Liquid Scintillation vs. Pancake GM

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George Schmermund
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Liquid Scintillation vs. Pancake GM

Post by George Schmermund » Fri Nov 11, 2011 12:19 am


Jon R offered the use of his fusor yesterday for testing one of my liquid scintillation/silver foil cells.The cell consists of a 2" PET container into which the Ag foil was spindled and the turns were separated to allow for maximum surface area in the LS cocktail. The outside cylindrical wall of the cell was then wrapped in white Teflon tape and then covered with Al foil. The bottom of the cell was coupled to a 2" PMT with Dow Corning Q2-3067 Optical Couplant. I'm currently testing other potential coupling compounds that are more readily available and much less expensive than the DC Q2 product.

The activation testing was done on a first approximation basis. There was no attention paid to great detail in these tests. Being a generalist, I was only interested in trends and any large difference that might show up between the two detection methods. There are several ways that the LS/Ag cell can be improved upon, but I was just interested in seeing if my idea about high sensitivity was valid using this fairly crude cell. The testing was conducted along with a pure silver dollar taped to a pancake GM detector as the target for comparison. Both detectors were surrounded with similar amounts of moderating material.

The fusor was operated at two rates: 100K n/s and 1,000K n/s. It was easy to follow the Ag 110 beta and allowed short time intervals between runs. After running for 100 sec., the detectors plateaued on the count rate meter. The short activation time allowed little contribution from the longer half life of the Ag 108 betas.

The distance from the center of chamber was ~5.5" for the GM tube and ~15" for the LS/Ag cell. After subtracting background for each detector, the rates were on the order of:

GM/coin @ 100K n/s = ~3 c/s

LS/Ag @ 100K n/s = ~37 c/s

GM/coin @ 1,000K n/s = ~25 c/s

LS/Ag @ 1,000K n/s = ~270 c/s

The lab BkG on GM tube for 100 sec. was ~0.6 c/s

The lab BkG on LS/Ag cell for 100 sec was ~2.5 c/s

My impression at this point (considering that the LS/Ag cell was ~3x further from the source than the GM/coin) is that the LS/Ag cell is dramatically more sensitive than the GM/coin.

Jon R - Thanks for the generous use of your lab and the help you've given me over the last year while I've been playing with these different cell designs.
Anything obvious in high vacuum is probably wrong.

Dan Tibbets
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Re: Liquid Scintillation vs. Pancake GM

Post by Dan Tibbets » Fri Nov 11, 2011 1:56 am

If no Gremlins are hiding in the setup, it seems like your investigation reviels an ~ 100 fold improvement over silver foil over a pancake Geiger tube.
Questions:

Was there a detection comparison to a good neutron detector? I presume your neutron flux was determined from dedicated aneutron counter- what counts was it achieving to derive the flux numbers?

Is it reasonable (perhaps with some tuning) to detect as little as 5,000 - 10,000 N/S fluxes with this approach?. With your data it looks like your setup would give a signal to noise (background) level of ~ 2:1 at this low flux of neutrons.

Is the scintillator better at detecting the betas than the geiger counter?
What is the relative surface areas of silver between the two systems?
Would a similar surface area of silver placed inside a custom G-M tube give similar results?
Was a control of the scintillator without the silver coating run?
How many betas were lost/ intercepted by the mica window of the pancake detector?

It looks like the major variables are -
the sensitivity comparison between the scintillator and GM tube,
the relative silver surface areas,
the effect of the mica window,
the distance (easily accounted for by applying the inverse square law),
the number of betas vs the number of direct neutron detections in the scintillator.

Dan Tibbets

George Schmermund
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Re: Liquid Scintillation vs. Pancake GM

Post by George Schmermund » Fri Nov 11, 2011 2:22 am

Dan - Some of your questions can be answered here:

Counting efficiency for pancake GM tubes is less than 25%. Ref: (Ag- 108 beta energy falls between Cl-36 and Sr/Y-90)
http://www.radpro.com/RSO-10-5-PRS.pdf

Counting efficiency for liquid scintillation ~ 100%. Ref: (P-32 beta is close in energy to Ag-108)
http://www.wikipedia.org/wiki/Liquid_sc ... n_counting

These references are from one of my recent posts, though the information has been generally ignored by the resident pancake heads. I'm a scintillatorian, but I have no interest in starting a range war with them.
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Frank Sanns
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Re: Liquid Scintillation vs. Pancake GM

Post by Frank Sanns » Fri Nov 11, 2011 3:16 am

Nice work George. I had a feeling the liquids would work well. Nice touch maximizing the surface area of the silver. Touche!

Frank Sanns

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Richard Hull
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Re: Liquid Scintillation vs. Pancake GM

Post by Richard Hull » Fri Nov 11, 2011 5:17 pm

The absoprtion of the betas by the mica window is effectively 0.00%; effectively, a non-issue.

Still a lot those other variables to clear up, but at first blush, it looks great for those wishing to spend the bucks and do the scintillator push. Most of the extreme cost will come for those having to buy enough AgNO3 as a compound to saturate the solution. A great savings could be had making your own with pure silver and nitric acid provided you are willing to work with the stuff, neutralize, wash and dry the resultant properly.

