Spectral Modification of Environmental neutrons due to the telluric radioactive gas diffusion

by Prof. Laszlo Sajo-Bohus (Universidad Simon Bolivar, Caracas (Venezuela))

Tuesday 25 Feb 2020, 11:30 → 13:30 Europe/Rome

M.B. CEOLIN meeting room (INFN-LNL)

The environmental thermal neutrons of terrestrial origin are mainly related to the alpha decay of the primordial radioactive families, which are present in the soil with variable concentrations above the world average of 30Bq/kg. The interactions between alpha particles absorbed by soil elements produce neutrons via nuclear reactions, (α, n). The environmental neutron spectrum is therefore modified via moderation during diffusion, with an intensity related also with the moisture content of the subsoil. Cosmic neutrons near the surface are also expected to have a thermal component in the energy spectrum. This can change in those cases where an exceptional accumulation of alpha emitting radioisotopes is measured due to a natural transport phenomenon mainly of 226,228Ra and 222,220Rn in the superficial geological layers. We report in this work the experimental values ​​obtained in collaboration between various Latin American countries in particular those in the areas of the Andes mountain, specifically Colombia (Tunja at 2800m asl), Ecuador (Riobamba at 940m asl) and Peru (Lima at the level of the sea). These points are selected to provide comparison of data obtained where cosmic neutrons have a different flux and to be able to determine the conditions under which the contribution of thermal neutrons of telluric origin could be not negligible. In some of the studied points a concentration of Rn gas has been determined in the soil, identified as a hot spot, above 200kBq/m3. We note that values in the surface soil, are around 20 kBq/m3. In the cases of radioactive anomaly typically found in these so called  hot spots, a modification of the spectrum of environmental neutrons is expected and therefore an additional dose to the background values, due to the production of neutrons following the phenomenology indicated above. This additional dose is studied by means of simulations via MC and applied to the selected accumulation points where the concentration of radio and radon were measured by  nuclear track methodology with passive detectors (NTM-LR-115) and alpha spectrometry within a portable system (Markus-10, Gammatech, Sweden). The spatio-temporal variability of the concentration of Rn gas in the soil and the consequent production of (thermal) environmental neutrons are objects of interest since it allows to evaluate the diffusion direction of the neutrons and the impact in measurements where a radiation background is required to be very low, for example in LNGS in relation to neutrinoless double decay.