building a scintillator

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Q
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building a scintillator

Post by Q » Thu Feb 02, 2006 12:19 am

i've searched the archives and come up with squat, though i'm sure i've seen this question before.
anyway, in making my scintillator i've come across a couple of problems that i'm trying to figure out. i'll get to those in a moment.

but first, as i'm working on the detection portion of this thing, i've connected the pmt (through the blocking cap) to my trusty (crusty) old vectorscope. i can see the signal coming from the pmt, but the pulses are of varying heights. my check source is an ore sample. is this differance in pulse height due to multiple events happening simultaniously or is it related to the energy of the incident gamma?

next, in making my power supply, i cant seem to get a clean output. i've tried filtering it with various bypass caps, but with little change. out of despiration, i've also made a notch filter with the notch frequency to match the operational frequency of the power supply. it helped, but not much. any other ideas?

thirdly, on most scintillation detectors, is the anode resistor usually at the pmt end or the detector end of the system. in other words, if i plan to use the pmt assymbly with other detectors, does the anode resistor need to be integrated into the pmt housing or is it normally part of the power supply/counter circuit? i would assume that due to the variations in pmt's and differance in dynode srting resistances the anode resistor should be part of the pmt asymbly. is this the case?

some basic questions from your basic Q

Richard Hester
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Re: builting a scintillator

Post by Richard Hester » Thu Feb 02, 2006 1:35 am

Gamma rays of varying energies will give varying pulse heights.

To clean up your power supply a bit, you can try running it through a couple of cascaded RC filters. For high frequency ripple (switching power supply), I'd try something like 100k and 0.01uF.

The anode resistor and output blocking/coupling cap should most definitely be at the PMT end of the system. You really don't want to have to deal with a BNC signal cable charged up to high voltage - they bite. This is assumng you have a separate connector for HV and output signal, which is the case for most PMT base assemblies.

Q
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Re: builting a scintillator

Post by Q » Sat Feb 04, 2006 6:18 am

thank you, richard!

soon, i plan to post some pictures of my progress in the images section. this project is coming along quite nicely- even though it's at a snail's pace.

Q

Jon Rosenstiel
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Re: builting a scintillator

Post by Jon Rosenstiel » Sun Feb 05, 2006 3:37 am

Q,

Just in case you haven't run across this site...

http://home.austin.rr.com/cthompson15/R ... Page2.html

Jon Rosenstiel

Q
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Re: builting a scintillator

Post by Q » Sun Feb 05, 2006 4:51 am

thanks jon,
that's a great site. actually, the ccfl inverter based hv supply is what i'm using. it works well, but i cant get a very clean output from it. richard's cascade rc filter suggestion seems to be working well though.

Q

Q
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Re: building a scintillator

Post by Q » Sat Feb 18, 2006 6:26 am

ever have one of those "duh!" moments where you realize you've beaten that dead horse into pulp for no good reason?
well, my hv supply for the scintillator project still puts out some ripple, but you know what? it doesn't matter! the reason i was having problems earlier is because my ripple was larger than the signal pulses coming from the pmt. i was trying everything that i could to clean up the 0.5 mv (p-p) noise. then it hit me... use a bigger anode resistor!!
now, the pmt signal comes in clear and nearly "drowns out" the power supply noise.
whew! now i can get on with this thing.

on a different note, i'm seeing some interesting pulses on the o'scope. at a somewhat regular interval (about once every 10 seconds or so) there is a massive pulse (at least 4x as large) that shows up. since i'm using a peice of u ore as my check source, is it possible that i am picking up a spontanious fission event? the scintillation material is general purpose plastic, 3 inches thick. would that be enough to catch the occational neutron? it just seems like a awful lot of events to be neutrons...

Q

Jon Rosenstiel
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Re: building a scintillator

Post by Jon Rosenstiel » Sat Feb 18, 2006 6:56 am

I would guess you're seeing cosmic ray muons, they produce huge pulses. As I recall, the average muon flux at sea level is one muon/cm^2/minute.

Jon Rosenstiel

Q
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Re: building a scintillator

Post by Q » Sat Feb 18, 2006 1:15 pm

hmmm, interesting. i had forgotten about cosmic sources... (another "duh" moment)
well, i am at approximately 2000 feet above sea level and the detector is 3 inches square. that might account for it, especially since the much larger pulses seem to be constant wether the check source is present or not. (i hadn't thought to try removing the source to see if that made a differance until this morning)

i was under the impression that muons were difficult to detect though.

Q

Jon Rosenstiel
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Re: building a scintillator

Post by Jon Rosenstiel » Sat Feb 18, 2006 7:33 pm

Muons pack quite the punch, about 200 times the mass of an electron and with an average energy (at sea level) of around 2 geV! They really tear up the scintillator.

Jon Rosenstiel

AnGuy
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Re: building a scintillator

Post by AnGuy » Mon Feb 20, 2006 12:31 am

>i'm seeing some interesting pulses on the o'scope. at a somewhat regular interval (about once every 10 seconds or so) there is a massive pulse (at least 4x as large) that shows up.

Did you check to see if there is any AC devices that might be kicking on which would be causing these periodic pulses? A high current device such as a refridgerator or a furnace could be triggering your detector. Muons or other external sources would not occur at regular intervals. They would occur at random intervals.

BTW: The best way I found to reduce noise in a DC supply is with a coupled inductor filter. This device is like a Pi filter but with an extra low resistance coil wound on the inductor core that is attached to a capacitor. See Link:
http://www.orc.ru/~igorg/SMPS/zero_rip. ... %20Feature

You can order Pot cores from this site if you wish to roll your own:
http://www.cwsbytemark.com/

A source for very low noise HV power supplies is emco:
http://www.emcohighvoltage.com/
Their CA series devices are designed for PMTs and they will cost about $250-$300 per unit.

Hope this helps!

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