Looking to restore a yellowed, non-functional NaI(Tl) crystal

[27rexr]

Hey there! I was recently at a local university's lab, and they had a detector that they had deemed broken. The crystal had become detached from the PMT, and the crystal was quite yellowed. I did some poking around and found that it *may* be possible to fix. I asked if I could take it home, and they said yes, so here I am. How would you recommend I go about this? I have read that the yellowing would probably be due to moisture being collected by the crystal. Below are my ideas ranked by how risky I think they are, from least to greatest:

Desiccant: Should I put it in a box with a ton of desiccant? It seems like the moisture might have to be actively pulled from the crystal with, so I doubt this would work.

Vacuum Chamber: I heard about people putting the crystal in a vacuum to pull the moisture out. Is this advisable? How well would this work?

Oven/Resting on heated printer bed: I have used my 3d printer bed and oven to dry filament before, and see no reason why it would not succeed in drying the crystal, but I have major concerns about introducing cracks to the crystal through the heat stress, which is why this is ranked as a more risky idea.

Solvent Cleaning/Mechanical Cleaning: I have heard of some people sanding down the crystal or exposing it to some kind of cleaning solvent to remove the outer layers of the crystal. This is ranked as my most risky idea as I feel there is a high chance of screwing this up, and I would rather not remove crystal.

Thank you for your help! Also, please keep in mind I have pretty limited knowledge about this stuff, and I apologize if any of these questions are stupid or ridiculous.
Attached are a few photos of the crystal, which also seems to show a solid layer about 2.5cm down? It seems like there is a clear space (of about 2.5cm) and then a solid gatorade-yellow block, but it could be a trick of the light or my imagination.

[Sesselmann]

Rex,

These are all good questions and suggestions, the expert on restoring crystals is Luuk, he used to work for Scionix, but recently retired and I'm not sure how often he checks the forum.

I don't have hands on experience with this, but in absence of a proper dry box your suggestions sound plausible to me.

In addition to drying the crystal you may also want to go over the surfaces with some fine grit sandpaper to clean it up.

Steven

[Svilen]

There are no stupid questions, but unfortunately Luuk, the expert on this matter reads rarely these days, as Steven mentioned above.

I also don't have any practical exprience with this, but just from what I've seen and read, I think you need the crystal out of the can first. When drying it will oxidize, which will stop the process so you need to clean it periodically with a sandpaper. Soaking in chloroform can be used to dissolve the glue seal. Having the crystal out of the housing will also allow you to see if it has cracks and how deep they really are. Professionaly it is dried in a dry-box/room for days, but I don't know the exact conditions (temperature, air flow...) inside.

I guess you know this already, but just in case you don't - chloroform and the thalium in the crystal are toxic and proper protection (gloves, ventilation, face mask...) should be used when working with them, in order to prevent skin contact and inhalation!

If the crystal is not big, if it is cracked, if you don't have the skills and equipment, it may be not worth restoring it (except for learning, which we all here surely consider as enough of a reason :P, but still).

[Rob Tayloe]

My suggestions to the original poster (27rexr) will be a bit different. Rather than spending time and exposing one's self to no small amount of risk to rehabilitate this crystal, I would suggest spending more time with the Physics or Nuclear Engineering folks at the local university. I presume this local university is the University of California at Berkeley (aka "Cal"). While they may have a poor football team this year, Cal does have some excellent academic programs. Oftentimes there are opportunities for high school students to begin taking college courses. I suspect that a willing student might find a professor or grad student that welcome some help. Early days, this "help" may seem to be more like cheap labor, but as one learns more and demonstrates an interest is is likely that more challenging opportunities will arise. Physics and Nuclear Engineering programs at major universities often have some very interesting equipment.

And as time goes on there may be opportunities to have an internship at a laboratory with a pretty good reputation. That lab is just up the hill from Cal; as you likely know it is the Lawrence Berkeley Lab. If one looks at the Periodic Table of Elements towards the bottom there are some elements called Californium and Berkelium. There is a reason for this. I can recall some years back being at LBL and getting ready to eat lunch in the cafeteria [this place has, in my opinion, the nicest view of all government labs]. A tall feller in a suit walked in, got lunch, and sat at a table with a number of grad students and asked about their work. This feller was Glenn Seaborg who had a hand in discovering the aforementioned and some other elements; he also won the Nobel Prize.

https://en.wikipedia.org/wiki/Glenn_T._Seaborg

https://www.berkeley.edu/

https://www.lbl.gov/

[27rexr]

Thank you all for your responses.

Sesselman: Your validation of my plans was greatly useful to me. Thank you for that.

Svilen: The same to you. I especially wanted to thank you for the tidbit about oxidization. I had not known that, and I imagine it will save me a ton of time. For the note on safety, I will be taking appropriate precautions to ensure that I do not end up on the news. I definitely think this is more of a learning experience than being worth it, but as you said, learning is a good reason too. Thank you.

Rob Tayloe: Thank you for your advice. I could have absolutely used something like that last year, but at this point, that’s exactly what I’m doing. I’ve joined the RadWatch team at Cal, where I do some work on the DoseNet system. I’m also constantly talking to professors, graduate students, and undergraduate students. You’re totally right about the interesting equipment part, there’s tons of awesome projects there. The whole experience has been incredibly valuable to me. This is where I ended up with this crystal, which is a secondary project to all the RadWatch stuff. Super interesting story about Glenn Seaborg. Thank you for your advice.

Thank you all again very much for your advice. I will be documenting my progress with this crystal as time goes on.

[luuk]

Hello Rex,

Restoring a wet NaI(Tl) crystal is “almost” impossible without the right professional
Equipment.
The NaI(Tl) that reacted with the moisture can only be mechanically removed, e.g. with sandpaper.
There is no other recovery for that problem (as far as I know).
The normal procedure is to remove all the sealing(glue) and take the whole detector apart.
Dry the crystal in a dry box (humidity as low as possible preferable <5%)
After the crystal is dried the damage part turns in to white(oxidation) this layer that can be several mm thick has to be removed with e.g. sandpaper every time a little depending on how bad the crystal is affected, this procedure can take several days, and in between the crystal has to be kept dry.
Once all bad material is removed the crystal looks white again, and can be rebuild in to its original dried and restored housing.
Some people claimed that when de slowly heated up the crystal the moisture disappeared, I tried it once at work just to prove that this is nonsense and of course not much happened with the crystal.
Using a desiccant sound nice but NaI is already a perfect desiccant so it is hard to beat that.
But even if you cannot retore the crystal it is still perfect for counting with a high efficiency because of the crystal size, so that could be a good use for a wet crystal.

I am sorry but that’s the way it is.
Luuk