In situ measurement of the electron drift velocity for upcoming directional Dark Matter detectors

TL;DR

This paper introduces a method for in situ measurement of electron drift velocity in TPC detectors, crucial for accurate 3D track reconstruction in directional Dark Matter detection, and demonstrates its application to specific gas mixtures.

Contribution

It provides a novel in situ measurement technique for electron drift velocity applicable to underground TPCs, improving accuracy over simulation-based estimates.

Findings

Adding CHF3 reduces drift velocity by a factor of 5 at 50 mbar.

The method is tested successfully on CF4 and CF4 + CHF3 mixtures.

Lower drift velocity improves track sampling for Dark Matter detection.

Abstract

Three-dimensional track reconstruction is a key issue for directional Dark Matter detection and it requires a precise knowledge of the electron drift velocity. Magboltz simulations are known to give a good evaluation of this parameter. However, large TPC operated underground on long time scale may be characterized by an effective electron drift velocity that may differ from the value evaluated by simulation. In situ measurement of this key parameter is hence needed as it is a way to avoid bias in the 3D track reconstruction. We present a dedicated method for the measurement of the electron drift velocity with the MIMAC detector. It is tested on two gas mixtures: CF4 and CF4 + CHF3. The latter has been chosen for the MIMAC detector as we expect that adding CHF3 to pure CF4 will lower the electron drift velocity. This is a key point for directional Dark Matter as the track sampling along the drift field will be improved while keeping almost the same Fluorine content of the gas mixture. We show that the drift velocity at 50 mbar is reduced by a factor of about 5 when adding 30% of CHF3.