Towards high efficiency solid-state thermal and fast neutron detectors

Variety of applications of fast neutron detection utilize thermal neutron detectors and moderators. Examples include homeland security applications such as portal monitors and nuclear safeguards which employ passive systems for detection of fissile materials. These applications mostly rely on gas filled detectors such as 3He, BF3 or plastic scintillators and require high voltage for operation. Recently there was considerable progress in the development of solid-state neutron detectors. These operate by detection of charged particles emitted from neutron interactions with a converter material. In order to increase neutron detection efficiency to a usable level, the thickness of the converter material must exceed the range of the charged particles in the converter, which limits the efficiency of planar detectors to several percent. To overcome this limitation three dimensional structured solid-state devices are considered where the converter can be thicker but still allow the charged particles to escape into the semiconductor. In the research described here this was accomplished by a semiconductor device that resembles a honeycomb with hexagonal holes and thin silicon walls filled with the converter material. Such design can theoretically achieve about 45% thermal neutron detection efficiency, experimentally about 21% was observed with a partially filled detector. Such detectors can be fabricated in variety of sizes enabling designs of directional fast neutron detectors. Other converter materials that allow direct detection of fast neutrons were also considered by both simulation and experiments. Because the semiconductor thickness is less than a few hundred microns, the efficiency of these detectors to γ-ray(s) is very low. With further developments these new solid-state neutron detectors can replace gas ionization based detectors in most applications.