Since HXT images are synthesized from sets of data from the 64 SC detectors, it is essential that the gain of each detector be set equal to each other with high precision (within 1 %). The gain calibration of HXT is conducted typically once in a month and the gain has been set constant since the beginning of HXT observations in October 1991, which enables us to obtain reliable hard X-ray images in the four energy bands.
In the calibration mode of HXT (HXT-CAL or Pulse Height (PH) mode), X-ray count data in 64 energy channels between 14 and 93 keV for each detector, instead of four energy bands in the case of observation mode (Pulse Count (PC) mode), are obtained. The gain of each detector is monitored by accumulating X-rays from an Am-241 calibration source (whose line profile peaks at 59.5 keV) attached on an aluminum case for the NaI (Tl) crystal.
The energy-channel relationship to which detector gains are adjusted is given as follows:
In normal observation mode, the four energy bands, L, M1, M2, and H, have the following energy range:
NOTE: Ch=0 is not included in the L band.