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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
Someone mentioned that PG + ZnS:Ag scintillator was about the best known low energy particle to light converter known. so far the biggest piece i can find is this. obviously 6mm will not work, i have managed to sand it down (after i split it to <0.2mm thickness) and confirmed that it works well.
Proud Mary informs me that carbon does not significantly add x-rays etc so in theory i could build a particle counter and spectrometer that is so small it fits into a pen. to make this work would require an open type CMOS sensor, ZnS:Ag powder or sheet, pyrolytic graphite and some sort of opaque (perhaps low melt alloy) casing.. to do spectrometry i'd need to deposit the metal layers onto the PG at right angles to the scan direction in an increasing density. Probably requiring numerous photoetch steps most likely.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
I've bought all these things on ebay, and so have others that I know.
The advantage of buying BGO or plastic scintillator second-hand on ebay is that it usually has one of more faces already machined, so you can couple it directly to a PMT will optical grease. (I use Canada Balsam, because I happen to have some, and because its refractive index is fairly close to that of common plastic scintillators and PMT end window glass.)
Then you just have to wrap the whole caboodle up in kitchen foil or metalized Mylar (cheap emergency 'space blankets') and you're ready to roll!
If you have practical skills of the mirror grinding type (which I do not) you can perfect all the faces of a plastic scintillator to get total internal reflection, but I just wrap it up in kitchen foil, or 'space blanket' - which admits less penetrating radiation, but is very easy to damage or scuff at the pinhole level which will dazzle your PMT, and perhaps even damage it beyond repair if the hole be large enough.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Proud Mary wrote ...
If you have practical skills of the mirror grinding type (which I do not) you can perfect all the faces of a plastic scintillator to get total internal reflection, but I just wrap it up in kitchen foil, or 'space blanket' - which admits less penetrating radiation, but is very easy to damage or scuff at the pinhole level which will dazzle your PMT, and perhaps even damage it beyond repair if the hole be large enough.
All aluminized Mylar I've seen has such a thin metal coating you can easily see a bright light through it even in the abscence of pinholes so I'm surprised it's good enough for the job! In fact I think the stuff used in cheap solar eclipse viewing glasses actually had a thicker than usual coating if I remember correctly from the last time I was in Cornwall! Are PMT's not as sensitive as I'd assumed? I thought they detected individual photons?
The quality of optical finish needed for decent total internal reflection should be quite easy to achieve. A series of progressively finer sheets of Wet & Dry/carborundum abrasive paper used wet and, followed by metal polish would do I think. Make sure you remove all scratches from the previous grade before moving to the next, scrupulously clean everything between grades, and look to finish with finer than 1000 grade at least (I think the grit scales vary - the last batch of 1000 grit paper I bough was more like the 600 grade of my previous experience. I had to get some 2500 grade to compensate. The process will feed the obsessive compulsive in you, but may result in repetitive strain injury After all that I think you'd want to surround it with something reflective anyway as not all photons will have a shallow enough angle to internally reflect.
I noticed some years ago scintillation plastic being sold which looked in the photographs just like the much less expensive decorative fluorescent coloured Perspex sheet and rod you can pick up easily. I don't know if the expensive stuff is really any better?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
Proud Mary wrote ...
If you have practical skills of the mirror grinding type (which I do not) you can perfect all the faces of a plastic scintillator to get total internal reflection, but I just wrap it up in kitchen foil, or 'space blanket' - which admits less penetrating radiation, but is very easy to damage or scuff at the pinhole level which will dazzle your PMT, and perhaps even damage it beyond repair if the hole be large enough.
All aluminized Mylar I've seen has such a thin metal coating you can easily see a bright light through it even in the abscence of pinholes so I'm surprised it's good enough for the job! In fact I think the stuff used in cheap solar eclipse viewing glasses actually had a thicker than usual coating if I remember correctly from the last time I was in Cornwall! Are PMT's not as sensitive as I'd assumed? I thought they detected individual photons?
The quality of optical finish needed for decent total internal reflection should be quite easy to achieve. A series of progressively finer sheets of Wet & Dry/carborundum abrasive paper used wet and, followed by metal polish would do I think. Make sure you remove all scratches from the previous grade before moving to the next, scrupulously clean everything between grades, and look to finish with finer than 1000 grade at least (I think the grit scales vary - the last batch of 1000 grit paper I bough was more like the 600 grade of my previous experience. I had to get some 2500 grade to compensate. The process will feed the obsessive compulsive in you, but may result in repetitive strain injury After all that I think you'd want to surround it with something reflective anyway as not all photons will have a shallow enough angle to internally reflect.
