Liquid Scintillation vs. Pancake GM

[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.

[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.

[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.

[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.

[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