B. L. LARSON AND K. E. EBNER
earths, such as cerium'™* and praseodymium, are not absorbed from the gut
and also are very poorly taken up by plants. Barium}?is not a problem, since
its half-life is only 13 days.
Cesium!7 (Cs!") is related chemically to potassium and is distributed in the
body somewhat like potassium (1, 19a). That is, it is found in muscle tissue,
blood. and other soft tissues. Cs!8? has a half-life of 27 yr., while its daughter,
Ba!*, has a half-life of 2.6 min. Beeause of its long half-life, Cs!8’ would be a
potential danger to the body, but fortunately it is removed fairly rapidly. The
biological half-life in the body is 200 days. Another factor lessening the absorption of Cs!8* in man is that plants take up little cesium from the soil colloidal
complex.
The pathway of Strontium-90 into man. Sr-90 is potentially the most dangerous nuclide to man that is formed in the fission reaction. Studies on the geo-
graphical distribution of the Sr-90 blown into the atmosphere indicate that the
majority has remained in the temperate zone of the northern hemisphere (10,19e).
Sr-90 has a long physical and biological half-life and is produced in relatively
high vields in the fission reaction. The Sr-90 formed probably combines with
oxygen to form Sr®°Qce, which gradually settles onto the earth’s surface. It has
been estimated that it takes 7-10 yr. to settle out one-half of the amount of Sr-90
formed from a blast (10, 14).
Chemically, strontium and calcium are similar, since they are in the same
group of the periodie table, but physically they differ, since strontium is about
two and one-half times as heavy as ealcium. There are several discrimination
steps against the uptake of Sr-90 in its passage from the atmosphere to the bones
of man and, apparently, this is due to the differences in physical properties of the
two elements.
The Sr-90 deposited on the earth either remains on vegetation which may
be eaten by animals or is washed into the soil. That which is washed into the
soil is bound to the colloid complex similarly to calcium and is retained in the
top few inches, thus being readily available to short-rooted plants. Plants discriminate against Sr-90 by absorbing it about 0.7 times as fast as calcium if
sufficient calcium is available (5,15). When plants are eaten by animals, a
portion of the Sr-90 absorbed is deposited rather permanently along with calcium
in the bone, though at a somewhat slower rate than calcium. This deposition
occurs mainlyat sites of active bone growth and bonetissue replacement. Sr-90
can also be passed from the blood stream into a developing fetus and into milk.
Sinee Sr-90 and calcium metabolism are closely associated, concentrations
of Sr-90 are generally expressed in terms of caleitum. The most common unit
used is the Sunshine Unit (S.U.), which is equal to the number of micromicrocuries of Sr-90 per gram of calcium (see Appendix). It is important to note that
the Sunshine Unit represents the ratio of Sr-90 to calcium present. In itself it is
not an absolute amount of Sr-90, although it is often used in this sense by assuming that the calcium level in the body is 1,000 g.
Estimated values of the discrimination against Sr-90 in the passage from
[4]