Safe and effective high explosives are critical to Lawrence Livermore National Laboratory's (LLNL) mission of stockpile stewardship. It is relatively simple to study the composition of such material before a detonation or examine the soot-like remnants afterward. But the chemistry in between, which dictates much of the detonation process, evades experimental interrogation as it passes by in a few nanoseconds or less.
In a study published in Proceedings of the National Academy of Sciences, researchers from SLAC National Accelerator Laboratory and LLNL triggered a slow decomposition of a high explosive and measured the effects on the molecules within it. The work provides the proof of concept for a process that could be extended to examine ultra-fast dynamic chemistry during detonations and illuminates intermediate structures that have never been experimentally seen before.
At the Stanford Synchrotron Radiation Lightsource, the team used X-rays to both trigger the chemical reactions involved in decomposition and measure the results.
"X-ray Raman scattering is a unique capability that can provide access to the bulk chemistry of these materials," said SLAC scientist and lead author Oscar Paredes Mellone.
The technique excites electrons in the cores of atoms by pelting them with X-ray photons that scatter and transfer some of their energy to those core electrons. A detector measures the new energy of the X-rays, and the difference reveals the electronic structure and composition of the material.
"We're not going to claim that X-ray-induced decomposition is identical to detonation, but we believe it may be fairly similar," said LLNL scientist and author Trevor Willey. "You're just exciting and breaking up the molecules with X-rays instead of with the shock of the detonation."
But only under cryogenic temperatures does that process freeze in place long enough to capture and measure transient chemical intermediates.
"By doing these much more slow, static, cryogenic experiments, we show that it's feasible to potentially use X-ray Raman spectroscopy in a detonation experiment," said Willey.
In addition to their experiments, Paredes Mellone searched the literature for molecular fragments theoretically proposed to be present during the initial stages of the detonation process. For plausible candidates, he worked with colleagues at the National Institute of Standards and Technology to calculate their molecular X-ray Raman signatures and compared the models to spectral data. The results showed a few most likely candidates and key decomposition mechanisms: certain bonds cleaving apart and leading to the opening of the internal, cage-like structure of the explosive molecule.
While this decomposition study provides the basis for similarly structured detonation experiments, certain advancements will be necessary.
"In order to take this dynamic, you would need an incredibly intense and pulsed X-ray source - an X-ray free-electron laser - like the Linac Coherent Lightsource at SLAC," said Willey. "We still need to do a fair amount of work to optimize detectors and improve their efficiency as well."
To this end, the community of scientists at SLAC, LLNL and elsewhere are exploring ways to incorporate X-ray Raman spectroscopy more routinely at the Linac Coherent Lightsource.
