Natural radionuclides as background sources in the Modane underground laboratory

The Modane underground laboratory (LSM) is the deepest operating underground laboratory in Europe. It is located under the Fréjus peak in Savoie Alps in France, with average overburden of 4800 m w. e. (water equivalent), providing low-background environment for experiments in nuclear and particle physics, astrophysics and environmental physics. It is crucial to understand individual sources of background such as residual cosmic-ray flux of high-energy muons, muon-induced neutrons and contributions from radionuclides present in the environment. The identified dominant sources of background are radioactive contamination of construction materials of detectors and laboratory walls, radon contamination of the laboratory air, and neutrons produced in the laboratory. The largest neutron contribution has been identified from (α, n) reactions in low Z materials (10^-7-10^-4 n s^-1 Bq^-1) and from spontaneous fission of ²³⁸U (1.1× 10^-6 n s^-1 Bq^-1).

Development of methods for the preparation of radiopure 82Se sources for the SuperNEMO neutrinoless double-beta decay experiment

A radiochemical method for producing 82Se sources with an ultra-low level of contamination of natural radionuclides (40K, decay products of 232Th and 238U) has been developed based on cation-exchange chromatographic purification with reverse removal of impurities. It includes chromatographic separation (purification), reduction, conditioning (which includes decantation, centrifugation, washing, grinding, and drying), and 82Se foil production. The conditioning stage, during which highly dispersed elemental selenium is obtained by the reduction of purified selenious acid (H2SeO3) with sulfur dioxide (SO2) represents the crucial step in the preparation of radiopure 82Se samples. The natural selenium (600 g) was first produced in this procedure in order to refine the method. The technique developed was then used to produce 2.5 kg of radiopure enriched selenium (82Se). The produced 82Se samples were wrapped in polyethylene (12 μm thick) and radionuclides present in the sample were analyzed with the BiPo-3 detector. The radiopurity of the plastic materials (chromatographic column material and polypropylene chemical vessels), which were used at all stages, was determined by instrumental neutron activation analysis. The radiopurity of the 82Se foils was checked by measurements with the BiPo-3 spectrometer, which confirmed the high purity of the final product. The measured contamination level for 208Tl was 8–54 μBq/kg, and for 214Bi the detection limit of 600 μBq/kg has been reached.

COMPARISON OF VARIOUS APPROACHES TO DETERMINING CO2 FLUXES FROM THE SOIL BY 222Rn CALIBRATED METHOD

Various approaches of determining CO2 fluxes from the soil based on 222Rn calibrated method were tested. It was shown that the exponential fits of CO2 and 222Rn depth profiles led to uncertainties up to 50% if the depth profiles of CO2 and 222Rn concentration were not measured at sufficient depths. However, CO2 fluxes determined from linear fits of CO2 and 222Rn concentrations in the surface layer of the soil are lower than the fluxes determined from exponential fits, and have lower uncertainties. The comparison of CO2 fluxes derived from 222Rn calibrated methods with directly measured CO2 fluxes showed that the latter are usually on the order of tens of percent higher than the former. CO2 flux from sandy-clay and uncultivated soil was estimated to be on the level of 3.0 mmol m-2 h-1 in the spring and around 7.0 mmol m-2 h-1 in the summer.