I noticed some years ago scintillation plastic being sold which looked in the photographs just like the much less expensive decorative fluorescent coloured Perspex sheet and rod you can pick up easily. I don't know if the expensive stuff is really any better?
With the 'space blankets,' it's a question of finding an area that is devoid of defects, which I have found quite easy to do, though sometimes tedious, but I suppose they may well be of variable quality.
Grinding away with progressively finer and finer grades of wet and dry is not a fruitful passtime for ungulates, so I find it less bothersome and probably less expensive to buy scintillator blocks ready made.
I don't know anything about decorative coloured Perspex sheet, I'm afraid, and so can't comment, but the material I use is from the St. Gobain premium plastic scintillator range - BC-408 and BC-412, whose data sheets can be viewed here:
Neither is good for X-rays <100keV, but there is Gd2O2S:Tb3+ and other instruments for that.
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
Proud Mary wrote ...
Grinding away with progressively finer and finer grades of wet and dry is not a fruitful passtime for ungulates, so I find it less bothersome and probably less expensive to buy scintillator blocks ready made.
With a full set of fingers and opposable thumbs and a strange sense of satisfaction from perfecting a surface, I might be able to help if you want to try your next scintillator ground and polished. Or maybe your ready made ones are already polished? Thinking on, I wonder if manufacturers save effort by using heat or a solvent to 'polish' the surface.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
IntraWinding wrote ...
Proud Mary wrote ...
Grinding away with progressively finer and finer grades of wet and dry is not a fruitful passtime for ungulates, so I find it less bothersome and probably less expensive to buy scintillator blocks ready made.
With a full set of fingers and opposable thumbs and a strange sense of satisfaction from perfecting a surface, I might be able to help if you want to try your next scintillator ground and polished. Or maybe your ready made ones are already polished? Thinking on, I wonder if manufacturers save effort by using heat or a solvent to 'polish' the surface.
Bye the way, is your Hippopotamus yawning?
That hippo is NOT yawning, but is SHOUTING ABUSE at Members of Parliament on the Thames Embankment as they queue up to receive their EXPENSES and other Free Money in the run up to the General Election. Fancy not knowing something as obvious as that!
Now, with the serious stuff out of the way!
There are accounts of making paddle-shaped plastic scintillators and how to grind and burnish them for total internal reflection here:
I've seen online several high school projects giving full accounts of scintillator paddle polishing for muon detection, but haven't been able to find them again today.
Yesterday I got a parcel in the post from Alex in Odessa from whom I'd ordered an SI-22G GM tube, and what a good tube it is too! It had the tired look of prolonged storage (since 1991, I suppose) but polished up beautifully after 10 minutes with 'Brasso.'
It is 215mm long, 18.5mm in diameter, with a metal skin, and brass electrodes at either end. Anode voltage is 390V; anode resistance 9-14M. I stuck 10M on the anode, and hooked it up to one of my Mini-Monitors (which have an HT adjust pot on the board inside the case) and it came in at about 600 cps at 3mm (now the standard test distance) from a pitchblende check source. The check source goes 100uGy/hr @ 3mm, so very roughly this works out at 6 cps = 1uGy/hr for the SI-22G. (I say 'very roughly' because at such low levels, radiation produced by the tube itself become significant.)
So it is as sensitive as can be without causing a lot of bother, and very good value. I've sent off for another one, so that I can configure the two of them together to have optional coincidence or anti-coincidence counting, or simply tube 1 plus tube 2 counts.
There's a refreshing new wave in instrument design going on, and my ideas for the physical layout of the coincidence counter have been influenced by this splendid radiation detector:
Registered Member #2261
Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581
No polishing surprises there, although I wouldn't use a Dremel!
On other aspects of scintillation crystal use, I'm a bit concerned about wrapping a scintillation crystal. Any contact with surface with a refractive index greater than air (i.e. anything) will degrade the range of angles over which TIR can occur. On that basis I would try to keep contact with the crystal to a minimum.
Also, my theory's not up to the analysis, but I wonder if a diffuse reflector would get more photons into the PMT than a specular one?
That SI-22G GM tube of yours looks almost identical to my SI-21G, but smaller. Do you think mine is likely to match your 390V Anode voltage & 9-14M Anode resistance ? Does the 390V figure indicate halogen quenching? (My Geiger counter build is on hold 'till the 'scope turns up...)
Yes, that's a nice looking detector. I bet it costs more than a car though!
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(Group Buy) Pyrolytic graphite
After all that I think you'd want to surround it with something reflective anyway as not all photons will have a shallow enough angle to internally reflect.
