How has the internal background behaved?
The internal background has generally been low, however, a low pulse-height component has increased above the initial turn-on value. For instance, when the first gas tank was exhausted, the system starting using tank 'B'. Initially, the background was identical to its initial value. During the past year this low pulse-height component has increased to a value comparable to that before the tank change. One possibility is that there is water outgassing from the walls of the tank, which is contaminating the gas. If this is true, then it explains the increase of the background in the lowest energy channels. The gain was reduced in October 1991 to reduce this affect (and to eliminate a detector "warm"spot). After 11 October 1991, the low pulse-height cutoff is channel 11, before it was channel 8.
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