The ionization yield in a methane-filled spherical proportional counter

TL;DR

This paper measures the ionization yield and fluctuations in a methane-filled spherical proportional counter, providing essential calibration data for dark matter detection experiments.

Contribution

It presents new measurements of ionization yield and fluctuations in methane gas within SPCs, aiding the development of accurate energy response models for dark matter searches.

Findings

Measured mean ionization yield at various voltages

Calibrated detector response using UV laser and radioactive sources

Provided data to improve energy response models for future detectors

Abstract

Spherical proportional counters (SPCs) are gaseous particle detectors sensitive to single ionization electrons in their target media, with large detector volumes and low background rates. The $\mbox{NEWS-G}$ collaboration employs this technology to search for low-mass dark matter, having previously performed searches with detectors at the Laboratoire Souterrain de Modane (LSM), including a recent campaign with a 135 cm diameter SPC filled with methane. While in situ calibrations of the detector response were carried out at the LSM, measurements of the mean ionization yield and fluctuations of methane gas in SPCs were performed using a 30 cm diameter detector. The results of multiple measurements taken at different operating voltages are presented. A UV laser system was used to measure the mean gas gain of the SPC, along with $\mathrm{^{37}Ar}$ and aluminum-fluorescence calibration sources. These measurements will inform the energy response model of future operating detectors.