Re: Supplier for Gamma Shield Material

[fbronson@canberra.com]

George:  You might consider just removing the contaminated graded liners and
leaving them out.  They are largely a holdover from the "olden days" when low
resolution NaI detectors were in common use, without enough resolution to avoid
confusion between the Pb X-rays and the peaks from the sample.  But today with
Ge detectors and good stable MCAs and good gamma spec algorithms, you can use
spectroscopy to "remove" the Pb X-ray lines.  Just create a "nuclide" with those
lines and let the computer do the work.  That will work quite well UNLESS you
are seriously interested in nuclides within approx. 2 keV of the same energies
as the Pb X-rays [approx. 73, 71, 84, 10 keV], which is not a very common
application, especially in the NPP area.

For those of you historically challenged, the purpose of the Cd is to absorb the
Pb X-rays, but in that process the Cd X-rays are generated [approx. 23, 26 keV],
which offended the earlier purists.  [note Sn does the same thing but at approx.
25, 28 keV]  So they added Cu to absorb these X-rays and regenerate then at low
energies [approx. 8, 9 keV].

If the bare Pb is a problem, seal it with a clear acrylic paint [no radioactive
pigments, please].  But, if you really like the look of copper, then try an old
roofer [they use copper for gutters] for the copper and to shape it into the
shield lining.  Don't let him use any solder, as it may be too radioactive for
your tastes;  try epoxy or RTV instead.  If you don't want to trouble yourself
with the Cd or Sn inner liner, then go a bit thicker on the Cu. That will very
slightly increase the low energy [<300 keV] background, but that may not be a
problem.  And, of course, if you only count Marinelli Beakers, then the liner
has no use, as the sample itself is the liner.

Sorry for getting off track;  now what was the question you asked originally??

frazier bronson

----------------------------

earlier today, George Crowley [gmcrowley@tva.gov] asked:

We have managed to contaminate one of our gamma detector shields.  We
believe we can just replace the copper liner and we'll be back in business.
There is a piece of cadmium on the back of the copper that we should replace
with tin to get around the toxicity issues.  Does anyone know of a supplier
that could provide me with a sheet of copper and tin that we could then fab
to fit our shield?


---------------
Frazier Bronson CHP
Canberra, 800 Research Pkwy, Meriden CT 06457 USA
tel: 203-639-2345 e-mail: fbronson@canberra.com


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