Avalanche gain and its effect on energy resolution in GEM-based detectors

TL;DR

This paper investigates avalanche gain and energy resolution in GEM-based detectors using different gases, emphasizing the role of the reduced first Townsend coefficient and providing a broad parameter space analysis for detector optimization.

Contribution

It introduces a comprehensive analysis of avalanche gain and energy resolution in GEM detectors across various gases and parameters, highlighting the relationship with the reduced first Townsend coefficient.

Findings

High-gain behavior is well described by electric field and number of GEMs.

The RFTC correlates strongly with electric field, influencing gain.

Energy resolution limits are estimated based on avalanche variance.

Abstract

We present avalanche gain and associated resolution measurements recorded with a $^4$He:CO$_2$ (70:30) gas mixture and pure SF$_6$, a Negative Ion (NI) gas. SF$_6$ is of particular interest to the directional dark matter detection community, as its low thermal diffusion helps to retain recoil ionization track features over long drift lengths. With the aid of a general form of the reduced first Townsend coefficient (RFTC), multiple GEM-based detector data sets are used to study the high-gain behavior of the $^4$He:CO$_2$ gas mixture. The high-gain data is well described purely in terms of the reduced electric field strength and the number of GEMs, and the robust relationship between the RFTC and the average, reduced, electric field strength across the GEMs is emphasized. The associated (pulse-height) resolution measurements are used to discuss the variance of the avalanche distribution and to describe and estimate the lower limits of energy resolution one should expect to measure using a simple relationship with the RFTC. In the end, a description of avalanche gain, its effect on energy resolution, and the contributing experimental parameters in GEM-based detectors is developed over a broad parameter space for further use.