Camera-PMT Calibration of Fluorescence Telescopes for the Telescope Array Experiment

Abstract

The Telescope Array is the largest cosmic ray experiment in northern hemisphere to study ultrahigh-energy cosmic rays at energies greater than 10(18) eV by using a hybrid detector technique. It consists of fluorescence telescopes to measure the longitudinal development of an extensive air shower due to a primary cosmic ray particle and plastic scintillation detectors to measure the lateral distribution of secondary particles that hit the ground. In this paper, we describe the camera-PMT calibration of the fluorescence telescopes by using YAP and xenon sources. For the energy and the shower profile measurements of an extensive air shower, calculating the correct fluorescence light-yield from each PMT light output is important. An absolute PMT gain variation related to aging is measured by using a standard YAP source and shows a variation of 3% or less over a three-year operation of the TA experiment. The relative PMT gain variation in each camera is measured by using a xenon flasher and turns out to be 4% or less.