Working around helium shortage

95 percent,” said Kevin Pritchard, one of the NIST engineers collaborating on the project and a Ph.D. candidate at the University of Maryland, Baltimore County. “The lithium-6 fluoride approach is down around 30 percent, which isn’t nearly good enough for our purposes. We spent several years trying to optimize it.”

The alternative approach demands that small crystals of zinc sulfide and lithium-6 fluoride be made at just the right sizes and mixed together with even distribution and spacing in a single medium. When a neutron hits lithium-6, it breaks into two smaller particles that strike some nearby zinc sulfide, making it glow. The team repeated past experiments indicating that with crystals of the correct size, the efficiency should rise to nearly 100 percent, but even with properly sized crystals it wasn’t lighting up brightly enough.

Figuring out why took the help of NIST’s Center for Nanoscale Science and Technology. Electron microscope images revealed that the crystalline particles were clumping together in ways that prevented the lithium-6 fragments from reaching the zinc sulfide. The team worked with industry to tweak the concentration of the binding agent holding the crystal mix together, and this change finally gave them an efficiency greater than 90 percent, rivaling helium-3’s.

The team details their work in a series of five scientific papers, some of which are in press. An overview of the approach is presented in a paper published today in Nuclear Instruments and Methods in Physics Research A.

Aside from freeing the neutron detection community from its dependence on hydrogen bombs, the method has other potential advantages. Most importantly, detectors can be made much thinner, as a gas requires more volume than a solid and needs to be kept at high pressure as well.

“This might allow us to fit more detectors into a space, so we can collect more neutrons and get a more accurate picture,” Pritchard said. “It would be an advantage even if we had plenty of helium-3.”

It might help with fossil fuel exploration as well, he said, as mining companies have used neutron detectors for years. Smaller, cheaper detectors could provide answers about rock structures deep underground more effectively.

For Pritchard and his co-authors, the advantages for neutron science will have the most personal impact.

“It’s hard to collect data efficiently when the intensity of your neutron beam is limited,” he said. “I think this is an incremental but substantive step in improving instruments for studying condensed matter.”

— Read more in A. Osovizky et al., “Design of an ultrathin cold neutron detector,” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 893 (11 June 2018)