-19-
UCRL-3644
and estimates of effects derived from this additional tissue burden will be
correspondingly small compared with other human problems.
At the present time, according to the Libby report (October 1956),
there is in the stratosphere about 2.2 megacuries of Sr?9, * and a similar
quantity of cesium-137.f If all the material in the stratosphere (in the fall
of 1956) were to descend upon the surface of the earth uniformly, the amount
of either Sr?9 or Cs137 would be about 12 millicuries per square mile. The
time of retention by the stratosphere of highly dispersed fission products is
on the order of many years. Measurements indicate approximately 10%fallout per year and 2.5% radioactive decay. As about 25% has been added to the
stratospheric reservoir of dispersed fission products during the past two years,
the level in the stratosphere has remained nearly constant over that time.
The quantity of Sr?° in the soil of the United States is somewhat greater than
expected from the fallout estimated on an average global basis; in the far
west it is 23 mC of Sr?9 per square mile. This is due to the heavier fallout
in the near vicinity of a nuclear explosion.
Strontium-90 distribution to September 1955
mC/square mile
World-wide except U.S.A. and Pacific Islands
U.S.A. except Utah,
3, 46
Colorado, New Mexico,
4, 9°
and bordering regions
12.5
Utah, Colorado, and New Mexico
20-23
The specific ratio of Sr? to normal calcium is a convenient way of
expressing the gr90 problem.§ This is because strontium closely follows
“Strontium -90 has a half life of 25 years and decays by emission of a B
particle of 0.54 Mev maximum energy to produce yttrium-90. Yttrium-90 is
short~lived (half life 65 hr); it decays to the stable zirconium by emission of
a 8 particle having a maximum energy of 2.24 Mev.
Because of the short
half life of the daughter product and the probable insoluble chemical form of
yttrium, the radioactivity of Sr?9° is equivalent to both its own beta decay and
that of ¥90,
. t Cesium-137 has a half life of 33 years and decays by B emission (0.52 Mev
maximum energy) with associated y emission (0.66 Mev energy).
8A convenient concept, established by relating irradiation of bone to bone
cancer, is that a maximum permissible concentration (1 MPC) of strontium-90
is equal to 1 pC Sr90 per 1000 grams of calcium. The concentration of calcium in the bones is such that 1 MPC can also be expressed as 1 pC Sr99 per
7000 grams of bone. The concentrations of radioactive strontium are usuall
expgessed in units of 0.001 MPC; the equivalence is 0.001 MPC = 1.4 x 107 ‘pC
Sr’
g of bone, corresponding to 0.0038 r/year.