The Japanese Association of Radiologycal Physicist (JARP) chamber is frequently used with Mix-DP phantoms to measure precisely the absorbed dose rates of X-ray and 60Co gamma-ray apparatus. Differing phantom and room temperatures sometimes cause changes in phantom temperatures with time. The temperature time constants for the JARP chamber were measured by Hiraoka et al., but few data have been obtained for the Mix-DP phantom's time constants. Our data indicated that when the initial difference between the phantom and room temperatures was 20°C, the rate of maximal temperature change of the Mix-DP phantom was 3°C h-1. Separating the 0.5 cm - 2.5 cm Mix-DP phantom slices and exposing them to air to decrease the temperature differences necessitated at least 5 hours to cause a temperature difference less than 1°C when the initial temperature difference was 15°C. The temperature of the Mix-DP phantom also changed by several degrees Centigrade in a room air conditioned only during working hours. Our dosimetry during conditions causing the temperature change rate from 3.2 to 2.0°C h-1 revealed differences in corresponding doses in air of 0.4% or less.
