RE: Ancient radiation levels were higher than today's levels

["Karam, Andrew" <Andrew_Karam@URMC.Rochester.edu>]

Am I out-of-date on natural radiation dose?  The latest UNSCEAR estimate I
have is two-thirds of the datum quoted here, and even that's rumoured to be
coming down by another 10% or so, largely because of a revision in the dose
coefficient for radon.

Dear Bruce:

We considered only beta-gamma radiation levels from geologic and biologic
materials.  Doses from rocks are higher than from soil because, in soil,
isotopes are "diluted" by the organic materials and the water that goes into
weathering products (clay, silt, etc.).  So dose from rocks will be higher
than from the soil overlying them.  Dose was calculated using the dose
conversion factors from Eisenbud and Gesell, corrected for changing U-235
abundance with time.

Dose from biological materials was based on "average" bacterial
concentrations of potassium (about 60% higher than in mammalian tissues).
We assumed that early organisms existed in mats that were thick enough to
stop the K-40 beta but that gammas escaped.  This seems reasonable, given
the fossil record.  Dose was calculated as beta energy deposition in terms
of MeV per gram of organism and converted to mGy/yr.

Radon and other alpha-emitters were not considered because early life lived
in the water and alphas have a very short range (about 1 micron) in water.
Since uranium solubility is very low in anoxic waters, we assumed that very
little U and very few U progeny would be present.  This does not, of course,
address direct alpha radiation from U, Th, and progeny in sediments, but wee
felt that dose would be low because of the low abundance of U in most rocks
and the fact that, even with sediments, there is often a thin layer of water
between an organism and the sediments.

Sincerely,

Andy
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