The gain and energy resolution of the detectors are dependent on the
gas mixture and the high-voltage (HV) setting. The detector
performance is regularly checked using a Fe
radioactive source.
There are 8 setting of the high voltage at 30 volt intervals that can
be used to compensate for changes in the detector gain due to
contamination (gain decreases) or leakage (gain increases) - each
step represents a change of a factor of 
1.28 in gain. The trims
on both HV units were set to the nominal value of 4 (1476 V) at
launch. Although both detectors are showing a small, steady decrease
in gain (1.95% p.a. in BCS-A and 0.92% p.a. in BCS-B), it has not
yet been necessary (Jan/94) to change the HV trim setting. The history
of the HV trim settings are contained in file
``$DIR_BCS_LOGS/hv_log.dat'' .
During the radioactive source calibration, in order to make it
possible to detect the 5.9 KeV photons emitted by the Fe
source, the gain of the detector for BCS-B is changed by setting an HV
trim level of 2 (1418 V).
0.1 in
The results of the PHA (pulse-height analyzed) pre- and post-launch detector calibrations performed
with a radioactive source are contained in the file
``$DIR_BCS_LOGS/gain_log.dat.''
A history of the the gain and energy resolution of the two BCS
detectors can be obtained by typing the IDL command:
IDL> .run gain_plot4
The variation of the gain with temperature (determined from measurements
made during the pre-launch Thermal Test) can be obtained by typing the
IDL command:
IDL> .run gain_temp_plot