Needing more than a spark test?

Regarding trying to restore a scintillator, there is, unfortunately, no easy way to remove the water. Usually, it is possible to reverse a physical process, but here, it may involve some chemistry happening. I suppose there is nothing much to lose by putting a very cheap (failed by water) Na(Tl) scintillator into an upturned jar on a metal plate with a outlet to suck a vacuum. Then, add to that, some arrangement of suitable heater wires arranged around it, and include a thermocouple.

I think, get a vacuum, and then heat, not too fierce at first. If there is actual free water in it, it may turn to vapor and crack the crystal. Maybe for a while a bit below 100C. Then, go slightly above 100C. I all is still well, then cook more, looking for transparency, but stay below 924K, or about 650C (or 1204°F), when it melts. If it was a good crystal before, it may remain so, without defects, is all that has happened is molecule hydration.

So - we try the AI engine, and this is what we find ..

Answer
To repair water-damaged NaI(Tl) crystals, one method involves baking the crystal at a high temperature to remove the absorbed moisture. This process can be done by heating the crystal in an oven at around 100-150°C for several hours. Another approach is to encapsulate the crystal in a hermetically sealed housing to prevent further exposure to moisture, ensuring its protection from water damage.


https://fusor.net/board/viewtopic.php?t=11360
Click to expand...
That is encouraging! There is more - it seems it has been done -> HERE
The main posting reads..
-----------------------------------------------
Repairing or reworking water damaged NaI(Tl) Crystals

Post by SM Packard » Tue Mar 14, 2017 5:22 pm
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?
------------------------------------------------
So maybe we don't have to cook it to near red heat :)

I don't have a temperature controlled (anything) yet, but I am tempted. Any photodiode that can see the one wavelength visible blue light can do the rest, even if it needs a low noise amplifier to help it.
 
@graham-xrf PID controllers are quite cheap, if you need to control heating (or cooling). I've bought a couple of them. One, I use for casting lead, the other I'm setting up to control a hot plate. You can buy them complete with a K-type thermocouple, a SSR, and a heat sink (for the solid state relay), for under $25 USD. The listing below is for $17.49USD.

www.ebay.com

REX-C100 Digital LCD PID Temperature Controller Set+ K Thermocouple+ Max.40A SSR | eBay

Output Current: 40A. K Thermocouple Probe Cable. Max.40A SSR. 1 Max.40A SSR. Detective temperature r-ange: 0 to 400℃. Temperature r-ang :0～400℃. Process value (PV), Setting value (SV). Length: 1M.
www.ebay.com

I made this using an ammo box and one of the PID kits. https://www.hobby-machinist.com/threads/making-a-pid-controller-box-for-a-20lb-melting-pot.106438/
 
Since NaI is hygroscopic it is always encapsulated. So my first thought is, where does the released water go when it's heated? In the case of very old crystals the water has had a long time to (presumably) diffuse through the seals. Attempting to drive it out in a relatively short time without removing the window sounds like a short-term solution to me. But, of course, removing the window adds all sorts of complications. If nothing else, safely maintaining a super-dry environment would be challenging for hobbyists. Then there's the small problem of replacing the window so the crystal doesn't go to heck soon afterwords. I would imagine that the sealing materials aren't going to be found in your local hardware store. Beeswax is supposed to be pretty good as a moisture barrier but I don't think it would be a good adhesive.

I have similar misgivings w/regard to drying cystals with a vacuum pump, since the window has to be hermetically sealed. A high vacuum certainly would be dry, but gaining access to the NaI and then replacing the window while under vacuum aren't going to be trivial problems to address.

Some conversations I read brought up the fact that thallium is very toxic, but I think that may be overstated, simply because the thallium content probably is quite low. The sodium iodide by its own probably is more toxic. That said, any rework procedure that is designed to keep the scintillator assembly in a totally dry environment will prevent contact with the crystal.

The other issue is that the procedure may not be compatible with a PMT+scintillator where the scintillator is bonded to the PMT.

Speaking just for myself, this is a rabbit hole I wouldn't want to go down. The time and equipment investment seems outsized compared to the cost of a new NaI(Tl) scintillator -- see this, for instance, where you can buy a new 1" scintillator for $65.

Note that the relatively thick aluminum windows on either of those (or, for that matter, the Scionix) will greatly reduce the sensitivity to low-energy xrays, like the ones emitted by iron, nickel etc. Just to repeat my justification for playing around with the Scionix: it is to verify that the rest of my hardware and software are working properly while not spending a lot of money in the process. It will be a bonus if it can be used for the purpose of this thread, "needing more than a spark test".

The final solution, especially for a portable system, probably will use a SiPM and less-hygroscopic scintillator like CsI(Tl), or maybe CdWO4 (10mm diam x .5mm thick for $63, also from OST Photonics).
 
I should mention that, in the speculated possible "treatment" involving some vacuum and some heat, the only way to get heat to the thing in the middle is by radiant from a glowing hot element, because there will not be convection in the jar. Nor will things cool off in a reasonable time unless you allow in some air

Control can get awkward too. I have done PID control for a vacuum furnace, though it was a much bigger thing. It required temperature feedback from both the filaments, and the workpiece, and used a cascade loop control. For good control, you need some heat loss as well, and this was not happening. In essence, the best the controller can do is put in all the heat needed to ramp the workpiece up to temperature, and then cut off, leaving it there hanging at about the set point, until one is done.

I do agree with @homebrewed about messing with these. If it was easy, lots of folk would do it, and be selling the results.
 
You must log in or register to reply here.
Back
Top