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thorium and votassiua in the ground and in our surr aindings.
We know these quantities amount to something like i 0 milliroentgens per year for an average person in this la itude.
But we also know that there are considerable variations
with conditions.
For example, a perscn living in a brick house may
very weil get 25 to 50 milliroentgens per year more than one
living in a wooden house, because of tne natural radioactivity of the bricks.
It is aiso very well known that whereas
-at sea level in this latitude the cosmic ray dosage is 37
.milliroentzgens per year, at 5000 feet altitude as in Denver,
Colorado, the dosage from cosmic rays is 60 milliroentgens
per year, or a difference of 23 milliroentgens per year.
is this in terms of strontium-90 body burden?
What
First, we must consider what part or the natural
radiation, if any, is similar to the radiation of strontium9C in biological effect so we can say without deoudt and
hesitancy that the physiological effects, whatever they
are, will be the same for the same energy adsorbed.
Fortunately, the cosmic rays seem to fit this bill.
in other
_ words, we are at liberty to compare the cosmic ray :adia_ tion dosages with the dosages from radiostrontium in our
bone structure. The reason this is permissiole is “nat the
ionization density along the tracks of the mu-mesons which
are the principal cosmic ray components at saa level and at
altitudes of 5000 feet are nearly the same as those of the
" yttrium-90 beta rays, the principal radiation which radicstrontium emits; that is, radiostrontium has a radicactive
daughter, yttrium-90, which emits a very energetic veta ray
and the ionization density along the track of this radiation
is very similar to that of the mu-mesons of the cosmic rays
and their disintesration electrons, and it is generally accepted by health physicists and radiobiologists tnat radia~
tions of the same ionization density have very similar, if
not identical biological effects for the same energy abe
serbed. The high energy of the vttrium-90 gives it an
average distance of penetration in tissue of 2 millimeters
so any effect of local noneuniformity of deposition of
strontium-90 in the bone is removed.
The cosmic ray ex-~
posure is, of course, uniform throughout the bone structure.
Therefore, we can equate cosmic ray dosage with strontiumn-
. 90 dosage and thus it is possidle for us to say that the
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