GAMBE: multipurpose sandwich detector for neutrons and photons

Detectors made with semiconductors such as silicon can be efficiently used for detecting and imaging neutrons when coated with suitable materials. They detect the charged reaction products resulting from the interaction of thermal neutrons with materials with high capture cross section like B-10, Li-6, and (LiF)-Li-6. This work describes the performance of a thermal neutron detector system, GAMBE, which is based on silicon sensors and a layer of neutron-sensitive material, such as a lithium fluoride film or a lithium-6 foil, in a sandwich configuration. This arrangement has a total detection efficiency of 4 +/- 2 %, 7 +/- 1 %, and 12 +/- 1 % for 7 mu m (LiF)-Li-6 film, 40 mu m and 70 mu m Li-6 foil respectively. Also, it enhances the rejection of fake hits using a simple coincidence method. The coincidence that defines a true neutron hit is the simultaneous signal recorded by the two sensors facing the conversion layer (or foil). These coincidences provide a very good method for rejecting the spurious hits coming from gamma-rays, which are usually present in the neutron field under measurement. The GAMBE system yields a rejection factor at the level of 10(8) allowing very pure neutron detection in high gamma background conditions. However, the price to pay is a reduction of the detection efficiency of 1 +/- 1 % or 0.9 +/- 0.3 % for 7 mu m (LiF)-Li-6 film and 40 mu m Li-6 foil respectively.