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Re: sodium iodide scintillation detectors
First, I assume the original poster was referring to the glue / sealant that
seals the glass into the metal crystal housing. Several manufacturers used
an electrometric seal that eventually fails and allows that yellowing from
humidity getting to the crystal as you mention. The very best crystals use
a metal to glass sealing process without any such "glue". There is of
course always optical coupling compound between the crystal and the glass,
then between the glass and the PMT window in the final detector assembly.
If your optical coupling is going to hell at 110 degrees F, you ain't doing
it right! In well logging tools, we routinely build detectors using off the
shelf silicone optical grease that go to 300 degrees F or higher. I like
the old GE silicone optical coupling grease better than the Dow stuff.
Interestingly, you can use almost anything in a pinch for testing, even
Vaseline.
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: "James Barnes" <james.g.barnes@WORLDNET.ATT.NET>
To: <radsafe@list.vanderbilt.edu>
Sent: Friday, August 06, 2004 4:42 PM
Subject: Re: sodium iodide scintillation detectors
> 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" <syd.levine@MINDSPRING.COM>
> To: <John_Sukosky@DOM.COM>; "Dave Oshlo" <doshlo@mgpi.com>
> Cc: <radsafe@list.vanderbilt.edu>
> 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: <John_Sukosky@DOM.COM>
> > To: "Dave Oshlo" <doshlo@mgpi.com>
> > Cc: <radsafe@list.vanderbilt.edu>
> > 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"
> > > <doshlo@mgpi.com> To:
> > <radsafe@list.Vanderbilt.Edu>
> > > 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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