next up previous contents
Next: Gain steps Up: Additional information Previous: Measuring aperture

PMT Voltage

When photomultipliers are used, the spectral response depends in a complicated way (involving several different phenomena) on the electrode potentials in the first few stages. If the high voltage is changed to vary the gain, as is sometimes done in DC or CI work, the spectral response varies. This must be treated exactly like a variable measuring aperture: only data taken under constant conditions should be reduced together, but sometimes this can be done if adequate calibration data have been measured.

The column label is PMTVOLTS. Once again, because several complex effects are involved, the effects are not predictable, so there is no point in trying to use quantitative values. Besides, the voltage settings on many power supplies are not very accurate, nor can one read an analog voltmeter precisely enough to obtain useful numbers. The values in this column will be treated as strings rather than converted to numbers.

Petra Nass