In Vivo Measurement of Radionuclides in the Skull: Factors Affecting K-40 Background in the Detector and Effects of the Control Group Type on Activity Assessment

Abstract: One of the grand challenges in the nuclear field is to establish the relationship between health risk and low-dose radiation. In vivo measurement of radionuclides in the skull, a promising method for studying the health effects of low-dose radiation, is being pursued by the TRACE (Trace-level RAdionuclides in vivo Counting Experiment) team at the China Jinping Underground Laboratory. Skull radionuclides exhibit low activity and emit low-energy gamma rays, making the measurements highly susceptible to background interference. This work aims to investigate human K-40 background to inform strategies for minimizing its interference, and to improve the accuracy of radionuclide activity assessment in the skull. Based on ICRP adult reference male phantom and Monte Carlo method, this paper examined factors affecting K-40 background in the detector. First, a key finding is that the top of the head is the best measurement position, yielding MDA values on average 2.12 times lower than the side and 1.56 times lower than the bottom position. This is attributed to the lowest K-40 background and highest counting efficiency for target radionuclides (Pb-210, Am-241, Th-234, U-235). Second, although K-40 is commonly assumed to be uniformly distributed in the body, the results of this study suggest that this assumption can underestimate background counts in the ROI by up to 10% compared with the actual distribution, which is mainly in soft tissues. Third, among all body parts, the head, as the largest source of K-40 background, is approximately proportional to the whole-body background. This suggests that a head phantom alone can represent the whole body for studying K-40 background, thereby simplifying phantom fabrication. Furthermore, this work developed a K-40 head phantom suitable for the Chinese adult reference male. Using this phantom, the effect of different control groups on the evaluation of skull radionuclide activity is investigated by comparison with the blank group and the normal individual group. The experimental results suggest that the phantom group offers both flexibility and accuracy, making it more suitable for activity assessment.