Testing a Lithium 6 glass fiber paddle

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Jim Kovalchick
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Testing a Lithium 6 glass fiber paddle

Post by Jim Kovalchick » Sun Apr 26, 2020 4:31 pm

This morning morning's fusion run was about testing out a different neutron detector. The PNC is of course portable and not sensitive by more modern standards. I would like to equip my installation with something better. About a handful of years ago when my son was the master mind of the fusion in my house, I bought a Li 6 paddle detector from an important member of this forum. My son tinkered with it a little, but mostly the detector has sat in a cabinet. I brought it out of hiding today.
There are some impure, sinful things I must confess to at the start:
1. I do not know the specs on the pmt with this detector, and I did not do a precise plot to find the plateau. I used an antique RCA technician radium source to find the knee and left the voltage there. It is about 1200 V positive.
2. I don't have a proper preamp. I passed the signal through a NIM spectroscopy amplifier pretty well cranked.
3. The signal went to a scaler by way of an SCA first. I tried dialing out the radium source with the SCA threshold to get a reasonably low background to start, but given this paddle probably sees its share of high energy natural sources I was afraid to take too much off. I started with a background of about about 50 cpm having dialed out about 30,000 cpm of radium daughters. When I started my fusion run, I found the initial count rate was ridiculous even with amplifier PUR chugging away. When I pulled the moderator block off, counts actually went up indicating I was seeing lots of xrays. I used the SCA to add more discrimination until removing the moderator dropped counts significantly. I had it down to about 5 percent left when I pulled the hdpe.
4. Speaking of hdpe, I dont have enough on hand to place some on both sides of the detector. I propped up the bottom of the detector flat on my work bench near the base of the fusor and put a very large block of 2" thick hdpe on top. I may get some reflection from the wooden work bench but I know it's not optimal.

I was relatively pleased with the paddle's performance. When my PNC read about 1000 cpm, the paddle was reading 16,000 cpm about 5 inches further away. This paddle reminds me more of an He 3 detector in sensitivity. I adjusted the fusor voltage and current and found the paddle to be fairly sensitive to small changes. I may try to quantify that relationship during future runs because after all it's not just about watching numbers turn.

Most importantly, as I stated before, removing the moderator block mades counts almost disappear.

The geometry of this paddle in comparison to the small chamber along with the lack of documentation makes it impossible to directly infer a fusion rate without correlating to another calibrated method. It does seem to be a decent relative meter though.

Jim K
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Richard Hull
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Re: Testing a Lithium 6 glass fiber paddle

Post by Richard Hull » Mon Apr 27, 2020 4:12 am

Sounds really good! The more counts you can honestly get, the lower you can detect the beginning of fusion, very weak neutron sources, and detect slight differentials in improvement or losses. It is why I use my big 3He tube to get thousands of counts per minute when the fusor is really doing its thing.

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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Jim Kovalchick
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Re: Testing a Lithium 6 glass fiber paddle

Post by Jim Kovalchick » Thu Apr 30, 2020 11:50 am

I am rethinking how I use this paddle with a fusor. I strongly suspect that the paddle is LiF:ZnS(Ag) bound to plastic light fibers. The ZnS scintillator is very difficult to use for energy discrimination by pulse height. Typically, it's done with pulse shape discrimination which requires hardware I don't currently have.

I had previously cranked up discrimination to knock out counts from a radium source. I noticed that during fusor operation, pulling the moderator block off the paddle caused counts to go up not down. I assumed this meant I was not using enough threshold discrimination so I raised it. This seemed to work. I was then able to see counts go down when the block was pulled.

Yesterday, at a higher voltage (38 kV) than my previous test, I pulled the moderator only to find counts would race up again. Raising discrimination only served to drop total counts and did not alter the moderator test behavior. I lowered voltage to 28 kV and found that there, pulling the moderator caused counts to go down.

I postulate now that the higher energy xrays making through my shielding are at an energy level that runs them into neutrons. I still have to resolve in my mind why I was able to use threshold discrimination to make a radium source invisible, but supposedly lower energy xrays are not.

I am also trying to wrap my head around the potential impact of proton recoil from fast neutrons. Because ZnS will react to proton recoil because there is a hydrogenous plastic within the detector, is there an effect of raising counts from fast neutrons when I remove the moderator? One might suppose that effect would exist at 28 kV as well as 38 kV.

My next steps will try to better understand the xrays impact on the paddle. My fusor is currently shielded by a patchwork of thin lead sheet wrapped on the chamber body. It attenuates the xrays to reasonable dose levels, but obviously at higher energies more xray energy makes it through. I have some larger sheets of 1/8" lead that I intend to use now to either shield the chamber or the paddle. Perhaps I will be able to discriminate by using lead.

Jim K

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Re: Testing a Lithium 6 glass fiber paddle

Post by Richard Hull » Thu Apr 30, 2020 7:43 pm

Wow! Good work in continuing to try to turn this detector to good purpose in neutron counting. Scintillation counters are tough to bring to bear on pure neutron counting. The Bicron, BC-720 is reliable in very high Roentgen high x-ray and gamma fields only due to its terrible neutron counting efficiency. (Lotsa' neuts needed to make it tick.) The BC-720 doesn't much want to count anything! In the Bicron literature, they note that if you don't sufficiently shield the PMT in very high gamma fields the PMT itself will add counts due to Compton scattering and even pair production from the PMT guts if the gammas are hot enough. It is rare in neutron counting to not have some attendant EM radiation. Fortunately for us, the fusor is 100% x-ray that is easily shielded.

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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