Re: Need help with gamma spec spectrum.

[Christoph_Hofmeyr/CNS1@cns.co.za]

Dear RADSAFERS,
I wrote to Stephen Wish, but he has not responded as yet.  I could not look
at his spectrum, but it sounds very much like the type of spectrum and at
the
position which one finds in thermal neutron capture in boron.
B-10(n,gamma)
makes a B-11 compound nucleus, which immediately breaks up,  resulting
in an alpha and  Li-7, which emits the 478 (?) keV gamma.  It is
symmetrically
Doppler broadened, and is registered in the 470 - 485 keV region with a
nice
flat top.   Pretty unmistakeable. The only problem: this is a prompt
reaction,
so where do the neutrons come from?   Assuming there is boron in the sample
(or in a boron glass bottle?), is there perhaps a (thermalised) neutron
source
nearby, for example Am-Be in a moderator?  The boron cross-section is
large,
about 1000 barn per (elemental) atom.  Lead shielding will let through
thermal
neutrons quite readily.  Still, the flux would be rather low...
Bingo! If Stephen uses a lightly shielded detector, the boron could be in
the
neutron source shielding and the gammas ..... So the sample could be boron
free.  Check the background.
I know this is an extremely long shot.  Stephen, if you could attach the
file as a
bitmap, JPEG or WORD graphics file, or just fax it.
Chris Hofmeyr
chofmeyr@cns.co.za
Council for Nuclear Safety
fax: 27 12 6635513

Stephen Wish  wrote:

Dear Radsafe:

I am having difficulty identifying a peak on a gamma spec spectrum.  The
peak
range is from 470-485 keV and has a peak shape of a wide plateau vice the
normal pointed peak you would expect.  I have verified that this is not a
detector or system problem and have found this plateau on 3 samples
arriving
from the same source.  The only sample history I have is that it is a water
sample that has/may have been used to stabilize high explosives (content of
HE described to be RDX, HMX, and 246 TNT).  A copy of the spectrum is
attached in a Microsoft works format (zip file).  Any help you could
provide
in identifying this would be greatly appreciated.

Please respond to me directly at arsgm@aol.com

Thanks!

Steven Wish
ARS of New Mexico
505-663-0363




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