Re: Background radiation levels

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

Excuse me for being unclear on the nuclear.  Of course the nuclear
physics is far from random for a specific isotope.  I was taking,
as a first order approximation, that in nucleosynthesis one makes
about the same number of atoms of radionuclides in each half life
category, since there are so many nuclides that are formed.  The
decay energies of beta emitters out at the neutron drip line have
very high decay energies, but are gone very quickly.  Once you
get into the realm of half-lives that are long compared to the
accretion time of earth, you still have beta decay rates 
proportional to E^5, but there are still a distribution of levels
of disallowed transitions that tend to keep a distribution of
half-lives in a large range of decay energies.

For beta emitters we have a pretty good experimental test in that we
see the 1/T ish behavior not only in the distribution resulting
from thermal neutron induced fission, but also from hig energy
spallation.  High energy spallation results in a substantially
different mass yield curve than fission, and nucleosynthesis is
different yet.  

The real error in my original post is that beta emitters with a
few exceptions K-40 etc. are no longer the bulk of the back ground
radionuclides.  Still if the age of the earth is 4.6 billion
years and the dominant progenetors of our background radionuclides
are U-238 and Th-232, my original assumption that the dominant
radionuclides in activity tend to have half lives comparable to
delta T since T zero still holds pretty well.

I agree completely that there is a lot more to it.  I was just making
a back of the envelope estimate to show that one would not expect
huge differences in background due to terrestrial sources of radiation.

Fun stuff ;)

Dale Boyce
dale@radpro.uchicago.edu