Effect of bias voltage on an He-3 tube's pulse height spectrum

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Jon Rosenstiel
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Effect of bias voltage on an He-3 tube's pulse height spectrum

Post by Jon Rosenstiel » Mon Oct 01, 2012 9:23 pm

Recently my dumpster diving friend showed up at my lab’s front door with an armful of small neutron detector tubes. My task, as fusor owner / operator, was to determine what portion of his neutron detector booty was scrap and what portion was good.

My friend’s booty consisted of four Reuter-Stokes RS-P4-0602-201 He-3 tubes (0.75” diameter x 2” effective length), two LND 25169 He-3 tubes (0.5” diameter x 2.5” effective length), one LND 2311 B-10 lined tube (0.625” diameter x 1.25” effective length), and one LND 2325 B-10 lined tube (1” diameter x ~2” effective length).

Of the eight tube booty only the two LND 25169’s turned out to be kind of iffy. By iffy I mean that they did work and they did produce a nice neutron capture peak at 764 keV, but they were extremely noisy… background counts were in the low hundreds of counts / minute range. As one would expect, the small LND B-10 lined tubes were in the count-rate basement. The 2311’s count-rate was ~30% lower than that of an N. Wood G-5-1 BF-3 tube (0.63” diameter x 1” effective length) and the 2325’s count-rate was ~20% higher than that of the N. Wood BF-3 tube.

On to the focus of this report, the four Reuter-Stokes tubes. As we didn’t have any specs on these tubes determining the proper operating bias took a bit of experimentation. (I guess we could have contacted Reuter-Stokes to obtain the information, but where was the fun in that?)

Using one of the R-S tubes I ran a series of tests that consisted of increasing the bias voltage from +800 V to +1350 V in 50 V steps in a constant (as constant as a fusor can be) neutron flux. After each 50 V increase in bias I re-adjusted the amplifier’s gain to keep the amp’s output pulse at 2 Volts so the graphs generated were at least somewhat consistent in appearance.

Electronics used in this experiment…
Canberra 3102D HV supply
Tennelec TC-175B preamp
Ortec 570 amp, 3 μs shaping
Canberra 2030 SCA
Ortec 773 timer / counter
Ortec 778 dual counter
(All of the NIM modules were powered by a Tennelec minibin)
Scope was a Tektronix TDS-210
MCA was a Canberra Series 35+

Image #1: +800 V to +950 V. Due to the large amount of amplifier gain required at lower bias voltages fusor electrical noise at the low end of the spectrum was a big problem.
Image #2: +1000 V to +1150 V. The neutron capture pulse height specta look pretty normal here. Based on the data at hand the bias voltage I’d recommend is in the neighborhood of +1000 V.
Image #3: +1200 V to +1350 V. As detector bias increases the height of the full energy neutron capture peak (channel 200, 764 keV) decreases. Maybe some of the more knowledgeable here can offer up an explanation as to why this is.
Image #4: Just for the heck of it I plotted amplifier gain vs. detector bias. An exponential trendline produced the best fit.
Image #5: One of the Reuter-Stokes tubes.

Jon Rosenstiel
Attachments
800 ~ 950.gif
1000 ~ 1150.gif
1200 ~ 1350.gif
V vs gain.gif
IMG_0045.JPG
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Doug Coulter
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Re: Effect of bias voltage on an He-3 tube's pulse height spectrum

Post by Doug Coulter » Tue Oct 02, 2012 12:10 am

Nice plots! I run my proportional tubes a bit on the high side to get more "gas gain", and tend to push them. This makes pulling the real signal out of a lot of EMI a bit easier.
Why guess when you can know? Measure!

Dustin
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Re: Effect of bias voltage on an He-3 tube's pulse height spectrum

Post by Dustin » Tue Oct 02, 2012 3:56 am

This might give some clues,
www.cedengineering.com/upload/Radiation Detectors.pdf

page 16... limited proportional region.

Steve.

Wealth of info here.

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Richard Hull
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Re: Effect of bias voltage on an He-3 tube's pulse height spectrum

Post by Richard Hull » Tue Oct 02, 2012 4:23 pm

Really nice work there Jon! Very professional and well delivered. Those small tubes seem to work well, but I wonder about the pressure in them? The noise floor is a bit close. I hate it when you have to work just above the scud.

See you in a day or two.

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.

Jon Rosenstiel
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UPDATE: Same test, but with known good tube

Post by Jon Rosenstiel » Wed Oct 03, 2012 8:22 pm

I suspect the dumpster find tubes may have been torture-test items, possibly subjected to temperature extremes, or vibration, or super high neutron fluxes, or super high gamma fields, etc, etc. So anyway, I thought it would be a good idea to run a similar test on a detector known to be good.

The Reuter-Stokes RS-P4-0820-103 (1" diameter x 20" effective length) I used for this test was purchased from O. E. Tech several years back and is one of my main neutron detectors. (The other is a Texlium He-3 tube of similar dimensions)

The setup used for this test was the same as before, but instead of using fusor neutrons I used a ~800 μCi AmBe neutron source.

Image #1: +850 V to +1150 V
Image #2: +1250 V to +1550 V
Image #3: +1450 V to +1750 V

Compared to the tube that was the subject of my original post the neutron capture pulse height spectrum for this tube doesn’t exhibit nearly as much degradation at low or high bias voltages. And what I find really interesting is that no matter what the bias the count-rate remains nearly the same. All that really changes is the height and width of the 764 keV neutron capture full energy peak.

BTW, I normally bias this tube at +1600 V.

Jon Rosenstiel
Attachments
850 ~ 1150.gif
1250 ~ 1550.gif
1450 ~ 1750.gif
I like old-timey heavy electronics that you don't have to hold down
with one hand while pushing the on/off button with the other.

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Richard Hull
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Re: UPDATE: Same test, but with known good tube

Post by Richard Hull » Tue Oct 09, 2012 2:51 pm

Jon, as you may know I also run a 1" X 21" Reuter and Stokes 3He tube and my bias is set almost at exactly the same point you bias yours. 1550-1650 volts.

Thanks for this neat info.

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.

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