-generation HiRAXS II spectrometer was specifically designed for plutonium experiments.

“We need to collect as many photons as possible,” Ping said, “because at 18 keV we don’t have a lot of x rays. We want a larger crystal, but when it gets really large, you have aberrations which blur your resolution. We need to shape it very precisely so we can demonstrate a good signal-to-noise ratio and maintain good resolution.”

Target Fabrication Issues

Fabricating the specialized targets for the project also posed challenges, said LLNL mechanical engineer Jacob Riddles. “As we work toward the next chapter in the EXAFS campaign,” he said, “our work in target fabrication focuses on tuning the smaller design details to ensure the team can confidently gather high-quality data when they make the switch to high-Z samples. This year we’ve designed a new physics package fixture design to help mitigate smaller VISAR data issues driven by plasma leakage around the physics package.”

The high-Z targets “will stretch the limits of our ability to maintain uniformity requirements,” added target fabrication engineer Anna Murphy, “as well as our limits for how much we can make cleanly during stock material production.”

Testing with Zirconium

Ping noted that EXAFS experiments on plutonium will be limited to studying the L-shell absorption edge. “We can’t use the K-edge of plutonium,” she said, “because it would require an x-ray source above 100 keV. There would be very few photons. We have to work with the L-edge.

“The zirconium K-edge happens to be at the same absorption energy as the plutonium L-edge, so it's a convenient material to test.”

On Feb. 22, 2025, in the first full trial of the HiRAXS II spectrometer, the EXAFS team measured the K-edge of zirconium ramp-compressed to a pressure of five million atmospheres. Ramp compression experiments apply a carefully tailored laser pulse shape that more “softly” compresses a material to prevent a rapid rise in temperature.

“We got these nice wiggles as our signal,” Ping said. “It is very high quality. That’s why it’s getting really exciting as we prepare for the plutonium shots about half a year from now.”

“We are moving through the process of getting EXAFS approved for plutonium use,” McNaney said, “and it will take us probably four or five months to work through that process. We spend a lot of time making sure that we're doing things in as safe a way as we possibly can. That process is one that we've used three other times, and it’s worked really well, but it does take some time.”

EXAFS is “a critical diagnostic that precisely measures the temperature across the spectrum for stockpile programs,” said NIF Director Gordon Brunton. “By optimizing the crystal to the energy range for plutonium, we’ve achieved amazing results.”

Along with the flexible imaging diffraction diagnostic for laser experiments (FIDDLE), Brunton said, EXAFS “is another example of how the advancements of platforms necessary to support fundamental science missions connect directly into enabling new stockpile stewardship capabilities.”

Joining Ping and McNaney on the EXAFS experimental team are LLNL researchers Hong Sio, Andy Krygier, Amy Coleman, Federica Coppari, Jon Eggert, Hye-Sook Park, Bruce Remington, Korbie Le Galloudec, Camelia Stan, Dave Bradley, Warren Hsing, and Andy Mackinnon.

Along with Riddles and Murphy, other LLNL team members are Dave Braun, Sebastien Hamel, Tom Lockard, Elijah Kemp, Stanimir Bonev, Rob Rudd, Randolph Hood, Kazem Alidoost, Christine Wu, Stanislav Stoupin, Daniel Thorn, Marilyn Schneider, Bernie Kozioziemski, Nino Landen, Jacob Corbin, Neil Ose, Nathaniel Thompson, Jay Ayers, Mark May, Justin Buscho, Neal Bhandarkar, Montu Sharma, Pascal Di Nicola, Abbas Nikroo, and Scott Vonhof.

External collaborators include Lan Gao, Ken Hill, Manfred Bitter, Frances Kraus, Novimir Pablant and Phil Efthimion from PPPL and Alex Chin, Matt Signor, Phil Nilsen, Ryan Rygg, and Rip Collins from the University of Rochester.

More Information:

“Measurements of K-edge and L-edge extended x-ray absorption fine structure at the National Ignition Facility,” Review of Scientific Instruments, October 22, 2024

“Probing the temperature of materials under extreme pressure at NIF,” NIF & Photon Science News, December 12, 2023

“Extended X-ray absorption fine structure of dynamically-compressed copper up to 1 terapascal,” Nature Communications, November 10, 2023

“New Target Facility Will Help Unlock Plutonium’s Secrets,” NIF& Photon Science News, May 31, 2022

“Design and expected performance of a variable-radii sinusoidal spiral x-ray spectrometer for the National Ignition Facility, Review of Scientific Instruments, September 3, 2021

“Scientists Collaborate on Novel Instrument for NIF,” NIF & Photon Science News, June 23, 2021

“The multi-optics high-resolution absorption x-ray spectrometer (HiRAXS) for studies of materials under extreme conditions,” Review of Scientific Instruments, May 10, 2021

“A new class of focusing crystal shapes for Bragg spectroscopy of small, point-like, x-ray sources in laser produced plasmas,” Review of Scientific Instruments, April 9, 2021

“Researchers Develop X-Ray Source to Measure Extreme Temperatures at NIF,” NIF & Photon Science News, January 22, 2021