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Re[2]: I-125, 131 sources
Forwarded-from: MIKEG
1. The mock iodine source issue (particularly the Ba-133/Cs-137 source) was
a particular problem for us at Oak Knoll and Bethesda. The problem was the
differential ratio of activity due to the relatively short half-life of
Ba-133 compared to Cs-137. True, it's still in the "years" range, but you'd
be surprised how many over 10 year old "mock" iodine sources are out there!
2. I subscribe to the at least once a year calibration using the real stuff
approach (I-131/I-125). The summing issue for the well crystals commonly
used in clinics today is not trivial. Also, a standard longer lived source
(such as the Cs-137 source we used for daily dose calibrator checks), prefer-
ably reasonably well matched for the photon energy (I-129 isn't too bad for
I-125), is reasonable assurance against missing significant window drifting.
Significant being the level specified in your license for that assay system.
(Of course, just to be sure when dosing, we ordered enough to check the
window everytime we scheduled bioassay--the real stuff that is.)
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Date: Thu, 28 Jul 94 16:23:40 -0500
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Subject: Re[2]: I-125, 131 sources
X-Listserver-Version: 6.0 -- UNIX ListServer by Anastasios Kotsikonas
X-Comment: Radiation Safety Distribution List
It is acutely embarassing that I can't provide more info
about mock iodine than what follows. The stuff was first
developed by the late great Marshall Brucer when he worked
at ORAU in the 1950s. The historical collection here
contains one of his primary mock iodine standard sets
assayed in 1959. As Albert surmized, the additional
component is Cs-137. The set says that the ratio of Ba-133
to Cs-137 was 12 to 1. An Amersham catalog I have lists
"simulated iodine" and says it contains Cs and Ba but
doesn't give the ratios. I'm sure they can provide the
data. Interestingly, the catalog warns that the mock source
emits a bunch of low energy photons that could lead to
summation losses - something that could be especially
significant with a well counter. They suggest a cadmium
filter be used when counting the mock source to cut out its
low energy component.
ANSI 42.13 might say something about this but I don't
have a copy and can't be sure.
Salutations and regards
Paul
HP trivia: Otto Hahn died on this date in 1968
and last Tuesday, the 26th, was the 99th wedding aniversary
of Marie and Pierre Curie