Dose Rate Estimation Around a ⁶⁰Co γ-ray Irradiation Source by Means of ^115mIn Photoactivation

Abstract

Photoactivation of nuclear isomer ^115mIn with a halflife of 4.48 h occurs by ⁶⁰Co γ-ray irradiation. This is because the resonance γ-ray absorption occurs at 1078 keV level for stable ¹¹⁵In, and that energy γ-rays are produced by Compton scattering of ⁶⁰Co primary γ-rays. In this work, photoactivation of ^115mIn was applied to estimate the dose rate distribution around a ⁶⁰Co irradiation source utilizing a standard dose rate taken by alanine dosimeter. The ^115mIn photoactivation was measured at 10 to 160 cm from the ⁶⁰Co source. The derived dose rate distribution shows a good agreement with both alanine dosimeter data and Monte Carlo simulation. It is found that angular distribution of the dose rate along a circumference at radius 2.8 cm from the central axis shows ± 10% periodical variation reflecting the radioactive strength of the source rods, but less periodic distribution at radius 10 and 20 cm. The ^115mIn photoactivation along the vertical direction in the central irradiation port strongly depends on the height and radius as indicated by Monte Carlo simulation. It is demonstrated that ^115mIn photoactivation is a convenient method to estimate the dose rate distribution around a ⁶⁰Co source.