Camera-PMT calibration of fluorescence telescopes for the telescope array experiment

The Telescope Array is the largest cosmic ray experiment in northern hemisphere to study ultrahigh-energy cosmic rays at energies greater than 1018 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.