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MDIR2 is a new OBC program, which is now available for general use. In its normal configuration it can provide intensity samples at a time resolution of 0.25 msec over a selectable range of PHA channels. If all the available telemetry is allocated to the MDIR2 program then the count rates in the selected PHA channels should be less equal 600 cts/sec for the data to be continuous. At higher counting rates the program will still work but the data will no longer be continuous. For each valid event the number of invalid samples since the last valid event is recorded as an 8 bit number, hence at a sampling rate of 4 kHz (the nominal value) if no events are recorded within 255 samples (ie. 62 msec) the counter overflows and is reset. This sets a lower limit to the source strength of ~ 200 cts/sec over the selected PHA channels for a probability of 10-5 per event of an overflow. For lower sampling rotates the lower acceptable count rate will of course be less than this value. Currently available Scientific Data Sequencer configurations support MDIR2 at 8, 4, 2 kHz giving time resolutions of 0.125, 0.25 and 0.5 msec respectively.

Within these constraints MDIR2 is a useful addition to the range of available OBC programs since it provides a higher time resolution than MHTR3 for continuous data with a background counting rate that is -20 cts/sec for 2-6 keV data compared to 80 cts/sec for MHTR3 Argon data.

2. Scientific Data Sequencer 35

The Scientific Data Sequencer controls the sampling of the experiment output channels. SDS 35 allows an improved monitoring of the MQE rate, and hence better estimate of the ME dead time, by sampling the MQE (which contains both Argon and Xenon data) at 32 Hz. This takes ~4% of available telemetry and 1% of the CPU procession power. For sources where dead time considerations are important, i.e. bright highly variable sources, use of MHTR3 with this SDS is recommended as a deadtime monitor. For a description of the ME dead times and how they can be corrected using the MQE rate see FOTH Chapter 8.

A. N. Parmar

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Last modified: Thursday, 26-Jun-2003 13:48:32 EDT