Characterization of a Gaseous Time Projection Chamber With an Internal ³⁷Ar Source

We report on a novel calibration method of gaseous detectors using an internal 37Ar source. The 37Ar is a fast-decaying and low-energy calibration source that provides a mono-energetic peak of 2.82 keV. A gaseous 37Ar source is injected and uniformly distributed in a Micromegas-based gaseous time projection chamber (TPC). Key performance parameters of the detector, such as electron transmission, gain, energy resolution, gain uniformity, and drift field evolution, are effectively and quickly calibrated. The gain uniformity, related to the homogeneity of the avalanche gap of Micromegas, is calibrated quickly, thanks to the event-by-event position reconstruction and quasi-point energy deposition of 37Ar. The energy resolution is improved with the obtained gain uniformity map. The most noticeable improvement in energy resolution, from 44.9% to 35.4%, is observed at a working pressure of 7 bar. The internal calibration source is also used to characterize the dependence of the detector’s electric field distortion on the drift field.