Radon Emanation Techniques and Measurements for LZ

TL;DR

This paper discusses radon emanation measurement techniques used in the LZ experiment, highlighting material screening, in-situ assays, and identifying the titanium cryostat as a significant radon source to reduce background noise.

Contribution

It introduces a portable radon collection system for in-situ measurements and identifies the titanium cryostat as a major radon emanation source in LZ.

Findings

Titanium cryostat is a significant radon source.

Portable radon collection system effective for in-situ assays.

Low emanation from $^{228}$Th source confirmed.

Abstract

Radon emanation was projected to account for $>50$% of the electron recoil background in the WIMP region of interest for the LUX-ZEPLIN (LZ) experiment. To mitigate the amount of radon inside the detector volume, materials with inherently low radioactivity content were selected for LZ construction through an extensive screening campaign. The SD Mines radon emanation system was one of four emanation facilities utilized to screen materials during construction of LZ. SD Mines also employed a portable radon collection system for equipment too large or delicate to move to a radon emanation facility. This portable system was used to assay the Inner Cryostat Vessel in-situ at various stages of detector construction, resulting in the inference that the titanium cryostat is the source of significant radon emanation. Assays of a $^{228}$Th source confirmed that its $^{222}$Rn emanation is low enough for it to be used, and that 14% of the $^{220}$Rn emanates from the source at room temperature.