The Effects of Anti-Coincidence High Voltage Settings

Fromm mid March 2003 until early November 2004, the HRC anti-coincidence shield was operated at a reduced HV level for most of the time when the HRC was not observing the sky. The HRC anti-coincidence PMT HV level (output read-back S2HVLV) is controlled via commanding its step level, S2HVST. For example, S2HVST = 8 corresponds to a HV read-back level of 83 ADU or 1700V (see HRC PMT HV Codes).

The HRC-I MCP "total" rate is not affected by the different anti-coincidence shield PMT HV settings; there is no mechanism for this to happen. However, the HRC-I MCP valid rate will change with step level as the response of the PMT is lowered with lower HV step level resulting in a lower veto rate and a larger valid/total ratio. The linear fit to the valid/total ratio is indicative of a change in the anti-co shield response - the increase in the ratio with time reflects a degradation in the efficiency of the anti-coincidence detector. This decreasing efficiency could be due to lowered performance of the PMT or due to degradation of the optical qualities of the plastic scintillater or joints between sections or to the PMT. Interestingly, there is also an affect on the event PHAs with the reduced PMT HV step; there is a tendency for a lower mean and narrower distribution.

Figure on right shows a plot of Valid/Total ratio against S2HVST. In order to generate this plot, the observed time trend seen in Figure 1 in the main page was removed first using a fitted line given in that figure. Because data are dominated by two S2HVST (5 and 8) points, the fitted line may not be accurate; still there is a trend. This linear relation might be used to determine a candidate update to the PMT HV step level to restore the Valid/Total ratio back to the start of the mission level.