building a scintillator

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
Q
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Re: building a scintillator

Post by Q » Mon Feb 20, 2006 12:51 am

thanks anonymous,
i have checked to see if there was anything around that might cause these pulses, so far, i've come up with nothing. the timing isn't exactly 10 seconds apart, but roughly some where between 6 and 15 seconds, sometimes as often as often as 2 in one second, but this is rare. the 10 second figure was a rough average.

as for the power supply, i have contacted emco, but they will only sell to oem's, government, or educational organizations... since i'm none of those, they refused to communicate with me beyond "try searching the internet for high voltage power supplies"- and they weren't very nice about it either. oh well, not to worry. i solved the issue. ebay also came through for me, giving me another supply for the next pmt project!

Q

AnGuy
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Re: building a scintillator

Post by AnGuy » Mon Feb 20, 2006 10:16 pm

>have contacted emco, but they will only sell to oem's, government, or educational organizations..

Just them that your an engineer developing or repairing a detector, and give them a company name (you can always make up one) and PO number. They did not ask for a Federal Employer ID number. I placed a order for three modules about 5 or 6 months ago. The sales rep asked me what they will be used for, I told her that they were for a detector. I think their minium order is 2 units.

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Re: building a scintillator

Post by AnGuy » Tue Feb 21, 2006 1:24 am

Here is another supplier of HV power supplies for detectors
http://www.picoelectronics.com/dcdclow/peHVP.htm

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Re: building a scintillator

Post by AnGuy » Fri Mar 10, 2006 1:04 am

In order properly us an detector such as a PMT, and HE3 or a BF3 tube the noise level must be very low > 0.005% The power supplies from Geiger counters operate with noise levels above 1%.

Its pretty difficult to make a low noise power supply. Basically you need to build DC-DC converter that operates at a very high frequency ( > 1 Mhz) so that you don't need giant caps and inductors to filter out the switching noise. The HV power supplies from EMCO are also very small, about 1" by 2" by 0.75" and are shielded to prevent EMI.

Richard Hester
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Re: building a scintillator

Post by Richard Hester » Fri Mar 10, 2006 1:59 am

You don't necessarily need extremely high frequency operation for low noise. Sometimes this can get in the way. A proper regulator design is important. In the files section, you will find a schematic for a HV supply based on a CCFL inverter, a pretty dirty source. With some shielding and a good regulator, I got 40 millivolts ripple for 900V out. This is 4.4 X 10^-3 %. If I set up the dropping resistors on the shunt regulator for lower quiescent current, no doubt I would get lower ripple, with lower power consumption to boot.

AnGuy
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Re: building a scintillator

Post by AnGuy » Fri Mar 10, 2006 2:24 am

>I got 40 millivolts ripple for 900V out. This is 4.4 X 10^-3 %. If I set up the dropping resistors on the shunt regulator for lower quiescent current, no doubt I would get lower ripple, with lower power consumption to boot.

I don't believe 40 millivolts isn't too hard, its getting it below 4 millivolts that becomes the challege. The EMCO supplies have ripple below 1 mv. You may also need to consider noise between your input supply connections, since there might be common mode noise between the two HV terminals, that isn't evident until you check against your DC inputs. Also what happens when you add a load? Does the noise level remain at 40 mv?

Richard Hester
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Re: building a scintillator

