Re: Mass Energy Absorption Coefficient

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

Alex, an addition to Dave's post re: the ICRP definition of
mass energy absorbtion coefficient.  Because it relates to
the energy loss of the incident beam only, it predicts the energy
deposited per unit volume at the entrance surface.  Since some
of the energy of the interacted photons is carried off by
lower energy brem photons, the mass energy absorbtion coefficient
is always numerically smaller than the mass attenuation
coefficient.  The are at least two useful uses of the mass
energy attenuation coefficients in everyday health physics.
One, since it allows the calculation of energy deposited per
unit volume, one can use the mass energy attenuation
coefficients for tissue to calculate entrance dose rates.
Secondly, by using the mass energy absorption (pardon my
previous mispelling) coefficient in simple shielding calculations
instead of the mass attenuation coefficient, you will
always get a result that corrects conservatively for
build up.  For thin shields the shielding calculations will
be close to what one gets using build up corrections.  For thick
shields it will be overly conservative.

Dale E. Boyce
dale@radpro.uchicago.edu