ИСТИНА

Photo detector is a sensitive part of the orbital extreme energy cosmic rays detector TUS. It composed on 256 pixels. Each pixel of the photo detector is a Hamamatsu R1463 photomultiplier tube (PMT) with a 13 mm diameter multi-alcali cathode covered by an UV glass filter and a reflective light guide with a square entrance of 15 mm size. All pixels have different sensitivities depending on the PMT gain and applied high voltage. The sensitivity can change during flight and has to be controlled. A special method of gain control and relative cross-calibration of the pixels was developed. It uses the measurements in a so-called METEOR mode (special mode of TUS operation with relatively low time resolution, 6.6 ms, thousands time larger than in EAS triggering mode: 0.8 μs) in which statistical fluctuations are minimal due to signal integration. In METEOR mode a-priory constant background pixel signals shows difference due to various sensitivity. The method of this relative cross-calibration is presented as well as examples of events measured in the METEOR mode.