NaI scintillator resolution and rejuvenation

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
richnormand
Posts: 288
Joined: Sat Aug 27, 2005 12:30 pm
Real name: rich normand

NaI scintillator resolution and rejuvenation

Post by richnormand » Thu Mar 16, 2006 4:54 am

I am having a difficult time resolving higher energy peaks with my Harshaw 3” by 1” NaI scintillator. For example I barely distinguish the double peaks of Co around 1.25MeV.

It is read by a Canberra series 3 that performs well when fed from a variable height pulse generator or with a Bicron x-ray detector. I have to conclude that it is the crystal or the PMT.

The crystal has a few cracks in it but it is still transparent with only a hint of yellow. Some places offer to “rejuvenate” old NaI crystals. Considering it is hydroscopic I would assume they simply leave it in an oven under low humidity and high temperature?

Any ideas from the gurus here?

Cheers and thanks

User avatar
Frank Sanns
Site Admin
Posts: 1666
Joined: Fri Jun 14, 2002 6:26 pm
Real name: Frank Sanns
Location: Pittsburgh, PA USA

Re: NaI scintillator resolution and rejuvenation

Post by Frank Sanns » Thu Mar 16, 2006 5:53 am

If the crystal is cracked you will loose your resolution forever. It will still detect but will no longer resolve.

Heating an already cracked crystal a great way to have it crack some more. I would not be too kean on heating up the crystal even slowly as the internal stresses from cracking will build up higher and the chance of more cracking is too great. Pulling a vacuum on the NaI will get the moisture out and I have even heard that it will remove some of the yellowness if you pump it down deep enough leave it under vacuum for half a day (or over night).

Frank S.

richnormand
Posts: 288
Joined: Sat Aug 27, 2005 12:30 pm
Real name: rich normand

Re: NaI scintillator resolution and rejuvenation

Post by richnormand » Thu Mar 16, 2006 3:07 pm

I have a Canberra Series 3 this is a full MCA with all the trimmings as well as a Canberra 1705 signal processor. Works well at high resolution with other detectors. Looks like it its the Harshaw NaI/PMT that is doing it. The crystal has multiple cracks at the bottom edge. I suspect multiple light scattering could be the culprit of built-in stress from the fracture. This was an e-bay purchase a while back and I only opened the housing recently to try to find out why the resolution is not that good.

How easy is it to cut and polish NaI? I have a very good region in the middle of the crystal (its a 3" diad by 2" thick)

User avatar
Richard Hull
Moderator
Posts: 11961
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: NaI scintillator resolution and rejuvenation

Post by Richard Hull » Thu Mar 16, 2006 3:09 pm

Note* If you are placing the source face-on to the PMT it might not do as well as a 2"! You only have 1" of stopping power of crystal from the front which is not good. Place the source to the side (shining in through the 3" section and the crystal) and see if you improve. 1" thick crystals are crap for high energy gamma spectro work. The absolute best gamma spectro NaI xtal is a 3"X3".

Finally, any cracks in any scintillator are just about a show stopper, as mentioned above.

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.

User avatar
Frank Sanns
Site Admin
Posts: 1666
Joined: Fri Jun 14, 2002 6:26 pm
Real name: Frank Sanns
Location: Pittsburgh, PA USA

Re: NaI scintillator resolution and rejuvenation

Post by Frank Sanns » Fri Mar 17, 2006 4:23 am

I had a thought about the cracked NaI crystal. It may be possible to repair it sufficiently to restore the original uncracked performance. From another current post ( viewtopic.php?f=13&t=5671#p34115 ) about coupling scintillators to PMTs, the role of optical coupling fluids is discussed. The thought is to repair the cracks with an optical coupling adhesive with the same refractive index as the xtal. I would imagine you could put the adhesive over the outside of the xtal, pull a modest vacuum, then release it. The adhesive should be sucked into the cracks and fill them. Wipe the residual adhesive from the xtal faces and let it cure. Should work. This is similar to the process used to repair small cracks in car windshields (windscreens).

