The HEXTE phoswich detectors and associated electronics make use of mono-energetic gamma-rays from an internal radioactive source (241Am) to continually maintain a constant overall gain of the X-ray energy to spectral PHA channel conversion. For this reason, the energy scale of the response matrix is independent of time, and no action on the part of the data analyst is required. The size of internal gain changes (e.g. due to PMT aging) that can be automatically compensated is limited to ±25%. Since the typical PMT gain has decreased by 10%, we have increased the high voltages, and thereby the PMT gains, so that the automatic gain control system can operate closer to the center of its range.
However, as the PMTs age, their efficiency at converting light from the NaI(Tl) and CsI(Na) scintillators to an electrical signal is reduced. This results not only in a loss of gain, but also in a slow degradation of energy resolution, which cannot be corrected by the automatic gain control system. Since launch, the response of an individual detector (expressed as a percentage Delta-E/E) has increased by ~0.2% (e.g. from 15.5% to 15.7% FWHM at 60 keV).
Therefore, the HEXTE team will eventually provide response matrices for various "resolution epochs" throughout the mission. However, the current set of response matrices is adequate for all AO1 and AO2 data.