Post by Richard Hester » Fri Mar 10, 2006 4:16 am

The supply I cited uses an output shunt regulator, so it is always loaded at a constant value, whether or not anything is attached to the output terminals. Also, if I remember correctly, I used an unmodified scope probe to measure ripple, so some of that ripple shown could have been pickup, always a problem when measuring switchers. Not too bad, anyway, for a 5 dollar investment for the inverter and some parts from the junk box.
Next time around, I'll pop off the end cap and ground lead from the probe, take my life in my hands, and try to measure ripple with a real short loop area. At any rate, the supply shown powered the PMT on a plastic scintillator fast neutron detector/SCA with no problems (yes, it detected neutrons, too). These days, I would use MPSW42 transistors instead of the TIP50s (higher gain, less space consumed), and something like 100-150k instead of 68k for the shunt regulator dropping resistors. One of these days, I'll try and post another circuit with less power consumption. If I did a blocking oscilllator with a hand-wound transformer, I don't doubt I could do better, but I wanted to post a circuit where people could just go and buy the inverter, as not everyone is set up to do their own transformers.
BTW, there's no real reason to be putting lots of money in the hands of folks like Pico or Emco unless you have it to burn. The power supplies used in the standard REM ball detectors aren't all that sophisticated. I have one of the Ludlum REM balls and a "Snoopy", both with schematics, so I know first hand. The manual for the Snoopy is posted on this site in "Files", so you can have a look and see what I'm talking about.

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Re: building a scintillator

Post by Richard Hull » Fri Mar 10, 2006 4:00 pm

I guess I am just too used to NIM steups for neutron instrumentation. I still retain but rarely use any of my portable neutron detectors anymore. I haven't used a scintillation based neutron counter since 2000 and haven't worked with my venerable Eberline rem ball since 2002. I did use my snoopy and my PNC-1 in 2004, but now I only have one portable (PNC-1) with batteries even in it! All the other systems are in storage with their batteries removed.

Once you use a stable nim supply and nim modules including a good preamp to cob up a neutron counter around a nice sized He3 tube, you will just never use much of any thing else,.... as anything else will be inferior.

Likewise, once you use a BTI bubble detector you will laugh at terawatt pulse noise in the vacinity of you detector and not be concerned about 100 rad gamma or X-ray fields giving errors in your neutron count either. They won't even make a single bubble.

For me, at this stage of the game, there are only two real neutron counters for the amateur encountering low level, fast neutron counting problems. This, inspite of my have invested about $3,000 in "other" neutron counting systems and efforts since 1997.

The noise in any NIM supply, regardless of type, (and I have used about 8 different), is not enough to impact an He3 tube count rate. You will receive far more heartache keeping noise out of the system from the electrically noisy fusor environment rather than a silly ole supply.

Old post of my current detector.....
viewtopic.php?f=13&t=5528#p33972



Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

AnGuy
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Re: building a scintillator

Post by AnGuy » Sat Mar 11, 2006 1:25 am

>The supply I cited uses an output shunt regulator, so it is always loaded at a constant value, whether or not anything is attached to the output terminals

What are you using as a Shunt regulator? I suspect that 900V is close to the max operating range. Can you operate it voltages near 2kv? For instance the BF3 tube I have requires an operating voltage near 1900 volts, the HE3 tube I have will operate at about 1600 volts.

> any rate, the supply shown powered the PMT on a plastic scintillator fast neutron detector/SCA with no problems (yes, it detected neutrons, too).

I don't think ripple is much of an issue if all you are doing is counting. As long as the ripple level is below the pulse level of the detector it can be managed with voltage discriminator or SCA. However I believe for MCA is a different story since the pulse height will be a summation of the ripple and the detector output, which will distort the results.

I always thought PMT scintillator detectors where even more sensitive to ripple than BF3/HE3 detectors but I've never worked with them.

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Re: building a scintillator

Post by AnGuy » Sat Mar 11, 2006 1:36 am

>The noise in any NIM supply, regardless of type, (and I have used about 8 different), is not enough to impact an He3 tube count rate

What is the average noise level of your NIM supplies? I assume that your using just a SCA instead of an MCA?

>You will receive far more heartache keeping noise out of the system from the electrically noisy fusor environment rather than a silly ole supply.

With the ripple & EMI filtering on the outputs of my DC power supply I have been able to eliminate spikes created by power cycles of large appliances (refrig, Washer, etc). I don't have my fusor in operation yet, but I am hoping that the spikes generated from the HV supply will also be suppressed. (I know I am probably overly optimistic!)

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