The DRIFT Directional Dark Matter Detector and First Studies of the Head-Tail Effect

TL;DR

This study investigates the Head-Tail effect in low-pressure gas detectors for dark matter detection, using simulations to analyze ionization asymmetry along nuclear recoil tracks relevant to the DRIFT detector.

Contribution

The paper presents the first simulation-based analysis of the Head-Tail effect in low-pressure gas detectors for dark matter, highlighting the influence of stopping power and range straggling on ionization asymmetry.

Findings

Evidence of ionization asymmetry along recoil tracks.

The asymmetry depends on ion type and energy.

The effect results from competing physical processes.

Abstract

Measurement of the direction of the elastic nuclear recoil track and ionization charge distribution along it, gives unique possibility for unambiguous detection of the dark matter WIMP particle. Within current radiation detection technologies only Time Projection Chambers filled with low pressure gas are capable of such measurement. Due to the character of the electronic and nuclear stopping powers of low energy nuclear recoils in the gas, an asymmetric ionization charge distribution along their tracks may be expected. Preliminary study of this effect, called Head-Tail, has been carried out here using the SRIM simulation program for Carbon and Sulfur in 40 Torr carbon disulfide, as relevant to the DRIFT detector. Investigations were focused on ion tracks projected onto the axis of the initial direction of motion in the energy range between 10 and 400 keV. Results indicate the likely existence of an asymmetry influenced by two competing effects: the nature of the stopping power and range straggling. The former tends to result in the Tail being greater than the Head and the latter the reverse. It has been found that for projected tracks the mean position of the ionization charge flows from 'head' to 'tail' with the magnitude depending on the ion type and its energy.