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

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.