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[Plucinsky
et al.1993, Plucinsky
et al.1996],
[MPE RSDC1993]
In 1991 June it became apparent that the behaviour of the PSPC
background had changed.
The background count rate in the lowest channels started to increase and
grew to more than ten times the previously observed values by 1991 October.
A hot spot also developed at the rim of the detector which was
easily observable in the real-time data.
Flushing the PSPC from a different gas supply and reducing the gain
by reducing the high voltage eliminated the hot spot and reduced,
but did not eliminate, the contamination in the low pulse height channels.
During this period, the flow rate was increased to about
as part of the attempt to decontaminate
the detector.
Unfortunately, the low pulse height contamination started to increase again after 1991 October and continued to increase until 1992 February, when the PSPC gas supply was changed to the third tank (three tanks were flown).
In 1992 January, problems were encountered with the regulating valve.
It apparently stuck a number of times during HRI observations
(during which the PSPC was routinely flushed).
As a measure to extend the valve's lifetime (it began working again
for unknown reasons), it was decided to lower the valve ``click-rate''
by a factor of two (which does not change the flow rate).
At the end of 1992 February, Tank A was empty and the gas supply was
switched to Tank C was started.
Since PSPC appeared to well-behaved, the flow rate was switched back
during 1992 mid-March to 
, where it remained
until the flow rate reduction made at the start of AO-4.
The PSPC gas flow rate was reduced on 1992 June 10, the same day
that the HRI was moved into the focal position.
This allowed the PSPC until June 15 (the beginning of AO-4)
to reach equilibrium at the new flow rate.
The flow rate was reduced from 3.8 to 2.5 cm 
min 
,
i.e., by about 35%, on 1993 June 10.