Bob,
The PMT bias voltage varies from detector to detector, this is due to the difference in gain between PMT's. PMT's are essentially amplifiers with a gain in the order of a million, this gain is a function of the coating thickness on the cathode (the brown coating on the inside of the glass) and how efficiently it converts photons to electrons.
The PMT manufacturers test each PMT and supply a data sheet specifying the gain. When I order PMT's for spectrometry, I specifically ask for tubes with low gain, in the range 150 to 500 ma @ 650V, this works best with the GS.
In the last batch of PMT's I ordered, the gain varied from 337 ma to 474 ma.
Note also that the Pulse Height Resolution (PHR) also varies from one PMT to another, so if you have a bad detector it may not be all due to the crystal. There can be as much as a 2% variance in the PHR @ 662 keV in a brand new PMT. The manufacturers test each PMT in a dark test chamber with a standard NaI(Tl) crystal and record the PHR, which can be around 6.5% for an excellent PMT and 7.5% for a not so good one.
The resolution of a PMT is a function of how well the dynodes focus the electron beam as it multiplies itself down the set of dynodes. Badly focused electrons fail to make it all the way to the anode, and cause the final pulse to be slightly low.
Attached is a picture of a box and grid from a PMT which I broke accidentally.
- ADIT PMT
- photo.JPG (158.73 KiB) Viewed 6831 times
The dynodes are the parts that look lice little scoops, and I think that round disc on the side contains a getter, the getter is made from a porous active metal which sucks up any remaining gas molecules after the PMT has been evacuated. Each dynode is held apart by a series of ceramic rings.