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Author: Subject: Chemicals for Scintillation detectors ( Radiation detectors)
Canuck
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Chemicals for Scintillation detectors ( Radiation detectors)

Hi there I am pondering about either going the route of a Organic scintillator or a NaI/Tl crystal based scintillator for doing uranium prospecting.

--
BTW: Does anyone here know how to synth Anthracene or Stilbene?

-- I just ordered a sample of BC400 plastic for my project.. but i am wondering about the chemicals.. and how they are manufactured.
IrC
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I have a lot of that plastic, does not work well. You need the NaI/Tl crystal with one face polished very well, and coated to aid light transfer. A friend of mine has much good info:

vulture
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Tl is going to be a bitch to acquire, given it's near chemical weapon status in these times of terror.

Although it was available commercially as a rodent or weedkiller IIRC.

One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
IrC
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There are a few places which still rebuild and repair the old scintillation counters who may have a source for the working crystals. If I were looking, I think I would search ebay for an old broken one cheap and salvage it for the PM tube/crystal assembly and build my own following some of the ideas on CT's page. Just a thought.

PS: as to making one, don't try. 25 years ago I had a compound of thallium salts burst into flames, a few gm in a test tube. I knew Tl was not good so I left the room until the air was cleared but it did not help. Within a few weeks I would start a low grade fever every evening and lay in bed soaking in sweat. I had multiple varied symptoms and kidney pain, none of which any doctor I went to could diagnose. I did not have the internet, nor any data on Tl or it's effects, and did not connect the two incidents at the time. All I knew was I was terribly ill and doctors treated me like it was all in my mind as there were so many varied complaints I went to them with. After a year or so the night fevers stopped and after 5 years or so I no longer noticed all the pains and problems, although to this day that combined with a major industrial chemical fire that happened 2 years after the Tl incident, my health has been generally poor ever since and this is over more than 20 years.

In short all my life before that day my health and energy was more or less ok, ever since I have never regained the fitness of my youth. While I cannot prove it I have no doubt the Tl incident caused major failures in my body which I have never recovered from. My advise to all is stay the hell away from any experiments involving thallium or any of it's compounds. If I had a choice between regaining my quality of life or doing science today I think I would choose life. Some chemicals are simply not worth the price you pay to experiment with and Tl is at the extreme top of my list!

[Edited on 11-6-2006 by IrC]
Marvin
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If you just want to detect ore NaI(Tl) isn't the best choice and I agree thallium isn't worth the risk. Even for spectroscopy in which this used to be king the much superior LaCe Halides are making inroads despite very high prices currently.

CsI(Na) may be a decent choice for home growing, but somewhat expensive. Growing from solution may not be possible, commercially they are pulled from a melt. You may well need a clean furnace for this one.

If you want to buy a crystal for prospecting that will perform well a 2x2 inch NaI(Tl) in a hermetically sealed case (Its moisture sensitive) will set you back about 220 USD from Saint Gobain. If you have the photomultipler and electronics allready or can build it yourself this may work out cheaper and much safer than trying to grow your own crystals. Ive allready said NaI(Tl) isn't the best, but relatively speaking its the cheap option and does perform well.

NaI by itself is a pretty good emitter but it has a problem. It has an absorbtion band that overlaps its emission band. These are so close that at room temperature NaI produces only about 2% of the light of NaI(Tl). The thallium introduces an emission band in the lattice the solid is transparent to. Think about doping in semiconductors and this may make sense.

For those people doing experiments at home this brings up an interesting possibility. Energy levels in crystals are thermally broadened, as you cool NaI the absorption and emission lines narrow and therefore overlap less, cooled enough the lines nolonger overlap at all and undoped NaI becomes a very good scintillator. At liquid nitrogen temperatures undoped NaI has twice the light yeild of room temperature NaI(Tl) (which gets worse as you cool it anyway). Photomultipliers also work better at liquid nitrogen temperatures. One of the things I want to try is growing an NaI crystal, which may be possible from solution (at low temperatures a hydrate is produced, neer 100C NaI crystals should be produced) and then put it in a cooled system for spectroscopy. This could be a homebrew way to produce a system as good or better than most commercial equipment. Not so useful for prospecting but other grown from solution crystals may perform better.

As far as I am aware it isnt possible to make doped crystals from solution, it needs to be done from a melt.

One more point for people playing with NaI, it should be as neer as possible potassium free. Potassium contains a radioactive isotope and hard as potassium is to remove from sodium salts, this will contribute a great deal of background pulses if not well purified. Methods of homebrewing NaI that involve recrystalising from KI are to be avoided.
Phel
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 Quote: Originally posted by Canuck -- BTW: Does anyone here know how to synth Anthracene or Stilbene?