Frank S.

richnormand
Posts: 288
Joined: Sat Aug 27, 2005 12:30 pm
Real name: rich normand

Re: NaI scintillator resolution and rejuvenation

Post by richnormand » Fri Mar 17, 2006 5:37 am

Good idea. Jon mentionned using high weight silicone oil (as used in RC car differentials) as coupling fluid for his plastic muon detector. I'll look up the index of refraction of lucite/plastic compared to NaI. If close this might work and seep into the cracks? Nothing much to loose unless the oil damages the crystal

richnormand
Posts: 288
Joined: Sat Aug 27, 2005 12:30 pm
Real name: rich normand

Re: NaI scintillator resolution and rejuvenation

Post by richnormand » Sat Mar 18, 2006 11:26 pm

You are right.
I usually place the source about 5cm away on the side to get the full 3" xtal cross section. The front has a stainless steel plug on it to position the assembly. This is the detector on the far right of my pix in the muon thread.

JohnCuthbert
Posts: 339
Joined: Mon Aug 19, 2002 8:30 pm
Real name:

Re: NaI scintillator resolution and rejuvenation

Post by JohnCuthbert » Sun Mar 19, 2006 10:00 am

Does anyone know if you can repair a cracked NaI crystal this way?
If you can then, so far as I can see, with a good refractive index match you could "glue" together powdered NaI with a suitable resin and make any size detector you wnted.
This would give you most of the sensitivity and resolution of a single NaI crystal and a lot of the convenience of a plastic scintilator.
This seems too good to be true, so I guess there's a mistake somewhere.

User avatar
Frank Sanns
Site Admin
Posts: 1666
Joined: Fri Jun 14, 2002 6:26 pm
Real name: Frank Sanns
Location: Pittsburgh, PA USA

Re: NaI scintillator resolution and rejuvenation

Post by Frank Sanns » Sun Mar 19, 2006 4:21 pm

I too would like to hear from Carl and some of the other spectroscopy experts here but here are some of my thoughts.

For gross count detection only, I would expect that the efficiency would be approximately the weighted average of the amount of NaI:dilluent ratio. The dilluent could be a matched refractive index glue or liquid or scintillator containing organic resin. So if you had 80% NaI and 20% non scintillating binder resin, you might expect 80% of the NaI effeciency.

The efficiency may be less than that because NaI(Tl) is essentially transparent to its scintillation wavelength. This is not the case with plastic scintillators.

For spectroscopy, my knee jerk reaction is that it would be very poor. It would be one thing to repair a single crack with a thin optical interface of fluid and something else entirely for 10^23 cracks. Then you have the problem of "wetting" the entire surface of the NaI powder. Any air void or places that the resin in not in atomic contact with the NaI would yield a path variation that would kill the resolution. Again, I am not the expert here and I would really like to hear from the others. No matter, it would still be an intresting experiment to do as I would really be curious of the spectroscopy result.

Frank S.

User avatar
Carl Willis
Posts: 2841
Joined: Thu Jul 26, 2001 11:33 pm
Real name: Carl Willis
Location: Albuquerque, New Mexico, USA
Contact:

Re: NaI scintillator resolution and rejuvenation

Post by Carl Willis » Sun Mar 19, 2006 9:50 pm

The idea of putting the crystal in an index-matched liquid (one that is dry and otherwise won't attack the material) is interesting. For small cracks I don't think you'd see much reduction in resolution compared with the undamaged crystal if this method were used.

But the suggestion gives rise to a more severe idea that I don't think would work well for resolution: what if you took a bunch of tiny grains or crystals of NaI(Tl) such as one could grow at home by evaporation, and stuff them in a jar of liquid with refractive index of 1.85 or so? I do think this would reward you with a monster scintillator, but electronic equilibrium within the NaI microcrystals is likely to suffer for high-energy gammas. Electrons from photoelectric and Compton processes will lose some energy traversing nearby inert material, with the end result that you have a broad full-energy peak. However, this idea has merit if you are looking for efficiency.

-Carl
Carl Willis
http://carlwillis.wordpress.com/
TEL: +1-505-412-3277

Post Reply