Vol. VII, No 3-4
131
RADICIODINE UPTAKE MEASUREMENT
Since all instruments except spectrometers
integrate the two peaks, it has been experimentally determined that the difference in
this portion of the spectrum causes 2 to 4
per cent difference in the comparative measurement of iodine and mock-iodine by most
instruments. In the X-ray portion of the
spectrum thereis slightly too much of the 80
key peak and slghtly too little of the 35 kev
peak. This causes another 2 to 4 per cent
difference in the comparative measurement
of iodine and mock-iodine (which may or
may not cancel out the medium-energy ditfference depending upon the instrumentused).
It has been empirically determined that in a
wide variety of instruments, the difference
in the measurement of iodine and mock-iodine is from O to 5 per cent
When the mock-icdine is used in its raw
state, both the barium 133 and the cesium
137 contribute entirely too large a proportion of the very low-energy X rays, however, this can be corrected by partially shielding the raw mock-iodine with a medium Z
metal. Figure 2 shows the results when suc-
cessive layers of babbit metal (a tin-antimony
mixture) are interposed between the mockiodine and the detecting crystal. The very low
energies decrease very fast. The higher ener-
gies are almost unaffected. Thus when the
correct thickness of babbit metal shields the
mock-iodine, a compromise absorption is ob-
tained. It has been found empirically that
a 0.82 mm thickness of babbit metal ts the
best compromise filter the raw mock-iodine.
All the mock-iodine sources are therefore
manufactured and used in a container made
of babbit metal.
Since barmmm 133 and cesium 137 have
widely different half lives, the mock-iodine
does not have a true half life. It does. however, have a useful life about 10 years. In
Fig. 3 the method of arriving at this useful
life is illustrated. The ideal mixture of barium
133 and cesium 137 has been shown to be
10.5 units of barium 133 to 1 unit of ce-
sium 137. A millicurie measure is used for
the cesium, but since the decay scheme of
barium 133 is unknown, an arbitrary milltcurie had to be defined for barium 133.
The barium 133 decays with a half life
of about 9.5 years. The cesium 137 decays
with a half life of about 33 years. Neither
of these half lives is exactly known but they
THE PHYSICAL DECAY OF THE TWO
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Fic. 3. The method of arriving at the useful life of
mock-iodine
are approximately true. If one starts out
with a mixture containing an excess of barium, the decay will extend through the ideal’
mixture and will eventually show an excess
of cesium 137. Many different mixtures of
barium and cesium were made up on an experimental basis and were empirically com-
pared with identical iodine 131 source. It
was found that, with some kinds of instrumentation, when there was an excess of ba-
rium 133 (much greater than a 15 to 1 ratio)
significant differences in the comparison of
mock-iodine with iodine 131 began to appear.
When the mixtures contained too little ba-
rium 133 (muchless than a ratio of 8 barium
133 to l cesium 137) variations again appeared in the comparison with iodine 131 with
different kinds of instrumentation. Therefore, an arbitrary cut-off point was made
with a starting mixture of 13.1 to 1, which
represented a 5-year decay period before the
ideal mixture was reached. An expiration
point was arbitrarily set up at a mixture of
8.2 to 1, which represented 5 years of decay
after the mixture had gone throughtheideal