3.
The exeretion rate is quite important.
A single dose in-
gested will be 50% excreted in approximately two weeks and 95% excreted
within a year.
The variously estimated 2-5% remaining may be considered
permanently fixed in the bone.
4.
19.9
Strontium-90 has a half-life of 20_to-30-years and decays
through the radioactive isotope yttriun-90 to the stable element
zirconium-90 by twobetaemissions.
Strontium-89 has a half-life of
55 days and decays by a singlebetaehiission’to stable yttrium-89.
A graphical comparison in Figure 2) shows that the strontium-89/
strontium-90 ratio of initial intensities is approximately 130/l.
If
this is reduced by a factor of 2 to take into account the two to 1 ratio
of beta particles emitted per disintegration by strontium-90, it results
ina 65/1 initial radiation intensity ratio.
This ratio by virtue of
the relatively long half-life of strontium-90, may be considered as
being reduced by a factor of 2 every 55 days.
The curves in Figure 21
indicate the relative intensities of strontium-89 and strontium-90
present at times after deposition in bone assuming the limiting extreme
of immediate deposition after detonation.
With proper graphical pre-
sentation, the areas under the curves give the relative proportions of
strontium-89 and strontium-90 radiations that will occur in the bone.
Calculations based upon the above considerations indicate that
strontium-90 will be the greater contributor of the total dose to
bone.
Pathway Through the Biosphere.
Some qualitative and quanti-
tative understanding of the behavior of strontium-90 as it passes
through the physical er.vvironment is necessary in order to interpret
the data relating to pathway through the biosphere.
No significant
studies have appeared which suggest that inhalationpathway into
the human is important.
Therefore, the mechanical and biological
transport of strontium-90 through the food chain will be given primary
consideration.
Intake of water contaminated with strontium-90 will
be considered as augmenting the food chain.
102