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).

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.