Measuring Drift Velocity in the Active Target Time Projection Chamber

The Active Target Time Projection Chamber (AT-TPC), constructed at the National Superconducting Cyclotron Laboratory (NSCL), serves as both a gas target and charged particle detector for nuclear reactions. As a charged particle traverses the detector, it ionizes the gas producing electron-ion pairs. The charged particle tracks are reconstructed from the collection of the electrons on the pad plane. An important factor in analyzing experiments is the drift velocity of electrons in the detector. It is required for accurate track reconstruction and also serves as an indicator of unwanted gas contaminants leaking into the detector. During a 2020 experiment at the NSCL, the detector was filled with helium gas. To determine the drift velocity, digitized signal of particles near the window and near the pad plane were isolated and fit. From this, the drift velocity throughout the week-long experiment was extracted. By doing this, we were able to assess the impact of changing environmental conditions (temperature, pressure, etc.) as well as the possibility of contaminants leaking into the detector.