Fluid-dynamics and transport of 210Po in the scintillator Borexino detector: A numerical analysis

Moving beyond the important contributions to neutrino physics obtained by the Borexino experiment during the last years, research activities are ongoing at INFN Gran Sasso National Laboratories to further improve the detector sensitivity in order to perform an accurate measurement of the subdominant CNO solar neutrino rate. To this purpose, the improvement of the detector fluid-dynamic stability is the key to further reduce the Po-210 background, that is continuously being transported inside the measurement fiducial volume by convective currents. In this framework, numerical simulations of the detector fluid-dynamics may help to better comprehend the Po-210 behaviour, and also to suggest effective countermeasures, able to minimize the natural convection inside the detector. In the present work, two-dimensional numerical simulations have been performed to improve the current understanding of Borexino thermal and fluid-dynamics. Adopted models have been optimized for different regions and periods of interest, focusing on the most critical aspects that were identified as influencing the polonium background concentrations. In particular, a Borexino-specific benchmark was constructed in order to validate the model temperature predictions. The derived inner vessel surface temperatures are successively used as boundary conditions for a more refined convective model of the inner most part of the detector. Based on the calculated convective currents, the transport behaviour of background Po-210 inside the detector active volume was investigated by means of a convection-diffusion model, showing a reasonable good agreement between calculations and experimental data.