attenuation coefficients in air

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

Dear Radsafers -

I have been doing some calculations using mass attenuation coefficients for
gammas in air for gammas with energies from 1 to 10^5 MeV.  I noticed that
the attenuation coefficients start to rise again for gammas above an energy
of about 50 MeV, so dose seems to decrease from higher-energy gammas.  Since
I have no direct experience with gammas above about 10 MeV, this is
something I'm unfamiliar with.  The coefficients I am using are those from
Table 5.1 of the 1990 Rad Health Handbook.  This raises the following
questions:

Are these mass attenuation coefficients accurate?

If so, what causes this increased shielding effectiveness for very
high-energy gammas?  This seems to contradict what I would intuitively
expect, but my intuition's been wrong before.  Does the pair production
cross-section start to rise above about 50 MeV?

As always, I appreciate your help!

Andy


P. Andrew Karam, CHP
Radiation Safety Officer
University of Rochester
(716) 275-1473 (voice)
(716) 256-0365 (fax)
andrew_karam@urmc.rochester.edu 

There is no Chase so pleasant, methinks, as to drive a Thought, by good
conduct, from one end of the World to the other; and never to lose sight of
it till it fall into Eternity, where all things are lost as to our
knowledge.
T. Burnet, The Theory of the Earth, 1697

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