Nedd help with divider network / datasheet of PMT Thorn EMI 9924 B/2

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scientificfun
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Nedd help with divider network / datasheet of PMT Thorn EMI 9924 B/2

Post by scientificfun » Tue Feb 19, 2008 1:34 pm

Hallo-

I have this devider network but I definitely have no clue how to set it up and all the standard circuits seem to be different from my findings.
A Datasheet for the PMT would help a lot also...
So- the network has 1 thick insulated clear wire which has a resistance to a thin black wire of ca. 7.4MOhm. Black one has no connection to housing.
Then there is a shielded cable- shield has connection to metal housing of network. Inner wire has 1MOhm to shield of cable.
What I tryed (from my guess...) connected black one to GND, thick insulated wire to -HV, inside of shielded to my amplifiers in, shield to GND. Increased HV to 1600V ca, increased amplification... 1 impulse all 10 sec (put a betalight with a light shield with just a small hole in fromt of PMT) with disriminator setting really low- further decrease of discr. level- just masses of noise (no change with change of light shield...)
Then tryed + HV: same also same when changing black and thick insulated wire... Checking with a scope on the shielded cable gave me some impulses all 10 sec also (and little 50Hz noise...)
Any ideas? PMT defect (Is it easy to destroy by wrong polarity, too much light,......?)? Divider defect? (especially- shouldnt there be a resistance from the inner shielded one to the black / thick insulated one also?)
Any help would be really apreciated!!!
Thanks
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Starfire
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Re: Nedd help with divider network / datasheet of PMT Thorn EMI 9924 B/2

Post by Starfire » Tue Feb 19, 2008 2:07 pm

A common circuit - check voltages all along the dynode string - should increase in steps - be careful not to expose the pmt to high incident light - it will destroy the tube if powered. Look on the final ( both sides of the cap ) with a good hi-impedance scope.
You should see any fluctuations if you move your hand in front of the pmt pin hole.
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scientificfun
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Re: Nedd help with divider network / datasheet of PMT Thorn EMI 9924 B/2

Post by scientificfun » Tue Feb 19, 2008 2:23 pm

Hi John-

thanks- but as I sayed... this does not seem to work.....
Also: in your circuit there should be a measurable resistance between HV and Sig out...

Thanks anyways..

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Re: Nedd help with divider network / datasheet of PMT Thorn EMI 9924 B/2

Post by Starfire » Tue Feb 19, 2008 2:39 pm

The O/P is usually taken from the low {Earth } side as a signal voltage across the final resistor in the chain, but most chains have a coupling capacitor for take off, unless direct coupled. The O/P is high impedance and will not suffer much of a load.

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Re: Nedd help with divider network / datasheet of PMT Thorn EMI 9924 B/2

Post by UG! » Tue Feb 19, 2008 3:49 pm

if it was me, i'd try to take appart that metal caseing and see whats going on inside, you never know how it was used previously. check all the resistor values if you do.

i'd test it with a proper scintilator. PMTs are designed to detect a few photons, if you give it too many you end up with a dc current, rather than a pulse and if the output is capacitively coupeled within that metal box(which it may well be), you will see nothing. as John noted give it much too much light, and you will kill the PMT.

also, you really need a digital scope to look at the output, its very easy to miss randomly occouring pulses or random magnitude when using a non-storage scope, and assume the PMT isn't working when it is.

when testing, use about 500-700V. the PMT will still work if the circuit is correct and there will be less chance of damage if it isn't.

one day i will get round to posting the writeup of the proton-recoil scint counter i made.

Oliver

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Carl Willis
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Re: Nedd help with divider network / datasheet of PMT Thorn EMI 9924 B/2

Post by Carl Willis » Tue Feb 19, 2008 8:14 pm

First order of business: 1600V is guaranteed to be TOO HIGH for a tube of this size.

Second: when you test a PMT, it *must* be protected from light or destructive currents can flow. Either put the PMT in a film bag or run it in the open with the lights out at no more than ~200 V.

Third: is the tube completely dead? Look at (a) the pumpout neck between the pins and (b) the face of the tube. If the former is cracked, no good tube. If the latter is not covered inside by a golden / brown film, no good tube.

Why do you think your circuit is any different from the "standard?" I'm willing to bet money it's pretty standard. You have two conductors, one of them an HV wire, plus a coaxial output lead. Between these leads you read ~7 MOhm, about what we'd expect if the dynode resistance is in the typical 560k range. This is undoubtedly just a passive divider network with no internal preamp to worry about. I'd say the thick HV wire is the +HV lead, probably requiring 700-1200 VDC, the black wire is a ground, and the coax is a pulse output AC-coupled off the anode. The pulses out will be negative pulses.

For testing a PMT, a counter / ratemeter setup is more useful than a regular analog 'scope, unless you have a huge countrate of fairly uniform pulses and can get the 'scope to reliably trigger on them. A good test light source is a glow-in-the-dark sticker such as can be bought just about anywhere. But no reason to do that if you have a piece of plastic scintillator around.

-Carl
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Re: Nedd help with divider network / datasheet of PMT Thorn EMI 9924 B/2

Post by scientificfun » Tue Feb 19, 2008 8:40 pm

Hi Carl-

thank you-

but do you mean the thick wire is a + (plus) HV connector? (with respect to the black GND one?) I thought standard would be to connect it to - HV?
And yes- guess now its standard too, forgot about coupling cap....

Greets
Roman

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Carl Willis
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Re: Nedd help with divider network / datasheet of PMT Thorn EMI 9924 B/2

Post by Carl Willis » Wed Feb 20, 2008 12:39 am

Hi Roman,

I've rarely met a PMT system that ran with negative HV. Granted, there are some (PET scanner detectors come to mind) but they are not in the majority.

The coupling capacitor, various divider and HV filter capacitors, and all the divider and anode load resistors should be present in the socket assembly with no additions required on your part.

-Carl
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Re: Nedd help with divider network / datasheet of PMT Thorn EMI 9924 B/2

Post by scientificfun » Fri Feb 22, 2008 8:16 pm

Thanks all-

it works the standard way as Carl sayed.... my signal out connector had a slack joint....
But now- great how sensitive these things are- and that with a 1" scintillator!

Greets

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