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Repairing or reworking water damaged NaI(Tl) Crystals

Posted: Tue Mar 14, 2017 9:22 pm
by SM Packard
I have acquired a number of high quality NaI(Tl) crystals for use in scintillation detectors. These crystals were made to very high standards of quality, but they are old, 50+ years.

The crystals are no longer clear. They have yellowed as a result of absorbing water, as the material tends to do. This obviously decreases the sensitivity and accuracy of the crystals.

Several years ago, there was a gentleman who said he could "rework" these crystals and restore them to their original state. I sent him a crystal and was very satisfied with the results. It went from looking like sweet cream butter to being as clear as glass.

Unfortunately he has since passed away, and I have no idea what method he used to draw the moisture out of the crystal.

I have hypothesized that it may have involved using a vacuum chamber or heating (or both, heating the crystal in a vacuum chamber) It may have also involved exposing the crystals to some kind of hygroscopic material, such as silica gel for some period of time with or without heat or a vacuum.

Whatever the method he used was, it seems to have worked very well and he seems to have taken it to his grave. This is deeply unfortunate, as high quality scintillation crystals are not easy to find, and these vintage crystals are excellent, but need to be dried.

For all I know, maybe he just put a hair dryer to them.

Does anyone have any ideas as to how I may be able to dehydrate these crystals so they can be used more effectively?

Thank you.

Re: Repairing or reworking water damaged NaI(Tl) Crystals

Posted: Wed Mar 15, 2017 2:12 pm
by Jeroen Vriesman
I have no idea what this NaI-reworker might have done, but just thinking about it, I would try freeze-drying.

Did it take a long time for him? (days?)

I think about freeze drying because heating or vacuum alone could damage the crystal. But it's just a wild guess. Maybe the migration speed of water through the crystal is way too low below 0 deg.C.

Re: Repairing or reworking water damaged NaI(Tl) Crystals

Posted: Wed Mar 15, 2017 2:31 pm
by Dennis P Brown
I would try putting it in a sealed plastic bag with desiccant. Even if it does not work, can't hurt. Then, maybe, try the more iffy methods.

Re: Repairing or reworking water damaged NaI(Tl) Crystals

Posted: Wed Mar 15, 2017 3:47 pm
by Richard Hull
Naked Crystals just do not exist in the real world of nuclear detection!! I assume these are canned and not loose crystals. If loose they would be a syrup by now, unless they were stored in the mojave desert. If canned, the canning is defective. There is nothing you can do in the can, I would think. It took years for a micro a leak to damage them.. I image they must be uncanned, treated and then re-canned. You might be on a quixotic quest.

Richard Hull

Re: Repairing or reworking water damaged NaI(Tl) Crystals

Posted: Wed Mar 15, 2017 8:47 pm
by Steven Sesselmann
Yes, old crystals can be improved, there is a guy on my forum posting under the name Luuk, a real crystal whisperer, I gave him my old 3x3" Alpha Spectra detector which was running at 18% resolution and got it back looking like new with an amazing 6.3% resolution.

I think they have a dry box and leave the crystal standing in it for several days, then they work on the crystal with some sand paper inside the glove box to remove the water damaged parts before encapsulating it.

Caution should be taken as the trace amounts of Thallium is quite toxic.


Re: Repairing or reworking water damaged NaI(Tl) Crystals

Posted: Thu Mar 16, 2017 6:01 am
by Rex Allers
To the best of my knowledge there is no way to recover, by drying, etc. the portion of an NaI crystal that has been damaged by absorbing moisture. One can and should dry the crystal to work on it but I think the only recovery is if the water damaged section is only the outer portion, the bad part can be removed leaving a smaller but functional crystal.

As Steven mentioned it needs to be done in a dry-box and is not an easy process. The thalium is poisonous and the NaI somewhat corrosive. The little that I know is mostly from reading posts by Luuk.

Luuk is in Europe and works for Scionix, if I remember right, so has plenty of knowledge, skills and equipment. I know of him from posts by him in the Yahoo group Gamma Spectrometry. If you become a member and post about this there, maybe you will get some good replies.

It wouldn't surprise me if there is nothing that can be done. It would be interesting to know what this person in the past did for you. I'm not an expert, but from what I have heard, you can't undo the damage, only (and if it is not extreme) remove the damaged parts and seal up the resulting smaller crystal to prevent future damage.

Re: Repairing or reworking water damaged NaI(Tl) Crystals

Posted: Thu Mar 16, 2017 9:05 am
by Jeroen Vriesman
I was talking to a chemist yesterday about this.
He says the same thing as Rex and others in this thread "no way to get the water out".

He said that diffusion of water is just way too slow, "it won't get out unless you kick it out".

So the only other "maybe try this" guess I have is to try MIVAC; drying with a combination of microwaves and vacuum.
Note that the absorption frequencies of water in a NaI crystal will probably be different from plain liquid water, so a house-hold magnetron might not work, or maybe it does, I don't have the data.

Real data is hard to find, most papers are about food drying. Some books mention hygroscopic solids in combination with freeze-drying, but only to state that dehydration is only complete for non-hygroscopic solids. I cannot find any reference to mivac for hygroscopic solids.

Re: Repairing or reworking water damaged NaI(Tl) Crystals

Posted: Thu Mar 16, 2017 9:45 am
by Dennis P Brown
Micro-waving might be a bad idea; diffusion is a random process and as much water will diffuse deeper into the crystal as towards the surface. Also, fast heating tends to cause water to collect at defect sites - a slow drying process is better.

Allex is correct that polishing the crystal is the only "easy" method to both remove the water damaged section and leave a clear face.

I have polished salt crystals with great success in room air using chemo-mechanical polishing. One uses dry alcohol (195 proof with salt added to further dry it; it must be new and sealed when ever not in use) and a fine sized grit to remove away the bad layers on a cloth "lap" mounted on glass (for super deep cutting, one first uses, ironically, some water(!) in the alcohol to speed up the process. Then a dry alcohol/fine grit clean up before polishing.) Then use a new cloth lap with a little dry alcohol and polishing compound to dress the surface and remove any trace water remaining. A final fast polish using a hint of water vapor on the lap can create a super clear finish that, again this may be counter intuitive,that helps prevent water from more easily getting in from a humid atm.

One aside: only the crystal face facing the photo-multiplier absolutely must be perfectly clear - some haze on the other faces will not prevent operation (but will decrease performance slightly.)

Again, I've had great success with pure salt crystals (NaCl) using this process but it is not certain if your crystal will respond the same way.

Finally, I would add that I have found very inexpensive crystals on e-bay and would never go to the trouble and expense of re-polishing a defective crystal. Considering the toxic nature of the waste from the pre-working, I'd avoid the whole process and just buy a new crystal.