Material radiopurity control in the XENONnT experiment

Abstract

The selection of low-radioactive construction materials is of the utmost importance for rare-event searches and thus critical to the XENONnT experiment. Results of an extensive radioassay program are reported, in which material samples have been screened with gamma-ray spectroscopy, mass spectrometry, and ${}^{222}$ Rn emanation measurements. Furthermore, the cleanliness procedures applied to remove or mitigate surface contamination of detector materials are described. Screening results, used as inputs for a XENONnT Monte Carlo simulation, predict a reduction of materials background ( $\sim$ 17%) with respect to its predecessor XENON1T. Through radon emanation measurements, the expected ${}^{222}$ Rn activity concentration in XENONnT is determined to be 4.2 ( ${}_{-0.7}^{+0.5}$ ) $\mu$ Bq/kg, a factor three lower with respect to XENON1T. This radon concentration will be further suppressed by means of the novel radon distillation system.

Fig. 1

(left) Render of the XENONnT cryostat and TPC, including the most relevant components investigated during the screening campaign. (right) The cryostat is surrounded by the neutron veto and muon veto which serve as shielding for background reduction. The U-Tubes, Guide Pipe, Beam Pipe, and I-belt are part of the calibration subsystem

Fig. 2

Schematic drawing of the xenon handling system and the TPC of XENONnT. The circulation of xenon through the purification systems is indicated with orange (GXe) and blue (LXe) flow lines

Fig. 3

The different subsystem contributions to the overall ${}^{222}$Rn emanation rate in XENONnT, adding up to 35.7 (${}_{-5.9}^{+4.5}$) mBq. The colors correspond to the scheme used in Fig. 2. Only central values from Table 6 in this work and Figure 2 in [10] have been used. The radon emanation from the Rn-DST system is not taken into account

Fig. 4

The UG-CR (ISO 6 class) is built around the cryostat in the center of the water tank. Filtered air is flushed first into the cleanroom in the center and is pushed thereafter through the water tank into the grey area, providing a sequence of decreasing cleanliness levels toward ambient air

Fig. 5

Calculated radon emanation rate after background subtraction of a 3 m${}^2$ large PTFE foil before and after a 105 d exposure to ambient air with a radon concentration of $130$Bq/m${}^3$. The initial emanation rate was not reached again after wiping the sample with acetone-soaked cleanroom wipes.