Lawrence Livermore Laboratory team achieves breakthrough detecting nuclear materials

A team of LLNL researchers has developed the first plastic material capable of efficiently distinguishing neutrons from gamma rays, something not thought possible for the past five decades or so.

As a result, the new technology could assist in detecting nuclear substances such as plutonium and uranium that might be used in improvised nuclear devices by terrorists and could help in detecting neutrons in major scientific projects.

With the material’s low cost, huge plastic sheets could be formed easily into dramatically larger surface areas than other neutron detectors currently used and could aid in the protection of ports, stadiums and other large facilities.

Natalia Zaitseva, an LLNL materials scientist, leads a team of Livermore researchers that has developed the first plastic material capable of efficiently distinguishing neutrons from gamma rays, something not thought possible for the past five decades or so. Photo by Jacqueline McBride

For years, plastic materials have been used in large, low-cost detectors for portals and high-energy physics facilities, and while they could detect neutrons and gamma rays, they have been incapable of distinguishing one from the other, which is key to identifying nuclear substances such as uranium and plutonium from benign radioactive sources.

“However, by studying mixed crystals and mixed liquids, we found that to achieve neutron discrimination from gamma rays, we had to increase the dye concentration in the plastics by at least ten-fold greater than would typically be used,” Zaitseva said.

In their paper, the team wrote: “Efficient pulse shape discrimination (PSD) (between neutrons and gamma rays) combined with easy fabrication and advantages in deployment of plastics over liquids may lead to widespread use of new PSD materials as large-volume and low-cost neutron detectors.” Zaitseva’s colleague, fellow LLNL materials scientist Steve Payne, noted that in some ways it is a particularly good time to develop a new method for detecting neutrons, given the advantages and drawbacks of current methods.

Organic crystals serve as one of the best neutron detectors, but the crystals can be difficult to grow and obtain in large volumes. Liquid scintillators present some hazards that hinder their use. Gas detectors that rely on helium-3, a byproduct of tritium’s radioactive decay, have run into problems because the United States now produces markedly less tritium.

Plastics have more flexibility in their composition and structure than crystals, as well as having none of the hazards associated with liquid scintillators.

“On balance, the plastic scintillators may turn out to be best for detecting neutrons once the factors of usage in the field, cost, and performance are taken into consideration,” Payne said.

In their work, Livermore scientists demonstrated a plastic scintillator that can discriminate between neutrons and gamma rays with a polyvinyltoluene (PVT) polymer matrix loaded with a scintillating dye, 2,5-diphenyloxazole (PPO).

They have found that plastic scintillators have a roughly 20 percent finer resolution for neutron-gamma ray discrimination than liquid scintillators. Crystals, in turn, are about 20 percent finer in resolution than plastics in their analysis.

“We do not see plastic scintillators as competitors with crystals because they serve different purposes. In another part of the program we are trying to grow crystals like stilbene in new ways,” Zaitseva said. Stilbene is the only crystal used for neutron detection and is expensive and difficult to obtain.

“We see our work as being at the beginning. We’re excited about where our research is heading. We would like to study and see whether the plastic scintillators can achieve results at the same level as the best crystals,” Zaitseva added.

The thought that plastic scintillators might be made with efficient neutron-gamma ray discrimination came about, in part, from mixing a scintillating chemical — diphenylacetylene or DPAC — with a stilbene crystal.

“As we mixed DPAC with stilbene at 5 percent, 10 percent and 15 percent, there was nothing,” Payne recalls. “Suddenly at 18 percent, we were able to distinguish neutrons from gamma rays. Once we hit 40 percent, we had the full function.

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