Response of an automatic air monitoring gamma-spectrometry station based on sample retention to rainfall-caused dosimetric alterations and to environments with a wide range of radon levels

Abstract An increase in ambient dose equivalent rate during precipitation events is a well known phenomenon that is easily identified by dose rate monitoring networks. Despite the invaluable information these monitors usually provide, they cannot identify the radioisotopes involved in these radiological alterations nor quantify their activity concentrations. Commercial systems based on passive gamma spectrometry using scintillation detectors allow to identify the radionuclides that increase during rainfall events but they cannot infer if they are deposited on ground or linked to the airborne particles. In 2016 an air monitoring station based on sample retention and scintillator gamma spectrometer was custom-designed and integrated into the automatic and quasi-real-time Radiological Alert Network of Extremadura (RARE) in south western Spain. In this work, it is presented how the capabilities of our system take advantage of the distinction between the radionuclides that are deposited and the radionuclides that are presented in airborne during rainy events. However, during the operation of RARE's homemade monitoring station based on sample retention and gamma-spectrometry, some shortcomings to meet the requirements appertained to real-time radiological networks, such as efficiency, were identified. Concretely, the station was targeted at monitoring the activity of gamma-emitting radioisotopes in airborne, but the relatively poor energy resolution of the gamma scintillation detectors employed meant that exceptionally high concentrations of radon decay products (such as 214Pb) in the area made it very difficult to identify and quantify the activity levels of some anthropogenic radioisotopes such as 131I, with there being a significant number of false positives and substantial increase in those isotopes' minimum detectable activities. This paper describes the simple approach that has been implemented in order to overcome the aforementioned analytical difficulty in situations where the activity concentration of radioiodine in air could be of the order of natural radon concentration, for example, when effluent releases from nuclear facilities permitted under authorities’ regulations.