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Re: mystery photomultiplier



At 23:59 10/25/99 -0400, you wrote:
Hello all.

We have what appears to be an old but good condition photomultiplier
tube. (Actually, two of them.) They are marked as being Amperex brand,
model XP1040. They are quite large: The front lens is about 12cm in
diameter.

The problem: we don't have the base or plug which would fit on the large
20-pin connector, nor do we have info about how the pins are connected.
As far as I can figure out, Amperex Corp. has long since bit the dust.

Would anyone have any idea on how to get info that would allow us to
make these things function?

--
Dr. James McLean

I expect that by this time, James has done a web search where he
certainly will have found trace of several suppliers of PMTs and
bases for researchers. But perhaps not.

If he needs to proceed on general principle, I offer these
tips.
A PMT is likely to have one cathode and eight to 12 anodes
(or 'dynodes'). Each electrode will provide a shower of secondary
electrons from the preceding stream impinging on the adjacent
electrode. The electron stream is motivated by a potential
difference of about 100 volts maintained beween the electrode
'rungs' of the ladder, so that one ends up with 800 to 1200 volts
at the final anode.

The cathode is unheated, and is of course a photo emitter.
You would expect the dynode pins to be arranged rationally in view
of the ascending voltage ladder, so that one may begin by expecting
the dynode connector pins to be arrayed in order between cathode
and final anode. It may even be possible to see some common
connection layout to these pins by looking through the envelope.

It is unlikely that a PMT will be damaged by a final current
of 1 milliamp so providing a current limiter at this value
would be helpful during testing. But I would be tempted to
arrange a 100 volt supply to a probe in series with a sensitive
microammeter, in an effort to define the first dynode.
(Selecting two adjacent pins of twenty, when sequence is
material is at most 38 checks, perhaps an even chance to find
the cathode and first dynode in ten judicious tests.)

I have in mind a lamp and a fan or light shutter playing on the
sensitive surface. Better even than an ammeter, a scope looking
across a 10 megohm resistor in the power supply lead might show up
the first electrode with a small square wave. One might then
progress to rigging a second electrode at one hundred volts more,
and looking for a slightly larger signal. And so on....

It would be good to reduce the light intensity as more electrodes
provide increased amplification. PMTs are in general blinded by
excess final current - perhaps permanently. 50 Amp/lumen+ is a
typical gain figure.

Sincerely

brian whatcott <inet@intellisys.net>
Altus OK