NaI scintillator resolution and rejuvenation

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Q
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Re: NaI scintillator resolution and rejuvenation

Post by Q » Sun Mar 19, 2006 10:29 pm

hmmm, a home grown scintillation crystal!
very interesting! since i'm not at the sprctroscopy stage yet, i may give that a try.
one question that comes to mind though, aren't NaI (and other crystals as well) usually doped with some other metal like titanium? how would one go about doing this in a home lab situation?

Q

Richard Hester
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Re: NaI scintillator resolution and rejuvenation

Post by Richard Hester » Sun Mar 19, 2006 10:39 pm

NaI crystals are doped with Thallium, which is exceedingly poisonous - not something I'd want around the house. Back when I was little (a long, long time ago), Thallium Sulfate was used as an ant poison - it's colorless, odorless and tasteless. A couple of kids died by Thallium poisoning through skin absorbtion playing with the "empty" tins. The stuff was withdrawn from the market. The doping percentage is pretty small for a NaI:Tl crystal, but I wouldn't want the stuff around unless I had a professional quality lab setup.

Jon Rosenstiel
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Re: NaI scintillator resolution and rejuvenation

Post by Jon Rosenstiel » Sun Mar 19, 2006 10:50 pm

Here is my $ .02 on this subject....
My original intent was to build a detector suitable for gamma spectroscopy using one of Frank S’s BGO xtals. Then, this thread came up and got me thinking about a multiple crystal detector. Being I had a small supply of the BGO crystals I decided to build and test a multiple crystal detector. Two goals: One, would multiple crystals optically coupled act as one large crystal? And two, to hopefully shed some light on how cracked and repaired crystals behave.

I coupled six of the 6 x 12 x 30mm BGO crystals (Frank S. BGO xtals) to the face of a 2" pmt. The xtals were positioned with their 6 mm edge against the pmt face. A small amount of silicone fluid (10,000 weight RC car differential fluid) was used between the xtals and the pmt and between the xtals 12 mm faces. (See upper photo)

Using my normal gamma spectroscopy setup (Canberra series 35+ mca) I did a comparison between the multi-xtal BGO detector and my Bicron 2M2 (2” x 2") NaI detector using the 662 keV gamma from a 10 μc Cs-137 source disc. (See lower photo)

Results: The resolution of the multi-xtal BGO detector was in the crapper! My Bicron NaI detector has a resolution of 7.3% FWHM at 662 keV. The multi-xtal BGO detector’s resolution ran from 14.7% to 17.4% FWHM at 662 keV.(For reference, a detector using a single 6 x 12 x 30mm BGO crystal had a resolution of 10.4%). With the detector faced up to the source disc the resolution was14.7%. With a source to detector distance of 5.6 cm the resolution was 17.4%. Intermediate source to detector distances gave intermediate resolution values. Also, if the source disc was moved laterally (with same source to detector distance) the resolution would change.

With five crystal to crystal interfaces this was an extreme example as far as cracked crystals go, but I do believe it shows what to expect with a cracked and/or a cracked and repaired crystal scintillator.

1. Degraded resolution.
2. Inconsistent resolution. (Dependant on source to detector geometry).

Same basic conclusion as mentioned earlier in this thread, a cracked crystal is curtains for spectrometry, but not a big deal if all you want to do is detect radiation. Take it U rock hounding, what do you care if you drop it, it’s already cracked!

Jon Rosenstiel
Attachments
BGO.jpg
BGO.jpg (46.31 KiB) Viewed 105 times
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NaI vs BGO.jpg (80.28 KiB) Viewed 105 times

Q
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Re: NaI scintillator resolution and rejuvenation

Post by Q » Mon Mar 20, 2006 12:16 am

thanks richard,
oops, thallium... not titanium. i knew it was one of those "t" elements. to many days with not enough sleep.
anyway, yeah, thallium isn't a substance that i really want to mess with in the current lab setup. but do i understand correctly that one could dope a NaI crystal simply by adding a tl salt to the solution?

Q

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Richard Hull
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Re: NaI scintillator resolution and rejuvenation

Post by Richard Hull » Mon Mar 20, 2006 3:20 pm

Cracked crystals are goners. Forget cracked crystal as useful items. They are forever degraded and pretty worthless.

This is why it is absolutely imperative to have an intellegent and cogent seller supply, in writing, that any PMT with xtal is in flawless condition prior to purchase with a full money back on defective items.

