Secondary scintillation yield of Xenon with sub-percent levels of CO2 additive: efficiently reducing electron diffusion in HPXe optical TPCs for rare-event detection

TL;DR

This study measures the electroluminescence yield of xenon-CO2 mixtures with sub-percent CO2 levels, showing that small amounts of CO2 can significantly reduce electron diffusion in high-pressure xenon TPCs, enhancing rare-event detection capabilities.

Contribution

It demonstrates that adding sub-percent CO2 to xenon maintains high electroluminescence yield while substantially reducing electron diffusion, improving TPC performance for rare-event detection.

Findings

EL yield remains high at 70% and 35% of pure xenon for 0.05% and 0.1% CO2.

Electron diffusion is reduced from 10 mm/√m to 2.5 mm/√m with CO2 addition.

Statistical fluctuations in EL are smaller than Fano factor below 0.1% CO2.

Abstract

We have measured the electroluminescence (EL) yield of Xe-CO2 mixtures, with sub-percent CO2 concentrations. We demonstrate that the EL production is still high in these mixtures, 70% and 35% relative to that produced in pure xenon, for CO2 concentrations around 0.05% and 0.1%, respectively. The contribution of the statistical fluctuations in EL production to the energy resolution increases with increasing CO2 concentration and, for our gas proportional scintillation counter, it is smaller than the contribution of the Fano factor for concentrations below 0.1% CO2. Xe-CO2 mixtures are important alternatives to pure xenon in TPCs based on EL signal amplification with applications in the important field of rare event detection such as directional dark matter, double electron capture and double beta decay detection. The addition of CO2 to pure xenon at the level of 0.05-0.1% can reduce significantly the scale of electron diffusion from 10 mm/sqrt(m) to 2.5 mm/sqrt(m), with high impact on the HPXe TPC discrimination efficiency of the events through pattern recognition of the topology of primary ionisation trails.