Scintillator and PMTs

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Scintillator and PMTs

Postby Jerry » Thu May 17, 2012 10:20 pm

I have been messing around with the Harshaw crystal and the PMT some. Got a socket and wired up the divider but I am just not getting any pulses out from the pmt. It does respond to light. I cut a tiny slit in the black tape I wrapped around it and saw a voltage change in response to the light level but not sensitive enough to pick up the scintillator pulses. I used the basic design from here, but using 10 meg and 5 meg resistors:

http://es1.ph.man.ac.uk/AJM2/e2e/hardware/hardware.htm

So to see if the scintillator is any good I strapped it to one of the little Hamamatsu HC120 PMT units I have. That definitely showed pulses. Hooked it to a counter and got 400 cpm background and when I sat a piece of U ore on it got 21500 CPM. But considering it is just looking though a tiny little 4*13mm window and not even using any optical coupling that is not that bad.

This is the basic data sheet for the PMT module, the difference being they are a custom version with a side looking window instead of edge.

http://sales.hamamatsu.com/assets/pdf/p ... series.pdf

Image
Scintillator on PMT by macona, on Flickr
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Re: Scintillator and PMTs

Postby Jerry » Fri May 18, 2012 2:32 am

I just got the old PMT working. After looking at someones else's pmt circuit for this tube I realized I didn't have any sort of current limit for the tube which was swamping out the pulses. Now it works and seems to be very sensitive. Adjusted it to a point where I started seeing noise and backed off on my scope and got something like 8900 cpm background and 164000 CPM with a piece if U ore I have. Which from what I have found seems about right for an unshielded 2" crystal. I wrapped lead around it and it slowed down significantly. Also found that "woods metal", a low melting point alloy is slightly radioactive.

-Jerry
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Re: Scintillator and PMTs

Postby Jerry » Fri May 18, 2012 4:13 am

My new (Well, to me) Tek TDS340A has a floppy drive so I can capture waveforms and it saves them as BMPs. Here is what the pulses out of the scintillator look like:

Image
Scintillator PMT output by macona, on Flickr

Going to try that PRA software and see what happens. Im not sure I have a known gamma source though.
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Re: Scintillator and PMTs

Postby Starfire » Mon May 21, 2012 9:50 am

Jerry,
\What size is the crystal? I am working with some 10 off PMT's with integrated NAL's and pre-amps at present. They where scavenged from a 10 cell Gamma counter. The lead shield around the ten PMT's was a brute. Also got five swiss made stepper motors.
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Re: Scintillator and PMTs

Postby Jerry » Tue May 22, 2012 4:13 am

About 2" in diameter. Maybe 1.5" thick.

Been messing around with the PRA software and just can't get it to work right. Steven Sesselmann has been trying to help me get this thing going. It should be working but it just is not. It might be that my pulses are too short. He measured them around 6us where he usually has them at about 25. The other possibility is the PMT is messed up. So I bought one off ebay. This one has a base with the divider circuity on it and what looks like a filter of some sort. I goofed though, the guy is in Israel, ended up spending as much in shipping as I paid for the tube!

Sounds like you got some nice toys to play with!
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Re: Scintillator and PMTs

Postby Doug Coulter » Fri May 25, 2012 8:46 am

PRA might be the answer for some, but I'm going to be developing a real PHA off our PIC uP board. Yes, you need to stretch those pulses, and you can't handle high count rates like what you list - it's a gosh-darn audio board with far too slow a sample rate to handle this job unless you get every quirk just so. Like the dancing bear, it's amazing it can at all - not how well it can dance.

A soundcard is a switched capacitor a/d with a big sparse-tap FIR digital filter on it. They don't like pulses outside the audio range...and they ring if you get close to the top of the range and do other nasty things that smear spectra.

Use Bessel RC filters to get the pulses stretched (which will lose you most of the original amplitude) and keep that count rate down in the very low khz or hundreds of hertz. Else it simply can't work. This means you can''t use it with hot samples, as just pulling the sample back farther will get you compton scattering in the air and lose your spectral peaks in background.

With a crystal that size, you'll have a lot of compton scattering in the xtal itself, energy will escape out the sides and create the pictures like on wikipedia, which is confusing at first.

http://upload.wikimedia.org/wikipedia/c ... ectrum.PNG

All that stuff in the middle is "artifact" from energy from the high energy line not being fully converted to captured light. CS 137 has just two very narrow lines...I get better spectra off the huge "gallon jug" xtals BillF found on Ebay - about 6" diameter by 4" deep. But thin xtals do better at the low energy end of things - less light loss and so on down there. So it's kind of a case where one size cannot fit all.
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Re: Scintillator and PMTs

Postby Starfire » Tue May 29, 2012 1:38 pm

Does this constitute a ' gaggle ' of PMT's? :
They have an integrated pre-amp and a well Nal crystal


DSCF0042.JPG
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Re: Scintillator and PMTs

Postby Jerry » Tue May 29, 2012 2:07 pm

I definitely was seeing the ringing from the card.

I was talking to a friend about using an atmel to do the job. He didn't think we could do it and get decent performance. And with no DMA access there is no efficient way to interface a ADC. I have seen a few pic based ones on youtube.

Starfire, that is quite a haul.
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Re: Scintillator and PMTs

Postby Doug Coulter » Tue May 29, 2012 5:12 pm

Jerry, there's a trick (actually, there is considerable prior art on this problem). You can do it with almost anything, I'm going to use a PIC because that's my favored toolchain and we already make a board here that will do most of it. Most uP's with a/d should do if you code the time critical parts (mostly realizing you need to do an a/d conversion and then re-open the gate) in asm.

You have to have a fairly smart hybrid front end (in this case hybrid means analog/digital). It doesn't take a ton of logic - my design uses a couple flip flops and gates, but it does take a super good, super fast peak catcher. Once you have a peak, you "close the gate" on the input, and a/d the peak at leisure. You don't have to get them all to get a good spectrum, especially true if there are a lot of pulses, you just have to make sure your "selection" of the ones you do take isn't non-random or affected by their height. If the count rate is quick, you have to sweat "pileups" which some nim bin stuff used a delay line to help detect. I won't need that trick (good deal, good analog delay lines are kind of scarce these days). I'm simply going to refuse more pulses if the signal doesn't hit baseline below threshold in between them. We'll have to see how that works out, but it seems all I need is a couple very fast comparators, a couple gates and flops, and a trick peak catcher (an emitter follower used as a diode, a fet to discharge that and the input later, and temp comp for the Vbe of the emitter follower). Most of the "work" is making sure you don't open the gate in the middle of another pulse and declare that a real pulse, since you might have just caught the backside, not the peak. It winds up looking deceptively simple on paper (but it's not, you have to handle every frigging case where the randomness can get you).

Sigma/delta a/d's are nice and great for audio, but not this crap. There's also a ringing prone highpass filter in digital in there on top of the anti-alias FIR....there are enough "issues" to make them seem wrong to me to use, the only advantage is that most computers have one already. Even the fastest ones you can get for pro audio aren't even close to good enough for this job, really, unless you have super stretched pulses and very low count rates.
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