Re: Background radiation levels

[rkathren@beta.tricity.wsu.edu (ronald kathren)]

Conceptually, Dale is on target, but the problem is a little more complex
than one might think.  There are likely radioactive species produced at the
time of formation of our earth (including the 4n + 1 (Neptunium) series that
have, for all practical purposes, decayed away by now.  The presence of
these nuclides would increase the dose rate.  But detrmining what was
present tens of millions of years ago might well be impossible.  

Also, I take exception to the statement that half-lives and decay schemes
are 'pretty random'.  Reference to any nuclear physics textbook will reveal
the well established rules governing nuclear decay.

Ron Kathren
rkkathren@beta.tricity.WSU.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
>
>