Neutron-Insensitive Proportional Counter for Gamma-Ray Dosimetry

A gamma-ray dosimeter with low sensitivity to neutrons has been developed for radiation dosimetry in mixed fields of neutrons and gamma rays. Neutron sensitivity in a 2.5- to 3-Mev H2(d,n)He3 neutron field has been shown experimentally to be ≤1.2% on the basis of first collision dose in tissue. Gamma-ray sensitivity is independent of energy to within ±5% from 1.25 Mev (Co60) to 200 kev and to within ±20% down to 47 kev. The instrument is a proportional counter which may be used as a dosimeter for gamma rays in the presence of neutrons by pulse-height integration of the small pulses due to secondary electrons produced by gamma rays and rejection of the large pulses due to heavy particle recoils from neutrons.