Abstract
Exposure rates due to gamma-rays from cobalt-60 and cesium-137 sources, which were prepared for establishment of the institutionally standardized method of calibration for exposure-rate-meters, were measured by using a 3″φ spherical NaI (Tl) scintillation spectrometer to discuss on the increased exposure rates caused by the backscattered gamma-rays from holders of the standard sources, and on the non-uniform angular distributions of the exposure rates given by the standard sources.
The exposure rates were obtained at points of interest as a function of atomic number of the source base or the holder material, and they had a maximum near the atomic number of the iron or the copper material. Another series of exposure rates were obtained as a function of angle θ between the base normal and the line from the source to the detector. The detectable amount of variations of exposure rates could be seen at an angle of around 60 degree in case of cobalt-60 source, and at an angle of around 30 degree in case of cesium-137 source.
