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HRC Radiation Issues - 6/16/03
- The microchannel plates of HRC-I and HRC-S will suffer gain reductions as a result of cumulative extracted charge due to radiation induced events. Laboratory measurements have shown that when approximately 3 x 108 pC cm-2 of charge is extracted, the modal gain will drop by 10%. The average charge extracted per event is about 5 pC. Thus a count density of approx. 6 x 107 counts cm-2 will cause a gain drop of 10%. At a count rate of 10,000 counts per second, this count density will be achieved in 170 hours of operation for HRC-I and 100 hours for HRC-S. Although the gain can be raised by increasing the operating voltage, we are limited by a finite number of possible increases. These voltages changes will affect the QE calibration since they are discrete levels and will produce discontinuous changes in the pulse height distribution.
- The HRC is a faint object camera. Excessive counting rates during an observation due to radiation will compromise the science.
- In response to a question about the total on-time for the HRC detectors, Jon Chappell will be providing this information, possibly by tomorrow's telecon.
- Mike Juda has found three events over the mission in which the HRC (S only) was at operating level during the radiation events: August 16, 2001 (DOY 228), November 20, 2001 (DOY 324), and December 26, 2001 (DOY 360). I have plotted these events:
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