Low energy radioactivity BG model in Super-Kamiokande detector from SK-IV data

The radioactivity background are among the most dangerous background for low energy neutrino analysis in Super-Kamiokande (SK), like the solar neutrino analysis. Among them, the main contribution is coming from $^{222}$Rn, which is spread in the detector's water due to the water source and to the photo multiplier (PMT) emanations. Up to now, its exact distribution in the detector was not known. Using our knowledge of the radon concentration in the detector water, and the SK-IV solar data, we developed a model of the radon distribution in the detector. The uncertainty on the Rn concentration associated with this model was estimated to be $\sim0.1$ mBq/m$^{3}$