Experimental investigation on the radiation background inside body counters

In vivo measurement of radioactivity based on various body counters is arguably the leading measure used to determine the distribution and activity of radionuclides in human subjects, such as I-131 in the thyroid, Am-241 in the lungs, and Pb-210 in the skull. Throughout the measurements, the radiation background is the key factor that determines the sensitivity of the counter. Therefore, to facilitate in vivo measurements, a well-designed shielding room is required to create a low-background environment. However, because the compositions of the radiation background are quite complicated, the respective contributions from each source remain obscure, which places a considerable burden on seeking an optimized design of shielding rooms that strikes the optimum balance between the construction cost and background suppression effect. In this study, we conducted a systematic experimental investigation on the radiation background outside and inside four representative body counters with assorted designs using a variety of radiation detectors, including high-purity germanium detectors, CdZnTe detector, radon emanometer, and gamma-ray dosimeter. By carefully controlling the experimental conditions and synergetic analysis of the measurement results, in conjunction with previous studies, we separated and determined the relative contributions induced by environmental radiation (4%), airborne radon and its daughters (2%), the normal radioactivity of human subjects arising from K-40 (58%), cosmic rays (12%), and radioactivity in shielding materials and measuring instruments (24%). Furthermore, based on these results, we discuss practical guidelines to design a shielding room for body counters.