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. 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.
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.
A plastic material was developed that is capable of efficiently distinguishing neutrons from gamma rays. The 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 plastic scintillator can discriminate between neutrons and gamma rays with a polyvinyltoluene (PVT) polymer matrix loaded with a scintillating dye, 2,5-diphenyloxazole (PPO). 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.
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 DPAC was mixed with stilbene at 5 percent, 10 percent, and 15 percent, nothing happened; at 18 percent, neutrons became distinguishable from gamma rays. Full function occurred at 40 percent.