One-year Measurements of Gamma-ray Background Using a High-purity Germanium Detector

Many studies of background levels in gamma-ray measurements using a high-purity germanium detector have been reported from various aspects such as its reduction and variation. In the present work, natural gamma-ray background was thoroughly measured in a year (283.5 days in total, n=271). One measurement time was almost all either 86,400 sec (1 day) or 100,000 sec. The data was first discussed in relation to radon concentrations in the laboratory. No correlations were found between the gamma-ray count rates from 214Pb and 214Bi and radon concentrations, meaning that radon just around the germanium detector was reduced to the negligible level by the introduction of nitrogen gas. Also, the count rates of major nuclides appeared to fluctuate with the normal distribution or its similar distribution, without seasonal variations. The coefficient of variance of a few up to several tens of percent was seen, which were larger than those calculated from counting statistics alone. Furthermore, summing of all gamma-ray spectra allowed us to see neutron-induced peaks that cannot be detected in usual short-term measurements. All data obtained here would be the knowledge useful for the practice of gamma-ray measurements.