The response characteristics of the 70 and 160 micron arrays change significantly as cosmic rays cause the slow buildup of residual charge in the detector material. These changes in response are monitored by frequent stimulator flashes (see 8.1.3.5) included in the MIPS observational sequences, at a nominal time interval of 2 minutes. The 24 micron array is of a type that shows minor changes with ionization damage, and calibration with stimulator flashes is a relatively infrequent event for that array.
In addition to tracking changes in the response of the Ge:Ga arrays due to ionizing radiation, the accumulated ionization damage needs to be periodically removed from them. Thermal anneal has proven to be the most effective technique for erasing the effects of cosmic rays. A full thermal anneal is required roughly every three hours, when the heaters raise the germanium detectors to temperatures of ~7 K (70 micron array) and ~5 K (160 micron array). The Si:As focal plane also includes a heater for annealing by raising its temperature to ~ 20 K. However, in this case, the operation is used only when the instrument is turned on to help stabilize the detector.
Unlike stimulator flashes, annealing is a separate operation from normal data taking. For example, anneals may be performed during slews between pointed observations or during downlinks. Observers should expect a significant (but well-characterized) change in the calibration properties of the arrays after an anneal. Although pipeline processing should remove most indications of the change, it is possible that the signal-to-noise ratio on sources will depend on the time elapsed since the last anneal.
