Last revision January 15, 2001 Decay of Gain for Large Area Detector 7 after TJD 10900 TJD 10900 = March 28, 1998 TJD 11690 = May 26, 2000 The Large Area Detectors (LADs) used automatic gain control (AGC) so that if the position of the 511 keV line (used for onboard automatic gain control and spectral calibration on the ground afterward) changed, the high voltage would be adjusted so that the line position in channel numbers would be constant. If the line position would begin to drop to lower channel numbers, the high voltage would be increased to compensate. However, the maximum possible value of the high voltage was slightly over 2000 volts. During the mission, the spectral response for LAD 7 decayed so that the AGC increased the high voltage until the maximum values were reached. Therefore, after approximately TJD 10900, the gain, i.e., the position of the 511 keV line in units of channel number, began to drop. By the end of the mission (TJD 11690), the position of the line had dropped from the usual linear channel number of 100 or 101 down to approximately 84. The other detectors generally did not suffer from an actual drop in gain although some of them did have voltages near the maximum by the end of the mission (especially LAD 1).