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.