The gamma irradiation system with variable effective half-life function has been developed, which provides the low dose rate gamma irradiation field by two 137Cs sources (3.7×1010 Bq and 7.4×1011 Bq) . This apparatus was designed to give an exposure at varying low dose rate in accordance with a biological half life of radioactivity in organisms by selecting one of two irradiation methods. The first irradiation mode uses the spatial attenuation by four movable irradiation tables and the other uses the radiation attenuation by an iron filter. Both of these are driven by pulse motors using the personal computer control. The characteristics on spatial gamma-ray irradiation field were measured using calibrated ionization chambers for gamma dose and NaI (Tl) scintillation ionization chambers for gamma dose and Nal (Tl) scintillation counter for gamma spectra. The irradiation field covers the dose rate between 2.5×10-3C kg-1 min-1 and 1.2×10-7 C kg-1 min-1. The energy spectrum information including scattered radiations are also obtained. The apparatus was proved to be useful in the experiments for the irradiation of mice at varying dose rate to simulate tritium in internal irradiation.
