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Subject: Re: Pentupler voltage multiplier info??
Date: Wed, 18 Dec 2002 00:25:09 -0500
Organization: Posted via Supernews, http://www.supernews.com
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
I did a bit of work with PMTs. We always had the anode near ground
potential. This made it easy to feed a more or less conventional current to
whatever (whatever being voltage, frequency or Log Amp...).
This forces the photocathode to be at something like -1 kv with the dynode
string decreasing toward ~ -50 volts on the last dynode near the anode. Gain
is proportional to the voltage applied. Keep a minimum of 50V per stage for
best linearity and speed. We used a 100V zener between the photocathode and
first dynode to force a high gain on the first stage. With every dynode
stage, the current needed increases as you move toward the anode. For this
reason, we used a resistive divider for dynode voltages and used lower
resistances for the last 3 stages. At high photocurrents, the dynode
voltages will try to redistribute and cause linearity problems. If you have
too much gain, you can cut down the number of stages and tie the un-needed
dynodes to the anode.
The majority of my experience was with side on PMTs similar to the Hamamatsu
R928 used in spectro-photometric measurements. I worked briefly with end on
tubes used in scintillation counters.
You can find some good application notes at www.hamamatsu.com
Thorn-EMI, Electro-nucleonics ENI and RCA used to have good apnotes. I don't
know who is in the business anymore except for hamamatsu. I do have a copy
of the RCA Photomultiplier handbook circa 1970's if anybody is interested.
For valid response address, remove the '1' following oppie
**** Check out the early February edition of "Design News" magazine. I've
got an article in it. The editors solicited entries from engineers that have
used their professional talents in a home project. I sent in my design for a
high reliability ignition timer for the boiler that heats my house.
"John Larkin" wrote in
| Multiplier strings like this are commonly used to drive
| photomultipliers. Each dynode is connected to a multiplier step, so no
| resistive voltage divider string is needed. This works nicely if the
| photocathode is the farthest from ground... it needs the least
| current, and successive dynodes need more, so that meshes nicely with
| the behavior of a C-W chain like this. The early stages of a PMT
| really need very little current.
| A few people make potted PMT sockets with the whole circuit inside,
| powered from 5 volts DC or whatever.
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