On Monday morning, April 30, 2012, observations with the Cosmic Origins Spectrograph onboard HST had been suspended when segment A of the COS FUV XDL detector experienced an excessively high count rate. Examination of the science and engineering data taken during this anomaly showed that the excessive counts were not due to illumination by any external source or calibration lamp, but instead appeared to be the result of field emission within the detector segment. A transient event first affected both detector segments, and over the course of several seconds field emission on segment A increased until it exceeded allowed bright object limits. However, the available data was inadequate to determine if the transient event seen at the start of the anomaly was due to some external source, such as the passage of a very high energy cosmic ray, or if it was instead due to a malfunction within the COS FUV detector. Similar field emission had been seen in ground testing when contamination was present on the detector's quantum efficiency (QE) grid which is used to produce an electric field that pushes photo-electrons back towards the micro-channel plates.
Because the event that suspended the detector was not fully understood, a cautious approach was adopted for the recovery of the FUV channel. The micro-channel plate voltages were raised in a series of steps, with each step being done first with the QE grid off, and then again with the QE grid on. Darks and lamp exposures were taken at each stage of the recovery and inspected in detail prior to proceeding with the next step. The final visits of this recovery were completed on 12 June 2012, when the COS FUV XDL detector was tested using full micro-channel plate voltages and with the additional QE grid voltage differential enabled. The behaviors measured in all engineering telemetry and seen in all of these exposures are consistent with the pre-anomaly behavior of the detector. The anomaly does not appear to have been caused by or resulted in any permanent damage to the COS FUV detector, and so the detector has been released for science. The first external observation after the recovery was taken on Thursday 14 June and was a repeat of the sensitivity monitor observation that had been taking place at the time of the April 30 anomaly. This visit executed successfully, and an examination of the data shows that the COS FUV channel's performance is consistent with expectations. GO observations with this channel resumed in the morning of June 15, 2012
Examination of a number of dark exposures taken over the last several months does show that there has been a region of slightly enhanced count rate on Segment A that roughly corresponds to the region of the excessive counts that caused the detector safing on 30 April. There appears to be an approximate correlation between this enhancement and the level of solar activity. During the recent recovery, this slight enhancement was also seen, but only when the QE grid was on, and there was no obvious correlation with the micro-channel plate voltage setting. This suggests that sufficiently high energy charged particles passing through the detector can induce field emission from the QE grid. As it is not understood what caused this field emission to run away on April 30, it is difficult to judge the probability of a similar event recurring in the future. However, since it is now apparent that this event did not damage the detector, in the event of another anomaly with similar signatures, it will be possible to recover the detector more quickly, with a much shorter disruption to COS science.