sno+ background study: polonium on acrylic vessel surface and radon assay

Abstract

SNO+ is a 780 tonnes organic liquid scintillator neutrino detector located at Vale’s Creighton mine, Sudbury, ON. 2 km overburden of rock above helps to achieve the low cosmic radiation background level of SNO+. Meanwhile, radioactive material in the rock can decay and produce radiation in the region of interest for the search of 0νββ decay. SNO+ is looking for neutrinos at very low energy and thus it is crucial to have a low background environment. My thesis evaluates two kinds of background sources: 222Rn and 210Po. 222Rn is the progeny of 238U in the rock. The water and gas assays are used to monitor the 222Rn concentration in the surrounding cavity water and other parts of the experiment or other gas volumes in SNOLAB. The analysis of the SNO+ data helps to understand the 210Po activity on the internal surface of the detector’s acrylic vessel. The 222Rn level in the cavity water is below the target of 4.5 × 10−13 gU238/gH2O. The Rn levels in the LN2 plant and international dewar are at a 10−4 reduction factor compared to mine air. The 210Po background level in the internal AV is holding a relatively constant level of about 1800 events per second. Spatially, the 210Po backgrounds are more active at the equator and the belly plate regions. The estimated number of 210Pb atoms deposited on the AV inner surface is 1.84 × 1012 .