Neutron-Insensitive Gamma-Ray Dosimeter

Abstract

A gamma-ray dosimeter for mixed radiation dosimetry with a neutron sensitivity of ≤ 2 per cent (on the basis of absorbed dose in tissue) has been developed. This instrument consists of a graphite-wall, helium-CO2-filled proportional counter operated at pressures of the order of 5–40 cm. Hg. Large pulses due to heavy particle recoils produced by neutrons, such as carbon recoils from the walls and helium recoils in the gas, are discarded. Small pulses due to secondary electrons produced by gamma rays are recorded and are weighted according to height (i.e., according to the amount of current represented by each pulse) (1). The counter acts as a Bragg-Gray cavity, the “current” serving as a measure of the gamma-ray dose. Gamma-ray sensitivity is independent of energy to within ±20 per cent from 1.25 Mev (Co60) to 47 kev. Some indication of the incident gamma-ray spectrum may be obtained from the gamma-ray pulse height distribution. The counter container (outside the graphite lining) is made of thin aluminum to minimize production of gamma rays in the walls by inelastic scattering of the incident neutrons. Detection of these gamma rays by the counter represents neutron sensitivity, since the initiating particles are neutrons. Tests were made with 2.5 Mev H2(d,n)He3 neutrons. The counter indicated an absorbed dose which was 2 per cent of the measured fast-neutron dose. This value is the sum of two contributions: (a) the gamma-ray contamination of the neutron radiation (the true gamma-ray dose) and (b) the neutron sensitivity of the dosimeter. Since we do not know the gamma-ray contamination of the neutron field, we obtain only an upper limit on the neutron sensitivity of the counter. Efforts are being made to decrease the gamma-ray contamination of the neutrons by redesigning the target assembly to minimize gamma-ray production by inelastic scattering. This instrument operates at low dose rates (≤200 mrad/hr.), below the useful range of the gamma-ray dosimeter of Sigoloff (2), which has been estimated to have comparably low neutron sensitivity. The instrument could be adapted for survey meter use by including a “single-window” pulse-height analyzer to accept pulses in a fixed range of pulse heights. This instrument is being used at the National Bureau of Standards in connection with an experiment to measure the neutron sensitivity of photographic film.