Thin Film Polymer Composite Scintillators for Thermal Neutron Detection

Thin film polystyrene composite scintillators containing and organic fluors have been fabricated and tested as thermal neutron detectors. Varying fluorescence emission intensities for different compositions are interpreted in terms of the Beer-Lambert law and indicate that the sensitivity of fluorescent sensors can be improved by incorporating transparent particles with refractive index different than that of the polymer matrix. Compositions and thicknesses were varied to optimize the fluorescence and thermal neutron response and to reduce gamma-ray sensitivity. Neutron detection efficiency and neutron/gamma-ray discrimination are reported herein as functions of composition and thickness. Gamma-ray sensitivity is affected largely by changing thickness and unaffected by the amount of in the film. The best neutron/gamma-ray discrimination characteristics are obtained for film thicknesses in the range 25–150？μm. 1. Introduction The development of efficient thermal neutron detectors is relevant to the fields of nuclear physics, nuclear power generation, imaging, and homeland security. Helium-3 filled proportional counters are widely used in radiation portal monitors to detect illicit transport of fissile materials, in neutron scattering experiments, in medical imaging, and in well logging. Due to the recent expanded use and reduced production of He-3, the supply of He-3 is dwindling such that the Department of Homeland Security has issued research funds to develop a replacement technology [1]. As described herein, efforts have been made to develop inexpensive and atmospherically stable polymeric composite materials to function as thermal neutron scintillation detectors that have low sensitivity to gamma-rays. The use of organic polymers as scintillators has many advantages over other scintillating materials such as single crystals and inorganic glasses in that selected polymers are air-stable, do not require high processing temperatures, have relatively low cost, are easy to fabricate in large areas in a wide range of geometries, and have fast response times [2]. Common commercially available polymeric scintillators are based on aryl vinyl polymers such as polystyrene (PS) and polyvinyltoluene (PVT). The aromatic pendant groups on these polymers have emission in the wavelength range 300–350？nm under both UV- and X-ray induced excitation [3, 4], so these polymers must be doped with wavelength shifters to shift the wavelength of scintillation to the region of sensitivity of common bialkali photomultiplier tubes (PMTs) (390–450？nm). In order for organic