Measurement of gas properties for the ion-TPC of N$\nu$DEx experiment

TL;DR

This study measures drift velocities and mobilities of charge carriers in SF6 gas at 750 Torr to aid the development of an ion-TPC for neutrinoless double beta decay detection in the NνDEx experiment.

Contribution

It provides the first detailed measurements of negative charge carrier properties in SF6 gas relevant for ion-TPC applications in neutrino physics.

Findings

Measured drift velocities of charge carriers in SF6 at 750 Torr.

Demonstrated z-coordinate reconstruction using charge carrier velocity differences.

Developed a specialized amplifier for charge readout in the TPC.

Abstract

In the N$\nu$DEx collaboration, a high-pressure gas TPC is being developed to search for the neutrinoless double beta decay. The use of electronegative $\mathrm{^{82}SeF_{6}}$ gas mandates an ion-TPC. The reconstruction of $z$ coordinate is to be realized exploiting the feature of multiple species of charge carriers. As the initial stage of the development, we studied the properties of the $\mathrm{SF_{6}}$ gas, which is non-toxic and has similar molecular structure to $\mathrm{SeF_{6}}$. In the paper we present the measurement of drift velocities and mobilities of the majority and minority negative charge carriers found in $\mathrm{SF_{6}}$ at a pressure of 750 Torr, slightly higher than the local atmospheric pressure. The reduced fields range between 3.0 and 5.5 Td. It was performed using a laser beam to ionize the gas inside a small TPC, with a drift length of 3.7 cm. A customized charge sensitive amplifier was developed to read out the anode signals induced by the slowly drifting ions. The reconstruction of $z$ coordinate using the difference in the velocities of the two carriers was also demonstrated.