Liquid Scintillation vs. Pancake GM

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George Schmermund
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Liquid Scintillation vs. Pancake GM

Post by George Schmermund »


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.
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Re: Liquid Scintillation vs. Pancake GM

Post by Dan Tibbets »

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
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Re: Liquid Scintillation vs. Pancake GM

Post by George Schmermund »

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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Re: Liquid Scintillation vs. Pancake GM

Post by Frank Sanns »

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

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Re: Liquid Scintillation vs. Pancake GM

Post by Richard Hull »

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.

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Re: Liquid Scintillation vs. Pancake GM

Post by Carl Willis »

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.

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Re: Liquid Scintillation vs. Pancake GM [PHOTOS]

Post by Jon Rosenstiel »

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

Images 1,2: George's scintillation cell ready for testing. (Moderator support NOT earthquake approved)
Image 3: Inspector Alert sandwich.
Image 4: One ounce 999 silver "coin" taped to my Inspector Alert. (2" pancake)
Jon R
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George Schmermund
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Re: Liquid Scintillation vs. Pancake GM

Post by George Schmermund »

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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Re: Liquid Scintillation vs. Pancake GM

Post by Richard Hull »

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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Re: Liquid Scintillation vs. Pancake GM

Post by Steven Sesselmann »

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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Re: Liquid Scintillation vs. Pancake GM

Post by Frank Sanns »

It may be better than you are giving credit Richard. To make a fair comparison, either price or detection area needs to be considered.

Using one of our large He3 dimensions of 22" x 1" that is a surface area of 2 x pi x r x h. Solving gives 69 square inches of surface area. This can be doubled with a little ingenuity in a liquid cell so both side of the silver/liquid scintillator emissions can be detected by a PMT. Silver can be rolled out into strips to construct such a cell and at $40 and ounce of silver that would allow for ten ounces of the metal to get to a discount price of an He3.

A cell with ten ounces of silver rolled out to 69 square inches should yield a hell of a cell with 138 square inches of effective detection area. Just sayin.

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Re: Liquid Scintillation vs. Pancake GM

Post by George Schmermund »

Frank - Thanks for understanding what I'm trying to do here. For reference, let me say that this cell used 3 quarters. That represents less than 20 grams of silver content. The rolling stretched them out to ~1" wide and ~15" long. The three of them then make ~45 sq.". Therefore, ~90 sq." total for both sides. I used a hand corrugator to zig zag one of the foils to make the spacer for separating the spindled other foils. This controls the uniform spacing between each facing foil and adds to the amount of available silver for activation.

I appreciate your 3He tube analogue and the heroic amount of silver you compared it to, but 1 ounce of foil at 2 or 3 mils thickness would cram this small cell. That's not to say that a bigger cell with more silver in it wouldn't be a better detector, I just don't want people to assume that large quantities of silver are necessary for getting good results.

As far as Richard's assessment that this detector would only be of interest to newbs because a highly sensitive detector is of little value or interest to the real players here, well, what can I say?
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Re: Liquid Scintillation vs. Pancake GM

Post by Frank Sanns »

Beta particles will only traverse the thinest of metal films so my guess is if you could get 0.005" to 0.015" silver foil you would be good to go. You might even be able to get away with using ammoniacal silver and a reducing sugar to plate out a thin film of silver out onto a plastic or glass structure.

Thanks for the work George. I will have to fire up my fusor soon and give this a whirl.

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Re: Liquid Scintillation vs. Pancake GM

Post by Dan Tibbets »

Perhaps a stupid question, but isn't Borno10 an excellent neutron absorber? Wouldn't that decrease the aviable neutrons left for detection within the detector? How good of a neutron moderator is B10? I would not think it great based on it's atomic weight. Does this effect have benefits greater than the neutron absorbing properties that would allow for a net detection benefit? Are you considering some resonance peak for the silver?

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Re: Liquid Scintillation vs. Pancake GM

Post by Carl Willis »

Steven's idea, which is an entirely different animal from the thread's topic, is to detect the 478-keV gamma ray from the product of the first reaction below:

B-10(n,a)Li-7* (94%)
B-10(n,a)Li-7 (6%)

For examples of this gamma ray being detected see:
viewtopic.php?f=13&t=5478#p33922
viewtopic.php?f=13&t=5280#p33724

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Re: Liquid Scintillation vs. Pancake GM

Post by Brian_Gage »

Being new, I had a time deciphering the acronyms. Okay, PET is a polyethylene plastic, the container was 2 inches. Was that diameter? And how tall? Did you improvise with a recycled food or medicine container? What is the liquid inside this container? Why was the scintillation counter placed so much further away from the neutron source? I'd like to build a scintillation detector, maybe even a counter (with help since I can't see). Right now I've only got a Monitor 4 EC linked to my computer for small, rather tame experiments.
Appreciate any feedback on your post. Regards. Brian
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Re: Liquid Scintillation vs. Pancake GM

Post by George Schmermund »

Brian - The container was from eBay: item #250810955009. I chose the ones with the white lids.