I've made t-stilbene in the lab by performing an Arbuzov reaction between benzyl chloride and triethyl phosphite, yielding diethyl benzylphosphonate. From there, a Horner-Emmons reaction with sodium methoxide and benzaldehyde in DMF gave t-stilbene in 49% yield. Unfortunately, the chemicals are expensive and somewhat exotic/restricted.
I remember using Beilstein to search for other more cheap and OTC preparations for this synthesis, but without luck. The closest I remember finding was a dehydrocoupling of toulene to stilbene, see USPT# 3,965,206.
Marvin
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Maybe worth mentioning that napthalene will give a signal on an ordinary photomultipler. Its spectrum is a bit furthur into the UV, but it does aparently work.

The organic scintillators are ok for beta detection but next to useless for gamma.

[Edited on 13-6-2006 by Marvin]
Canuck
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I have heard the BC400/NE103 plastic is actually very good and up to 50% yeild of NaI/Tl.
I just got some prototyped plastic (fast counting) used for detecting Gamma and beta from Uranium 235 and U238 as it is less \$ than NaI/Tl & not hydroscopic!
Just got some from Dr.Zarkoff of Atomic Rocks who has a highly thought of reputation.
Also GEOelectonics is getting me a plastic 3" dia sensitive tube and preamp that'll work to FAR blue to near UV specs. I'll post pics when the assembling and adjusting is all done.

I tried Naphalene as a scintillation material but it didn't work by itself very well.. I think I need a shifter.. a solvent? What about combining the naphalene with Sodium Flourescene in a polystyrene round to shift the spectrum to green?

[Edited on 13-6-2006 by Canuck]
unionised
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I realise that you are after gammas not neutrons but some of the ideas in this
http://www.doylegroup.harvard.edu/neutron/publications/These...
might be interesting.
Salicylates are OTC and downshift UV to blueish visible quite nicely (though I think fluorecsein is a prettier option).
Has anyone done anything with really silly quantities of simple scintillators? I could scarcely afford a few inch
Na(Tl)I crystal but I could afford a bucket of sodium salicylate solution. I could even afford to dope it with NaI or Pb(CH3COO)2 to give the gammas something heavy to hit.
Marvin
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The best photomultipliers are most sensitive to blue usually, so a blue glowing dye would be preferable to a yellow of similar light yeild. CsI(Tl) for example actually has a higher light yeild, in terms of photons than NaI(Tl) but that the photons are (mostly) yellow reduces the photomultipler signal to either a third or half of the NaI(Tl) signal (Offhand I can't remeber exactly).

I'm not convinced a few heavy atoms in the scintillator will improve matters, though wrapping it in a metal foil may increase its gamma response. In the thesis the boron atoms are there to fission on contact with neutrons and its the charged heavy fragments that then stimulate the scintillator. The other thing to remeber about that thesis is that the performance is utterly rubbish.

For BC400, 50% of NaI(Tl) sounds overly optimistic. ITs data sheet says 65% of anthracene, which is the strandard most plastics are measured against being the brightest organic scintillator. Crystaline anthracene has about half the light yeild of NaI(Tl). I'd put BC400 at about 1/3rd of Na(Tl) under the best circumstances and with nowhere neer the gamma stopping power.

[Edited on 14-6-2006 by Marvin]
Canuck
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New plastics are higher yeild

 Quote: Originally posted by Marvin The best photomultipliers are most sensitive to blue usually, so a blue glowing dye would be preferable to a yellow of similar light yeild. CsI(Tl) for example actually has a higher light yeild, in terms of photons than NaI(Tl) but that the photons are (mostly) yellow reduces the photomultipler signal to either a third or half of the NaI(Tl) signal (Offhand I can't remeber exactly). I'm not convinced a few heavy atoms in the scintillator will improve matters, though wrapping it in a metal foil may increase its gamma response. In the thesis the boron atoms are there to fission on contact with neutrons and its the charged heavy fragments that then stimulate the scintillator. The other thing to remeber about that thesis is that the performance is utterly rubbish. For BC400, 50% of NaI(Tl) sounds overly optimistic. ITs data sheet says 65% of anthracene, which is the strandard most plastics are measured against being the brightest organic scintillator. Crystaline anthracene has about half the light yeild of NaI(Tl). I'd put BC400 at about 1/3rd of Na(Tl) under the best circumstances and with nowhere neer the gamma stopping power. [Edited on 14-6-2006 by Marvin]

Well, this turns out not to be BC400 that I have but some prototype russian plastic that is high yeild. It has both Anthracene and Stilbene with a plastic that is by itsself a scintillator and a spectra shifter, it also has infused Pb to stop gamma.
I'll get the company's name.. it is stuff you just might want to try!

 Sciencemadness Discussion Board » Special topics » Technochemistry » Chemicals for Scintillation detectors ( Radiation detectors) Select A Forum Fundamentals   » Chemistry in General   » Organic Chemistry   » Reagents and Apparatus Acquisition   » Beginnings   » Responsible Practices   » Miscellaneous   » The Wiki Special topics   » Technochemistry   » Energetic Materials   » Biochemistry   » Radiochemistry   » Computational Models and Techniques   » Prepublication Non-chemistry   » Forum Matters   » Legal and Societal Issues