As it was advertised at the time of the Pisa collaboration meeting,
we are planning to perform a detector-wide adjustment of the MACRO
PMTs gains that currently run rather low. We plan to perform this
operation starting sometime during the second week of January 1998.
Keeping the gains and PMT quality at their best state is crucial
for the wide spectrum of sensitivities we are addressing in MACRO.
Consistent PMT gain reseting over the last three years and continuous
servicing of problem counters has not left many PMT problems that we
will have to address. In a quick check of a sample of ~200 counters,
5-10 tankends might need extensive work. Approximately an equal number
of tankends suggest that the two PMTs in the same tankend are running
at different gains. In this occasion, where gains of the PMTS within
a tankend seems to have drifted apart, each PMT will have its gain
reset individually. For the vast majority of tankends, gains will be
adjusted on a per tankend basis, if their gain is found less than 3.7mV.
The 3.7mV test has been traditionally followed during the last two
PMT gain resetting periods and it reflects basically our accuracy
in setting the PMT gain by eye. Among the ~200 tankend sample, more than
30% of them was found running below the 3.7mV level with typical values
between 3.0 and 3.5mV.
We plan to proceed on a per-uvax basis, having first the gains of the
PMT in that uVax reset within 1-2 working days. Any tank-end work will
be left after the commission of the bulk of the uVax.
Threshold adjusting for the ERPGC and PHRASE will be inevitable.
Please let me know if you have any questions or comments regarding the
proposed plan.
--Erik