Radon production in the EXO-200 recirculation loop

Abstract

EXO–200 was a 0νββ experiment which has completed data taking and its next generation nEXO is currently in the design phase. In the enriched liquid Xenon of EXO–200 a steady state of 170 ± 31 222Rn atoms was ob- served which contributed to the background of the detector.

This thesis is concerned with finding the source of the radon atoms in the Xenon of EXO–200. It is done through a post-mortem analysis of the plumb- ing emanation. A majority of the plumbing was reconstructed replacing the detection chamber with a Radon detector known as an Electrostatic Counter (ESC) that detects alpha decay products. Several experiments were done with different configurations of the plumbing. Different configurations were compared against each other by blank subtraction to single out components of interest, specifically the purifiers and gas purity monitors (GPMs). New efficiencies for the ESC were calculated for the new detector configuration.

The reassembled plumbing generally, showed an average steady-state 222Rn population of 222 ± 62 atoms which is consistent with what EXO- 200 observed during operation. The purifiers averaged a 222Rn activity of 33 ± 23 atoms (6.1 ± 4.2 dpd, 1 dpd= 0.012 mBq, where dpd is decays per day). The GPMs showed a 222Rn activity consistent with zero. An estimate was made of the activity from wetted stainless steel surfaces, it was found, but not proven conclusively, that these surfaces could be the dominant back- ground contributor. Thus the major contributor to the background appears to be external to the purifiers and GPMs, it is expected that this contributor is the wetted stainless steel surfaces.