Design Considerations for a Semiiconductor Sandwich Fast Neutron Spectrometer

Semiconductor sandwich fast neutron spectrometers, using the Li6 or He3 reactions, are considered for use in measuring reactor type neutron spectra. Extensive digital computer calculations were performed in order to analyse the performance of these spectrometers. Areas treated were: 1. Response Functions - Characteristic resolution response functions were calculated for various layer thicknesses of Li6F and gold at a number of different energies. Results compare favorably with existing experimental data; 2. Detection Efficiency - The relative detection efficiency, for detection of the reaction products in coincidence, was calculated for both Li6 and He3 spectrometers as a function of energy, detector orientation angle and low level discriminator setting. The net relative detection efficiency, for any incident angular neutron distribution, can be obtained by weighted integration of these curves. The detection efficiency effect was found to be significant, especially for the He3 spectrometer; 3. Silicon Background Response - The true coincidence background response, due to reactions occurring in the silicon of the diodes, was calculated for the cases of incident fission and incident Pu-α-Be spectra. The background was found to be primarily due to the ground state (n,p) and (n,α) reactions. The results are in good agreement with available experimental data. A circuit arrangement, using fast (<30 nsec) electronics to minimize gamma ray pileup effects, low level discriminators to eliminate thermal neutron response if desired, and fast gates to minimize pulse pileup in the microsecond part of the electronic chain (multichannel analyser), has been designed and built for use with Li6 spectrometers.