A team of Lawrence Livermore National Laboratory researchers has used computer models to develop 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.
The LLNL team's studies, detailed in an article appearing in the journal Nuclear Instruments and Methods in Physics Research Section A, was published online in late December.
"It has been established opinion since the 1950s that organic crystals and liquid scintillators can work for detecting neutrons, but that plastics are not suitable for neutron detection," said LLNL materials scientist Natalia Zaitseva, the paper's lead author. Scintillators are special materials that light up when excited by ionizing radiation.
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. And without that capability, they could not be used to detect nuclear substances such as uranium and plutonium.
"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.
Currently, 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 in the United States because the country produces markedly less tritium than in the past.
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).
"In order to make progress on the scintillating plastic, we needed to understand the migration of energy in the material," said Livermore computer simulation scientist Sebastien Hamel. "In particular, we needed to understand the rate at which energy moves around and – it turned out – more importantly the processes that governed its motion."
The team ran computer simulations to gain that understanding, using the Hera cluster. Hera boasts 13,824 2.3 GHz CPUs.
"Since we were comparing the impact of using various dye molecules as dopants in the plastic, I had to perform several such simulations," Hamel said. Hamel estimates that each simulation used 64,000 CPU-hours; according to Hamel, the simulation was performed for "tens of compounds."
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 five percent, 10 percent, and 15 percent, there was nothing," Payne recalled. "Suddenly at 18 percent, we were able to distinguish neutrons from gamma rays. Once we hit 40 percent, we had the full function.
"It was a painful process and it took several months. Natalia was the one who made the connection between the stilbene/DPAC mixtures and saw a hypothetical route for the use of plastics. This insight was important, and it was what we needed to make the breakthrough."
Along with discovering plastic scintillators with efficient neutron-gamma ray discrimination, the team has also found that this discrimination can be very sensitive to certain impurities in crystals.
"We had cases where we tested a crystal and it had pulse shape discrimination (PSD) between neutrons and gamma rays, and then we tested the same type of crystal and it had no PSD," Zaitseva said.
"When we started this work, there was little understanding of how PSD was affected by the chemical composition of the scintillating materials. We have found some of the major principles of molecular interaction that determine the presence or absence of PSD properties in organic scintillators," Zaitseva said.
Zaitseva, who joined LLNL in 1993 after pioneering work on rapid crystal growth at Moscow State University, noted that her first work at Livermore was to produce large-scale potassium dihydrogen phosphate (KDP) crystals of high optical quality for the National Ignition Facility laser.
"Now we have applied these techniques for growing pure organic crystals for neutron detection. By studying these crystals, we are starting to understand new physical phenomena that can be applied to discover new plastic scintillators," she said.
The team's research to develop plastic scintillators has been funded by the
US National Nuclear Security Administration's Office of Nonproliferation and Verification Research and Development, which recognized the importance of these materials while they were in an early, formative stage.
In addition to Zaitseva, Payne, and Hamel, other members of the LLNL multi-disciplinary team that achieved the advance are: materials scientists Nerine Cherepy and Leslie Carman, synthetic chemists Benjamin Rupert and Paul Martinez, nuclear scientists Andrew Glenn and Iwona Pawelczak, University of California, Davis graduate student Michelle Faust, and mechanical technician Keith Lewis.
"Altogether, our technically diverse team has the capability and knowledge needed to come up with this breakthrough plastic scintillator," Zaitseva noted. "Everyone on the team added something necessary for our success."
One of the next steps for the team will be to find the right commercial partners. As Payne noted, "We're very good at inventing technologies, but we need commercial partners to bring our innovations to market." Currently, active negotiations to license the technology are under way with two companies, one of which is already engaged in process development.
A version of this article first appeared on the LLNL website.