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

["Heinmiller, Bruce" <heinmillerb@aecl.ca>]

So then the statement, "presently, humans receive...", actually means,
"presently, biota in contact with geologic and biologic materials
receive..."?

Bruce Heinmiller
heinmillerb@aecl.ca

> ----------
> From: 	Karam, Andrew[SMTP:Andrew_Karam@URMC.Rochester.edu]
> Reply To: 	radsafe@romulus.ehs.uiuc.edu
> Sent: 	Thursday, December 16, 1999 9:53 AM
> To: 	Multiple recipients of list
> Subject: 	RE: Ancient radiation levels were higher than today's levels
> 
> Am I out-of-date on natural radiation dose?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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