The Telescope Calibration of Fluorescence Detectors at Telescope Array Experiment

Abstract

Physics goal of Telescope Array(TA) experiment is to observe ultra high energy cosmic rays witha "hybrid" detection technique using fluorescence light detectors (FDs) and surface particle detectors (SDs). The TA has three stations for FDs where two stations named “Block Rock Mesa(BRM)" and "Long ridge (LR)" have been newly constructed for 24 new detectors (12 each), and the other named “Middle Drum(MD)” with 14 detectors has been migrated from the HiRes experiment. In this report, we describe detector calibrations for the BRM and LR FD stations. The telescope calibration includes mirror reflectances, absolute gains of reference phototubes, "relative gains" in a cluster of 256 tubes, (non-)uniformity of photo-cathode sensitivity of the tubes and transparency of the filters attached to phototubes and camera. The mirror reflectances are measured using a spectrometer, and the absolute gains or uniformities of the tubes are measured seasonally with a well-calibrated standard light sources. The relative gains are hourly monitored during observation by using a Xe flasher and an alpha-ray scintillator pulser (YAP). We discuss the calibration data qualities, and evaluate systematic errors in the air shower reconstructions and the energy determinations using the calibration data.