β線吸収線量率測定器の開発 ディスクリミネータ・フィルタ併用法

Abstract

A method and an instrument were developed for measuring directly the β-ray absorbed dose rate almost independent of β-ray energy and corresponding to the epidermal thickness of tissue. A discriminator, of which discrimination level was set according to the epidermal thickness of tissue, and a filter, controlling the number of incident β-rays, were used for obtaining a constant dose sensitivity (counting rate per absorbed dose rate). The filter was composed of two semicircular plates of acrylic resin, whose thickness was 25 and 1, 000mg/cmcm2, respectively. The shape of filter is a disk. The dose rate was obtained by subtracting the counting rate measured with the filter from that measured without the filter and by multiplying this remainder by a constant factor (reciprocal of the constant dose sensitivity).The dose sensitivity of instrument was constant at 40cps/(mrad/hr)±15% for β-rays with the maximum energy 0.35-3.5MeV (epidermal thickness: 7mg/cmcm2) and 0.3-3.5MeV (epidermal thickness: 40mg/cmcm2). The constant factor was 2.5×10-2(mrad/hr)/cps. The lower limit was 3.5×10-2mrad/hr(7mg/cmcm2) and 2.1×10-2mrad/hr(40mg/cmcm2).