This is the LS cocktail that I used: http://www.mpbio.com/product_info.php?f ... y=01882470

The !5" distance was chosen because that was as close as the detector would fit. The next detector will hopefully fit on the oven platform where the GM/coin was tested.

A new and improved design is already in the works, but the basics are the same. I'll post some pix and new info soon.
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Re: Liquid Scintillation vs. Pancake GM

Post by Richard Hester »

Interesting - using the liquid scintillator, you could count the proton recoil traces that happen in the process of moderation, and then verify your findings by looking at the activation induced by the moderated neutrons.
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Re: Liquid Scintillation vs. Pancake GM

Post by George Schmermund »

Frank - Your idea of using Tollen's reagent / reducing sugar Is a good way to silver the inside of the cell. This would provide the inside surface with a highly reflective deposit and also add more Ag for activation. My next cell design incorporates the same idea, but I'll be coating the inside wall with evaporated Ag from a hot filament. I have this method ready at hand and it will require no wet chemistry except for the cleaning process to prep the cell for deposition. This method will also allow for considerable film thickness which is easy to monitor and control. The more silver the better. Your method is also excellent and within reach of those who don't have a vacuum deposition system available.
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Re: Liquid Scintillation vs. Pancake GM

Post by Frank Sanns »

It wasn't liquid but back in the days when we were still looking at Hornyak buttons for neutron detection, I always wanted to make one with silver foil convoluted into the scintillator. Looks like you have taken it way beyond. Making me itch to get back to some of this stuff and stop putting out the fires that others light.

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Re: Liquid Scintillation vs. Pancake GM

Post by George Schmermund »

Carl - I finally got around to doing an accurate measurement of the foil surface area. The weight was easily available from the mint website. But I guessed at the surface area since it wasn't simple to get a really good number from the foil because it ends up shaped kind of like a scimitar after extensive rolling. Today I rolled out more quarters to the same thickness, length, and width as the test samples. They come out remarkably similar each time.

Anyhow, I took a piece of 8.5" x 11" paper and used it as a template to cut a piece of aluminum foil. This makes it easy to get a good number for the weight of a sq." of Al foil. I then used an Exacto blade to trace the rolled out coin on another piece of Al foil. It works out to be 11.54 sq." of Ag foil for each face. That makes a little over 69 sq. " foil area in the cell that Jon R tested. This number is encouraging because it will be easy to put much more surface area into the same size cell by using squared up foil stock.
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Re: Liquid Scintillation vs. Pancake GM

Post by Richard Hester »

Toluene is available in gallon lots at the Evil Orange and other hardware stores. A few grams of fluorescent materials transforms it into a scintillation cocktail, though some treatment (bubbling through nitrogen to get rid of fluorescence-quenching dissolved oxygen) may help to optimize things. Though the toluene is not near as dense as polyethylene or polypropylene, it will moderate fast neutrons. The process of moderation generates recoil protons that can be detected by a PMT from the havoc they induce in the scintillator. The point is one could use this as a moderator for a silver foil, count the prompt recoil proton reactions, then turn right around and count the delayed silver activity with the same solution and PMT.
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Re: Liquid Scintillation vs. Pancake GM (Inexpensive Cocktail)

Post by Richard Hester »

It's amazing what you can find if you nose around a bit using Google - in this case a citation regarding a less expensive/toxic/flammable/volatile liquid scintillator cocktail based on medicinal paraffin (mineral oil to U.S. readers - the scientists were Brits...). It's about 1/3 as sensitive as a standard liquid scintillator, but not flammable/volatile. It doesn't suck a lot of expensive fluors, either The mixture consists of mineral oil with 0.8 g/liter p-terphenyl and 0.008 g/liter POPOP. The purified oil from the drug store works better than the industrial variety. As a comparison, one of the sources I saw was talking about 100g/liter of PPO in toluene - that's a lot of money!

Another citation mentiions a mixture of 3g/liter PPO and and 0.02g/liter POPOP in mineral oil. This particular mixture has 28% of the activity of a toluene solution, but perks up to 67% if one adds 90g/liter of naphthalene.

Both of these mixtures look like attractive candidates for a combination moderator/scintillator, both for simple activation experiments and a combination of recoil proton/activation measurements. The lack of volatility and compatibility with plastics like perspex/lucite is a plus.
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Re: Liquid Scintillation vs. Pancake GM (Inexpensive Cocktail)

Post by Richard Hester »

Another interesting twist in the standard detector might be to use the mineral oit scintillator as a moderator for one of those Russian corona tubes, monitoring the scintillator with a PMT the same time. That way you get prompt readings from the fast neutrons via recoil and the same neutrons moderated via the corona tube.
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Re: Liquid Scintillation vs. Pancake GM (Inexpensive Cocktail)

Post by Dustin »

This is very interesting, thanks George.
Perhaps silver wire wound on a frame would increase surface area
or treating the foils with sandpaper.
Maybe even silver plated coiled "flywire mesh".
Some good coax has silver plated braid that would have a huge surface area. Carefully removing it, you could fill your container with tubes of braid.
Steve
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