M. BruceR
132
point. Thus the useful life of mock-iodine
is 10 years. A 15 or 20 year period could
easily have been set up that would meet
the requirements, of most instrumentation,It
is felt, however, that for calibration purposes,
Acta Radiolégica
Interamericana
what instrumentation is used (excepting spec-
trometers) the results from measuring any
iodine sample should be about the same as
the result from measuring an identical mockiodine sample.
THE RELATIVE PROPORTIONS OF LOW, MEDIUM AND HIGH ENERGIES IN Ba*—C,187 MIXTURES
MAKING UP MOCK IODINE
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{. The expected differences between iodine 131 and mock-iodine at the extremes of the accepted intercalibration mixture.
the accuracy of the 10 year period should
be in excess of the accuray that is obtainable
with most instrumentation.
In Fig 4 the differences that can be expected between iodine 131 and mock-iodine
at the extremes of the accepted intercalibration mixture are shown by dividing the spectrum into 4 energy regions. It is seen that
not quite 40 per cent the total energy emit-
ted from iodine 131 is in the arbitrarily defined X-ray region. There is a slightly lo-
wer percentage in the X-ray region with the
13.1/1 starting mixture and thereis a slight-
ly greater percentage in the 8.2/1 expiration mixture. About 20 per cent of the energy from iodine 131 is in the low-energy
region. Another 40 per cent is in the medium region. The mock-iodine mixtures have
about the same distribution of energy. Only
about 1-1/2 per cent of the distribution is
in the high-energy region. Thus it can be
seen that mock-iodine matches the gamma-
ray spectrum of iodine 131, and no matter
It was necessary to suspend the mock-io-
dine in extended sources of a variety of shapes in order to mimicclinical conditions. To
do this the mock-iodine was suspended on
an ion-exchange resin. The ion-exchange
resin is a tissue-like, unit-density material
that not only matches soft tissues but also
tightly binds the isotopes so that their use,
even when the containers are broken, is re-
latively safe. After the material was bound
on the ion-exchange resin, it was measured
in small lots, against samples calibrated by
the National Bureau of Standards in a high-
pressure ionization chamber. These small lots
of active material were then mixed with large amounts or small amounts of inactive re-
sin to obtain a uniform dispersion of activity throughout any required volume of material.
The mock-iodine was then put into a calibration manikin. A great deal of trouble
was taken to math as exactly as possible a
simulated clinical condition. The manikin