Quote:
Originally Posted by Sandy G
The light from an AMBIENT source could damage the photosynthetic coating ?!?
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The reason to keep them pointed down is, as stated, to avoid spots from loose particles.
However, when working with common 931a photomultiplier tubes for a flying spot scanner in the 70s, I found that they could be damaged by exposure to bright office fluorescent lighting [edit - before ever being powered up. Taken out of the box in a dimly lighted room and installed in the scanner, they were good, but if exposed to bright light in the process of opening the box or replacing a tube to test another one, they would get worse.]
We had two brands, RCA and Hamamatsu. The Hamamatsu's, out of the box, were less noisy than the RCAs, but could be made noisier if you exposed them to the office lighting. [Edit: IIRC, the RCAs were just noisier whether exposed to bright light or not.] I never made a test to determine if that was because the output was reduced and subsequently we had to turn up the multiplier voltage, or if they were just noisier at the same voltage (i.e., by measuring the voltage required before and after). I suspect that the output of the photocathode was less after exposure and we had to turn up the voltage to get normal output.
Photo-emissive surfaces are made from compounds / alloys that hold their electrons loosely and that seems to involve the compounds holding together loosely as well - characteristics are easily degraded by high temperature, but I have never seen a description of the effect we saw with the Hamamatsu's. I suspect that other tubes like the iconoscope could have the same problem, but if they were always manufactured or handled in a bright environment, no one ever saw the original higher performance.