PARTICLE PHYSICS AND NUCLEAR DECAY: CANCELLATION OF PHOTOMULTIPLIER GAIN DRIFT FOLLOWING SOURCE INTENSITY CHANGE.
PITTSBURGH UNIV PA SARAH MELLON SCAIFE RADIATION LAB
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Photomultiplier tubes as used for measuring light pulses or intensity in nuclear applications or other situations are notoriously subject to gain drift following change in light intensity. The complete set of tests run in this laboratory on several representative tubes, Du Mont type 6292 and RCA type 6342-A is covered. A principal result is that a cancellation method is evolved which uses a deliberate change in the high voltage to immediately erase the memory of each source intensity change. For several tubes the empirical characteristic curves, relating anode current to high voltage subject to zero gain drift, are determined and extensively tested. The range of validity of the cancellation method in terms of ambient temperature, spectrum, intensity level, is explored. Although a limitation is found at higher temperatures 50C, the characteristic curves are valid down to 0C. It is established that the seat of these gain drifts seems to be distributed over the ten dynode surfaces. Included are discussions of temperature sensitivities of photomultipliers, NaI scintillators, light-pipers, NaI afterglow, and grid control of tube current. Grid control, itself a method of preventing gain drift, is validated by a set of tests and presented. Author