4.3.1 On-line Calibration
The well-defined endpoint in the Michel positron spectrum at 52.83 MeV allows
an on-line calibration of the detector between data taking runs. Here a set of
reference histograms, which can be either a set of previously calibrated data
or simulation generated histograms, is compared to the actual data using the
minimum c 2-method. The histograms are taken during data runs
using the Master Trigger Unit (MTU) pattern of the prescaled Michel events.
When a significant gain drift would be noted, a change in the HV setting
accordingly to the formula[19]
,
where g is the necessary gain factor, was applied. During the test beam run in
1996 it could be shown that 44 PMT readout channels worked stable for a period
of a week [Rit96]. The average deviation of one channel was about (0.5±1)%
per week. This also demonstrates the stability of the temperature control system.
[19] This formula is valid for a 10 stage PMT.