A Rn-220 source for the calibration of low-background experiments

TL;DR

This paper characterizes Th-228 sources that emit Rn-220 for calibrating low-background detectors, demonstrating their effectiveness and safety for use in advanced experiments like XENON1T and nEXO.

Contribution

It introduces a new Rn-220 source based on Th-228 for detector calibration, with detailed emanation limits and filtration efficiency assessments.

Findings

Sources efficiently emanate Rn-220 for calibration.

Filters significantly reduce long-lived isotope activity.

Sources are suitable for next-generation noble gas detectors.

Abstract

We characterize two 40 kBq sources of electrodeposited Th-228 for use in low-background experiments. The sources efficiently emanate Rn-220, a noble gas that can diffuse in a detector volume. Rn-220 and its daughter isotopes produce alpha, beta, and gamma-radiation, which may used to calibrate a variety of detector responses and features, before decaying completely in only a few days. We perform various tests to place limits on the release of other long-lived isotopes. In particular, we find an emanation of <0.008 atoms/min/kBq (90% CL) for Th-228 and 1.53 atoms/min/kBq for Ra-224. The sources lend themselves in particular to the calibration of detectors employing liquid noble elements such as argon and xenon. With the source mounted in a noble gas system, we demonstrate that filters are highly efficient in reducing the activity of these longer-lived isotopes further. We thus confirm the suitability of these sources even for use in next-generation experiments, such as XENON1T/XENONnT, LZ, and nEXO.