Gamma-ray Exposure Rate Monitoring by Energy Spectra of NaI(Tl) Scintillation detectors

Abstract

Background: Nuclear facilities in South Korea have generally adopted pressurized ion chambers to measure ambient gamma ray exposure rates for monitoring the impact of radiation on the surrounding environment. The rates assessed with pressurized ion chambers do not distinguish between natural and man-made radiation, so a further step is needed to identify the cause of abnormal variation. In contrast, using NaI(Tl) scintillation detectors to detect gamma energy rates can allow an immediate assessment of the cause of variation through an analysis of the energy spectra. Against this backdrop, this study was conducted to propose a more effective way to monitor ambient gamma exposure rates. Materials and Methods:The following methods were used to analyze gamma energy spectra measured from January to November 2016 with NaI detectors installed at the Korea Atomic Energy Research Institute (KAERI) dormitory and Hanbat University. 1) Correlations of the variation of rates measured at the two locations were determined. 2) The dates, intervals, duration, and weather conditions were identified when rates increased by 5 nSv•h -1 or more. 3) Differences in the NaI spectra on normal days and days where rates spiked by 5 nSv•h -1 or more were studied. 4) An algorithm was derived for automatically calculating the net variation of the rates. Results and Discussion:The rates measured at KAERI and Hanbat University, located 12 kilometers apart, did not show a strong correlation (coefficient of determination = 0.577). Time gaps between spikes in the rates and rainfall were factors that affected the correlation. The weather conditions on days where rates went up by 5 nSv•h-1 or more featured rainfall, snowfall, or overcast, as well as an increase in peaks of the gamma rays emitted from the radon decay products of 214 Pb and 214 Bi in the spectrum. This study assumed that 214 Pb and 214 Bi exist at a radioactive equilibrium, since both have relatively short half-lives of under 30 minutes. Provided that this assumption is true and that the gamma peaks of the 352 keV and 1,764 keV gamma rays emitted from the radionuclides have proportional count rates, no man-made radiation should be present between the two energy levels. This study proved that this assumption was true by demonstrating a linear correlation between the count rates of these two gamma peaks. In conclusion, if the count rates of these two peaks detected in the gamma energy spectrum at a certain time maintain the ratio measured at a normal time, such variation can be confirmed to be caused by natural radiation. Conclusion:This study confirmed that both 214 Pb and 214 Bi have relatively short half-lives of under 30 minutes, thereby existing in a radioactive equilibrium in the atmosphere. If the gamma peaks of the 352 keV and 1,764 keV gamma rays emitted from these radionuclides have proportional count rates, no man-made radiation should exist between the two energy levels. JRPR rates in various regions. -The 1,764 keV peak count rates maintained a proportional ratio to the 352 ...

Precipitation-related increase events of the electromagnetic component of secondary cosmic rays: Spectral analysis

“…Эта линия и является индикатором распада K-40. В базовом спектре можно заметить также слабые линии Bi-214 (609 и 1764 кэВ [Григорьев, Мелихов, 1991]) и Tl-208 (2614 кэВ [Lee, 2017]). Иных заметных линий в базовом спектре нет.…”

“…В работе [Lee, 2017] при проведении спектральных наблюдений использовался спектрометр, аналогичный использованному нами, с близкими характеристиками. Он был установлен в кампусе корейского университета, около года с его помощью проводился мониторинг рентгеновского и гамма-излучения.…”

“…О том, что при осадках возрастает поток гамма-излучения, говорит сам автор. На приведенном в работе [Lee, 2017] рис. 5 четко прослеживаются события возрастания, связанные с осадками, но автор сосредоточился на линиях радионуклидов и вычислении дозы, которую эти радионуклиды дают, оставив без внимания изменение фонового уровня.…”

“…На рис. 8 мы приводим скан рисунка 10 из работы [Lee, 2017]. Четко виден аналогичный эффект: спектр во время осадков (оранжевый) подобен фоновому спектру (голубой), смещенному к бо ́льшим значениям.…”

“…Наблюдения проводились недалеко от Сеула (Корея). Рисунок заимствован из работы [Lee, 2017], там же можно найти подробную информацию об используемом спектрометре и сводную таблицу с десятками подобных событий Lee [2017] этот факт никак не комментирует и не отмечает, для него это не имеет значения. На рис.…”