If adopted, and to be useful in cross machine comparison, (the real thrust of this entire effort originally), every thing would have to be replicated closely in any setup. One formula needed for everyone. No variance allowed.

Richard Hull
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Carl Willis
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Re: Liquid Scintillation vs. Pancake GM

Post by Carl Willis » Fri Nov 11, 2011 5:53 pm

Hi George,

Good report on this project and its comparison with the coin-on-pancake method.

I'm curious what the dimensions of the foil were. My guess is that most of the advantage in your method relates to the careful geometry, which on a weight or volume basis makes much more effective use of silver for activation purposes than the coin. There is a silver sandwich detector described in Glenn Knoll's radiation detection textbook that takes much the same approach as you do, with the goal being to homogenize the silver into the detector medium as thoroughly as practicable.

-Carl
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Re: Liquid Scintillation vs. Pancake GM [PHOTOS]

Post by Jon Rosenstiel » Fri Nov 11, 2011 7:23 pm

Performance of George's liquid scintillation cell was impressive considering it was nearly 15-inches from the poissor.

Image 1: One ounce 999 silver "coin" taped to my Inspector Alert. (2" pancake)
Image 2: Inspector Alert sandwich.
Images 3 & 4: George's scintillation cell ready for testing. (Moderator support NOT earthquake approved)

Jon R
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George Schmermund
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Re: Liquid Scintillation vs. Pancake GM

Post by George Schmermund » Fri Nov 11, 2011 9:40 pm

For starters, let me say that I'm taking this cell off the table as a "standard detector". The cell design will be described in more detail in another thread. It will be there for anyone to copy or modify as they wish and can utilize it as they see fit. I'm not seeking anyone's approval on the design and construction. But, as far as can be judged at the moment, the LS/Ag cell is vastly more sensitive as a neutron detector than a pancake GM tube/coin. I'll leave the "standard detector" for others to bicker about.

For clarity I'll repeat some of what I've posted previously when I was considering this type of cell for more exacting purposes. I did try using an LS cocktail mixed with a saturated AgNO3 solution. The results weren't as promising as putting BC 412 turnings into a saturated AgNO3 solution w/o the LS. This latter experiment convinced me that the detection of Ag betas was better with just plastic as the scintillator. The problem was in the uniform packing of the turnings.

I still was interested in the LS approach and decided that the silver was best utilized as a foil and not as an AgNO3 solution. Somehow I didn't make it abundantly clear enough that this new cell is made with Ag foil and not an AgNO3 solution. I promise not to refer to saturated solutions again.

I see no need to experiment with more combinations than just LS and some type of foil. Silver seems to be the de facto standard at this point in activation experiments. Indium is in second place. I have a few pounds of extremely pure Indium that I use to make temperature calibration cells, but it's very expensive. Silver fills in nicely here because it is relatively cheap and readily available in any coin store. Many people have some earlier 90% silver coins around the house. That's where I found mine.

The cell that Jon R and I just tested was made from 3 quarters that were rolled into foil. The foil stretched to 15" and was ~0.0035" thick per coin. The shape is oblong on both ends. No effort was made to improve the shape at this point. As stated earlier, the foils were spindled up and spaced to maximize the surface area exposed to the LS cocktail. The first big improvement that is easy to make (though more expensive) would be to start with silver foil 2 or 3 mils thick and make a ribbon of some predetermined length. The plan is to maximize the foil's surface area exposed to the LS while still keeping the foil faces separated.

Of course there will be straw men erected by the naysayers to disqualify this detection method as too expensive, complex, etc. but it's no longer in the running as a "standard", at least by me, so the dissension is moot. I'm presenting it only as a very sensitive neutron detector for those who wish to have one.
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Richard Hull
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Re: Liquid Scintillation vs. Pancake GM

Post by Richard Hull » Fri Nov 11, 2011 10:07 pm

This method may be the third best method of neutron detection for low count systems. I would imagine it would be vastly cheaper than the best two preceeding it; 3He or BF3, unless someone fell into an awfully cheap tube, (unlikely). From what I have seen so far and going on the results stated, this is probably #3 best method in sensitivity to low flux neutrons.

A standard system, as we were talking about earlier, was solely for cross comparison of systems near a peak of operational performance. To be used in research comparisons across systems for newer system designs or major design modifications to detect definite improvements in performance if there should be any. Sensitivity is not such a big deal here near the top of the heap.

It is newbs with small purses that might best benefit from the sensitivity and reduced cost of the PMT/Ag/Scintillator. They could build it any way they choose to hold costs down and still detect real fusion with some ease and at a price far below the proportional tube's sky high prices. They could easily calibrate it at some future date using the tried and true BTI bubble detector route if they wished.

I would imagine a solid scintillator disk could be only 1/4" thick and catch, (stop), all the betas from a foil

Richard Hull
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Steven Sesselmann
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Re: Liquid Scintillation vs. Pancake GM

Post by Steven Sesselmann » Sat Nov 12, 2011 12:47 am

George,

Great experiment, and nice way to do it. Would you by any chance be able to post a picture of the cell?

Another idea that I want to try, is to make a borated moderator into which I place a NaI scintillator, then with PRA I simply discriminate above and below the 480 Kev peak from the B+n reaction.

I was thinking of casting the moderator with a mix of Borax and epoxy, will report when I get a chance to try it.

Steven
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