Monitor for 4 to 8 MeV neutrons suitable for (n, p) and (n, α) experiments

Abstract

Abstract A monitor counter for fast neutrons is described based on recoil protons emitted from a thin 60 μm polyethylene radiator within a small solid angle and detected by a silicon surface barrier detector. It is capable to select the incident neutrons by their energy as well as by their direction. The absolute neutron flux is measured with a precision of 4% and its relative fluctuation over several days is better than 1%. The electronic equipment required is simple because of its quite unsensitive threshold setting The relationship4) D =const.(Vρ)1/2 between the depth D of the depletion layer of a silicon surface barrier detector and the applied bias V was checked by counting the alphas of the 29Si(n,α0)26Mg reaction produced by a fixed number of 6 MeV incident neutrons within the depletion layer of the silicon diode for a set of eight values of D; this relationship was found to be confirmed. Also agreement was found between the measured cross section of the29Si(n,α0)26Mg by Anderson1) and our value σ = 30 ± 4 mb for En = 6 MeV. As an appendix the energy loss ΔEx has been computed for a charged particle x of incident energy Ex passing through the depletion layer of a silicon surface barrier detector having a thickness D (in μm). The computations were done for protons, deuterons, tritons,3He and alphas for a set of 15 different thicknesses D in the interval between 50 and 1000 μm and for 20 values of the energy of the incident particle x changing from 2 to 20 MeV.