Properties of PACE-I HgCdTe detectors in space: The NICMOS warm-up monitoring program

We summarize the results of a monitoring program which was executed following the cryogen exhaustion of the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on board the Hubble Space Telescope. During the subsequent warm-up, detector parameters such as detective quantum efficiency, dark current, bias offsets, and saturation levels have been measured over the temperature range 62 K to about 100 K. The measurements provide a unique database of the characteristics of PACE-I HgCdTe detector arrays in the space environment. A surprising result of the analysis is the fact that all three NICMOS detectors showed an enhanced dark current in the temperature range between 77 and 85 K. However, a subsequent laboratory experiment designed to replicate the on-orbit warm-up did not reproduce the anomaly, despite the fact that it employed a flight-spare detector of the same pedigree. The mechanism behind the on-orbit dark current anomaly is therefore believed to be unique to the space environment. We discuss possible explanations for these unexpected observational results, as well as their implications for future NICMOS operations.