He-3 Tube Pulse Height Spectrum

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Carl Willis
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He-3 Tube Pulse Height Spectrum

Post by Carl Willis »

This past weekend I went to Pittsburgh and visited Frank S. with some NIM stuff, the main purpose of which was to check out the new He-3 proportional tubes he got from OETech. I ended up going home with one. This is a sweet tube and hopefully a few details here will be of use to all who aspire to get a He-3 or already have one.

Tube Specs: Reuter-Stokes RS-P4-0820-103
Length: 22 inches Diameter: 1 inch
Fill Gas: P4 (proprietary mixture) at 5.3 Atm.
(contains ~4 Atm. He-3, ~1.2 Atm. Ar, ~0.1 Atm. methane)
Recommended Bias Voltage: 1680 V

Argon improves the stopping power of the fill gas, causing a reduction in wall effects. It also may increase the ion mobility to reduce ballistic deficit. Methane is a quench gas that improves proportionality of response.

The top photo below shows my NIM setup with the He-3 tube sheathed in a 6" diameter paraffin-filled acrylic pipe a la Frank S. A very weak neutron source is taped about half-way up the pipe. The preamp is an Ortec 118 with 500 MOhm load resistance. The shaping main amp is a Princeton Gamma Tech 346, recently repaired (!) from a major power surge that blew nearly every tantalum cap on the board and a few other odds and ends. The Tracor-Northern MCA is hooked to my PC by serial cable. Its TTL circuits must be supplied with external 5 V from the power supply on top of the bin, because parts of the MCA got toasted in the same power surge that roasted the amp. It still has a lot of life left in it so I have bought the parts necessary to make a complete repair. The HV supply on right is providing a bias of 1680 V.

Because He-3 ions migrate slowly, a long shaping constant is required in the amp or ballistic deficit will smear down the full-energy peak. I use a 4 uS shaping time.

The second image is the pulse height spectrum obtained by counting with the neutron source attached as shown for 8725 s. During this time, 16857 neutrons were counted in the part of the spectrum to the right of the red LLD line...almost two per second! The spectrum is self-calibrating. The full-energy peak always shows up at 764 keV, the Q-value of the He-3(n,p) reaction. The low-energy end of the neutron capture spectrum is defined by a step at 191 keV, the energy deposited by the H-3 product only (if the proton is lost into the counter wall). The FE peak has a continuum on its low energy side caused by some wall effect (loss of the triton to the tube wall) and also some ballistic deficit which is really unavoidable in the circumstances. You know when you see this shape of spectrum that you are detecting thermal neutrons.

Finally, I should mention that the bare He-3 tube can be used as a fast-neutron spectrometer. Although capture cross-section drops with neutron energy, you can still get a peak at FE + En where En is the incident neutron energy > ~1 MeV. Folks with both a fusor and a He-3 tube should keep this in mind!

-Carl
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Frank Sanns
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Re: He-3 Tube Pulse Height Spectrum

Post by Frank Sanns »

Here are 2 more.

Frank S.
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Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Re: He-3 Tube Pulse Height Spectrum

Post by Frank Sanns »

Arrrrrg!
Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Re: He-3 Tube Pulse Height Spectrum

Post by Richard Hull »

This is great work Carl and Frank. it is nice to see this stuff being done.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Carl Willis
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Re: He-3 Tube Pulse Height Spectrum

Post by Carl Willis »

I'm going to post my pics temporarily at a Gmail account, and others are welcome to use same account.

www.gmail.com
Username: americium.241
Password: Am-241

Hope it works.
-Carl
Carl Willis
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TEL: +1-505-412-3277
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Carl Willis
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Re: He-3 Tube Pulse Height Spectrum

Post by Carl Willis »

The pictures seem to be working now, so I have cleaned up my redundant posts. All the images are evidently resized for display now, and the pulse height graph has to be clicked on to see in sufficient detail, but it is much improved from having nothing. Thanks Ryan!!

Now I can see Frank's pictures of his He-3 spectra taken using a delay-line amp (this was before I had repaired the PGT gaussian shaping amp I used last night.) There is a lot of down-smearing of the peak due to ballistic deficit, which occurs when charge collection in the tube is slower than the pulse shaping time and hence you lose proportionality on some pulses. Max shaping time on the DL amp is only 0.25 us and cannot be changed. I'd recommend 1 us at least. I will have another post shortly that shows how shaping time influences the spectrum.

-Carl
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