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Re: sodium iodide scintillation detectors
Let me add that the NaI is also thermally sensitive and can crack if it undergoes rapid temperature changes. I am not sure how sensitive a crystal would be to cracking when going from very hot (<100 F) to say about room temperature, but when going from approximately room temperature to sub-zero, they will crack unless you are careful.
James Barnes <james.g.barnes@WORLDNET.ATT.NET> wrote:
Some minor points . . . .
The "glue" is actually "optical coupling." Dow-Corning makes the best, but
my experience is that the coupling made in the 80's is a bit superior to
that made in modern times (go figure). We have a jar of the old coupling
jelly that we treasure and only let the most initiated view (but not touch).
The point of the humidity is valid. NaI is hydroscopic, and as it gathers
more moisture, the crystals will acutally turn yellow. It's the
discoloration that drops the efficiency as the scintillation light cannot
penetrate through the discoloration as easily as it can through a clear
crystal.
If well-sealed with some dessicant, the crystals should have a long
self-life(years). Leave them in the open and they'll be discolored in a
matter of weeks.
Another point (that may not be directly in answer to the question), if you
use these crystals in hot weather (say So. California desert), the optical
coupling will thin out from the heat and will run. This makes the detector
operate very erratically; let the probe cool down a bit, and all will return
to normal. You can run into this in temperate climates if the technician
leaves the detector in the cab of a pickup on a sunny day. The effect seems
to start about 110 deg F.
Hermetically sealed is the best, but sometimes you can rehabilitate a probe
for not a lot of money just by refurbishing the optical coupling. It may
take some trial and error, but it isn't hard to acomplish.
Jim Barnes, CHP
james.g.barnes@att.net
----- Original Message -----
From: "Syd H. Levine"
To: ; "Dave Oshlo"
Cc:
Sent: Friday, August 06, 2004 12:28 PM
Subject: Re: sodium iodide scintillation detectors
> There is no excuse for a crystal failure due to the glue. Though more
> expensive, we use special ruggedized hermetically sealed crystals, usually
> in stainless steel, in well logging sondes. I have some of these built in
> the 1970's still working perfectly. If I were you, I would talk to other
> manufacturers than just St. Gobain (they own what was Harshaw / Bicron /
> Solon). There is an outfit in that same area called Rexell that is more
> responsive, and usually a bit cheaper. Also try Gintek in South Carolina,
> and a new outfit down near Huntsville (the name slips my mind).
>
> Syd H. Levine
> AnaLog Services, Inc.
> Phone: 270-276-5671
> Telefax: 270-276-5588
> E-mail: analog@logwell.com
> URL: www.logwell.com
>
> ----- Original Message -----
> From:
> To: "Dave Oshlo"
> Cc:
> Sent: Friday, August 06, 2004 6:58 AM
> Subject: Re: sodium iodide scintillation detectors
>
>
> >
> >
> >
> >
> > Dave,
> >
> > Based on our experience with two 4 x 4 x 16 inch NaI detectors used in
our
> > standup whole body counter, the life expectancy is about 12 years. The
> > limiting factor is not the crystals themselves but the glue used to seal
> > the detector housing the NaI crystal. The glue starts to degrade after
> > about ten years allowing humidity to invade the crystal. When the glue
is
> > degrading and conditions are humid (e.g., summertime in Virginia), the
NaI
> > crystal absorbs water and you'll see a significant decrease in detector
> > efficiency. When conditions are dry (e.g. wintertime), the NaI crystal
> > will start to dry out and you'll see a corresponding increase in
detector
> > efficiency. The good news is that you can send your NaI detectors back
to
> > the manufacturer (e.g., St. Grobain) and they dry out the crystal and
> > reseal it into another detector housing. By doing this you can
> > indefinitely extend the life of your NaI detector. The drying out and
> > resealing process takes about a month for the large 4 x 4 x 16 NaI
> > detectors.
> >
> > I'm not sure what the shelf life of NaI detectors are in a warehouse.
> > However, it's likely to also be dependent on the glue that's used as a
> > sealant. Check with a crystal manufacturer such as St. Grobain to get a
> > definitive answer to this. Hopefully, the glue they're using today is
> > better than what they used 20 years ago.
> >
> > John M. Sukosky, CHP
> > Dominion
> > Surry Power Station
> > (757)-365-2594 (Tieline: 8-798-2594)
> >
> >
> >
> >
> > "Dave Oshlo"
> > To:
>
> > Sent by: cc:
> > owner-radsafe@list.Van Subject: sodium
> iodide scintillation detectors
> > derbilt.Edu
> >
> >
> > 08/05/2004 02:46 PM
> > Please respond to
> > "Dave Oshlo"
> >
> >
> >
> >
> >
> >
> > Dear radsafers,
> >
> >
> > I’m looking for two pieces of data on a typical sodium iodide
> scintillation
> > detector but can find inadequate documentation.
> >
> >
> > 1. What is the life expectancy of a sodium iodide scintillation
> > detector in an operational nuclear power plant environment?
> >
> >
> > 2. What is the shelf life of a sodium iodide scintillation detector
> > (in the warehouse as a spare part)?
> >
> >
> > An exhaustive web search has not provided any solutions. If someone
could
> > point me in the right direction, I would be much obliged.
> >
> >
> > Thanks,
> >
> >
> > Dave Oshlo
> >
> >
> > MGP Instruments
> >
> >
> >
> >
> >
> >
> >
> > ************************************************************************
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> >
> >
>
>
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