Dear Photomultiplier Doctor,
What is the significance of nominal A/lm?
Mr. Derek Tam Sing
Rapiscan
This is the gain for which the photomultiplier is suited. You can go lower say down by a factor of 10 for
high light level applications, but you need to be careful about linearity and sensitivity to magnetic fields. The collection from cathode to first dynode also falls off at low HV. If you find you are operating below 105 gain, then you need to consider either a photomultiplier with fewer stages or reducing the gain of your electronics. See technical reprints RP093 and RP094, in PDF format, which you can download from this page.
You can operate higher than the nominal gain by a factor of about 10. The linearity may also be affected so check this in terms of the maximum energy you want to detect.
A/lm is a step function of HV volts, V. In fact the A/lm or the gain follow a power law g = a Vn where n is about 10 . Applying an extra 50 V to a photomultiplier increases its gain by ~2. Please have a look at the gain curve in the data sheet to verify this.
Regards,
Photomultiplier Doctor.
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