An Improved Method of Neutron Spectroscopy Using Threshold Detectors

Abstract

An improved technique of measuring neutron-energy spectra in the range 2. 5 to 30 MeV has been developed. In this technique threshold detectors are exposed to the unknown neutron flux. The ac-tivities of residual nuclei from (n, p), (n, a), and (n, 2n) threshold re-actions are measured by gamma-spectrum analysis. Experimental excitation functions are used. Measured activities and cross sections form a set of integral equations which are solved for the neutron spectrum by using a least-squares technique, Neutron-reaction cross sections are not well known above about 20 MeV, and 30 MeV is selected as the highest energy for which this method is feasible based on available data. An approximate correction is made to measured activities to account for activation caused by neutrons with energies above 30 MeV. The previous technique, on which this work is based, was im-proved by: (a) substitution of experimental reaction cross sections (from re-cently published literature) for those calculated from the continuum model of the nucleus; (b) solution of the spectrum from the set of integral equations by a least-squares technique; ' (c) calculation of the amount of activation of threshold detectors caused by neutrons with energies above 30 MeV within a thick concrete shield exposed to a 6.2-BeV proton beam. Application of this improved technique is demonstrated in ex-periments with a Be 9 (d, n)B 10 neutron source, and the neutron spec-train a thick concrete shield exposed to a 6.2-BeV proton beam.