Simulations of the Nuclear Recoil Head-Tail Signature in Gases Relevant to Directional Dark Matter Searches

TL;DR

This paper presents detailed simulations of the head-tail effect in nuclear recoil tracks within gases, crucial for directional dark matter detection, revealing the influence of stopping power and range straggling on track asymmetry.

Contribution

First detailed simulation study of the head-tail effect in gases relevant to directional dark matter searches, combining SRIM and Monte Carlo methods.

Findings

Head-tail asymmetry exists in nuclear recoil tracks.

Magnitude of asymmetry is influenced by stopping power and range straggling.

Results inform detector design for dark matter experiments.

Abstract

We present the first detailed simulations of the head-tail effect relevant to directional Dark Matter searches. Investigations of the location of the majority of the ionization charge as being either at the beginning half (tail) or at the end half (head) of the nuclear recoil track were performed for carbon and sulphur recoils in 40 Torr negative ion carbon disulfide and for fluorine recoils in 100 Torr carbon tetrafluoride. The SRIM simulation program was used, together with a purpose-written Monte Carlo generator, to model production of ionizing pairs, diffusion and basic readout geometries relevant to potential real detector scenarios, such as under development for the DRIFT experiment. The results clearly indicate the existence of a head-tail track asymmetry but with a magnitude critically influenced by two competing factors: the nature of the stopping power and details of the range straggling. The former tends to result in the tail being greater than the head and the latter the reverse.