Re: Background radiation levels

["Dale E. Boyce" <dale@radpro.uchicago.edu>]

While I do not have a direct reference to radiation levels during
prehistoric times, I can point out some of the input parameters
you need to specify to estimate the activity on a first order
back of the envelope calculation.

1.  Time zero.  The time when the materials that accreted to form
the solar system were formed.  This is probably between 5 aned 20
billion years ago.  I don't have a reference, but it would be sometime
prior to the 4.6 billion year age of earth and less than the estimated
age of the universe.

2.  In nucleosynthesis the neutron capture rate during R process is
so great that you make every neutron excess radioisotope.

3.  Half-lives tend to be pretty random.

4.  Decay schemes tend to be pretty random.

5.  Because of 3 and 4 the dominant radioisotopes will tend to
have half-lives comparable to the time since time zero.  

6.  Because of 5, total activity tends to behave as 1/time since
time zero for times long compared to the synthesis time.  For example:
if you irradiate U235 with thermal neutrons, you will see activity
fall off as T^(-1.1).

7.  Because of 4, the exposure rate will behave similarly to the
total activity.

So if for example you pick 8 billion years ago as time zero, the
background from radioactive material 4 billion years ago would
be about twice what it is now.

This ignores cosmic radiation which probably has changed somewhat,
but if one presumes that life evolved out of the ocean, then the
ocean would tend to overwhelm in difference in atmospheric pressure
and in soft cosmic ray flux.  One then is left with the question
of whether or not there have been significant changes to the
hard component of cosmic rays.  I can't speak to this question.

It is an interesting thing to think about the collective historical
dose to DNA.  Pick your number for your oldest ancestor:

1 billion years * 0.3 Rem/yr = 300 million Rem

1 million years ....         = 300 thousand Rem

100,000 years                = 30  thousand Rem

6,000 years                  = 1,800 Rem

Dale Boyce
dale@radpro.uchicago.edu