For an independent check, Mike used another X-ray source, which is always in each HEXTE phoswich detector's field of view - namely the 241Am gain calibration source, whose decay X-rays are tagged by coincidence with an alpha particle detector. A spectrum of these events is telemetered every 512s.
Since the calibration source events have the same signal processing (and associated deadtime) as good events, we can use them to test the dead-time model on the 512s time-scale. For each detector, the number of counts in the 60 keV line peak is compared to the number seen during final ground calibrations, and to the number predicted by the Crab-based dead-time model. The results, from a single day in 1997 December, show that the two independent estimates for of deadtime agree well. The largest deviation is for detector PWA2, at 5%, while the smallest is ~0.6% in two other detectors.
Thus, the HEXTE team concludes that the deadtime correction, as modeled in hxtdead is accurate to ~2 percent.
These analyses will be extended to other epochs within the mission to refine the current deadtime model to accuracies of a fraction of a percent. In addition, the method will be be used to check for possible variation in livetime vs energy by comparing the rates in the low energy line complex in the 241Am spectrum against preflight values.
Related links:
HEXTE Dead-time model explanation (from the 1996 September 3 status report)
HEXTE
data reduction recipes, including an explanation of
hxtdead (at GSFC).