Of course on e-bay or in a surplus situation this is going to happen about .0000002345% of the time. Earl or Elmo don't know nuthin' about no things like that............I got this thing and I want money and you ain't gittin' yers back once you got my thing.

Scintillation crystal are far too easy to damage through mishandling or temperature gradients. Most are surplused due to cracked or otherwise horribly degraded crystals, but they sure do look purdy from the outside.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: NaI scintillator resolution and rejuvenation

Post by richnormand » Sun Mar 26, 2006 3:52 am

Interesting.

I used one of my $9 e-bay 2” PMTs for this test, so results may vary…!

I first used one of the BGO with a polished 12mm face on the PMT and a Cs source at 5cm. Resolution was a bit better than 11%. This I think is consistent with what is to be expected. Also in agreement with this measurement,
viewtopic.php?f=13&t=5677#p34121
First pix.

Then considering the comments here

viewtopic.php?f=13&t=5672#p38002
Second pix.

I wondered about polishing the other two crystals since it takes me about half a day to do a good job with diamond paste for all surfaces on each one.

Well I tried it anyway. With three crystals with RC diff fluid coupling between the xtals and the PMT ( thanks for the tip Jon about RC diff silicone stuff), I got about 11% resolution but a lot faster acquisition. I’ll repeat the whole process in a more systematic way to check these results in the next few days.

Here is what I got with three xtals:

See pix #3.

with that geometry that was put in a cardboard box (with light leaks too) for a quick test. The Al foil is also seen :

Pix #4


Comments welcome but it seems that with a good optical interfaces the resolution is not degraded as much as when adjacent surfaces are of poor optical quality.
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clip_image003.jpg (10.47 KiB) Viewed 1944 times
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clip_image002.jpg (12.46 KiB) Viewed 1944 times

Jon Rosenstiel
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Re: NaI scintillator resolution and rejuvenation

Post by Jon Rosenstiel » Sun Mar 26, 2006 5:59 am

Very interesting, Rich. You've done an awesome job of polishing those xtals. How's the fingers feeling?

I did a real quick polish test with my buffer, and it appears that it will do the job, but those crystals are a bear to hold onto. To keep flying crystals to a minimum I'll need to make some type of holder.

Keep us posted.

Jon Rosenstiel

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Frank Sanns
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Re: NaI scintillator resolution and rejuvenation

Post by Frank Sanns » Sun Mar 26, 2006 6:43 am

Wow, you sure made the xtals sparkle. How would you like to do the 39 that I am tiling my 5" PMT???? I think not.

I am curious how the interface LOOKS with the coupling fluid between the xtals. Can you see the interface at all or is essentially totally gone? If the refractive index is close to the substrate, the interface will dissapear and your three xtals will look like one big one. It is this condition that really interests me.

Good work Rich! When I get my interface completed in the next day or so, I will run my MCA / BGO monstor xtal to see what the limit of resolution will be.

Frank S.

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Re: NaI scintillator resolution and rejuvenation

Post by richnormand » Sun Mar 26, 2006 3:31 pm

Here is a photo looking through the side. The interfaces do vanish.

I put a microscope slide at the entrance and exit surfaces since the RC silicone fluid was creating distortions and I did not want to remove the whole assembly for a cleaning. The black bars in the back were traced in pencil on a ¼” sheet placed about 4 cm away.
The coupling fluid does effectively remove the slight haze and orange peel effect left by my polishing job.

(Edit:) Looks OK in the visible but it is at the UV emission wavelength that the interfaces and coupling fluid matching would be important.

I started with 6um diamond paste and moved immediately to the 0.5 um paste and used a polishing pad that was a bit too soft and thick. Not the best way to do it but I was afraid of breaking/dropping the xtals so I skimped on the polishing time as much as I could. Also my finger tips still hurt from the sharp edges holding these suckers! If I was to do a larger quantity I would invest in creating a suitable jig and move to a motorized lapping polisher. That would also preserve the surface flatness better although this does not look as a problem.
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Re: NaI scintillator resolution and rejuvenation

Post by HighVoltageFox » Sun Mar 26, 2006 11:01 pm

I don't know if this has been said yet but anyway; the solution to the water absorbing crystal is a vacuum oven, commonly found on ebay and labX on the cheap. Of course those of you with extra vacuum fittings can just throw a small vacuum chamber together and just pump it down and heat it or bake it out with the crystals inside, quick cheap vacuum oven.
Oh, yeah and I have some experince in glass blowing and generally broken glass is a lost cause unless you want to melt it down.
,Andrew

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