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: Jacqueline McBride |
When a young man was
advised to pursue a career in plastics in the 1967 movie, “The
Graduate,” people could not have envisioned one of the material’s uses
developed by Lawrence Livermore National Laboratory (LLNL) scientists.
In a key discovery, 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.
Their studies, detailed
in an article appearing in Nuclear Instruments and Methods in Physics
Research A, was published online.
“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 Natalia Zaitseva, the paper’s lead
author and an LLNL materials scientist. 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, 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% finer resolution for neutron-gamma ray
discrimination than liquid scintillators. Crystals,
in turn, are about 20% 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%, 10%, and 15%, there was nothing,” Payne recalls.
“Suddenly at 18%, we were able to distinguish neutrons from gamma rays.
Once we hit 40%, 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
also has 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 National Nuclear Security
Administration’s (NNSA) Office of Nonproliferation and Verification Research
and Development (NA-22), which recognized the importance of these materials
while they were in an early, formative stage.
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.
