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Re: Mass Energy Absorption Coefficient -Reply



Howdy:

First, a few general comments intended to provide a gut feeling for 
the mass energy absorption coefficient.  Then, some thoughts on Dales
question. Apologies for the length of this post.

General comments.

1. In a simple sense, the linear absorption coefficient, uen, is the
fraction of the photon (i.e. gamma ray or x-ray) energy absorbed per
unit distance travelled. Its units are per cm.  

2. This number is usefull for all sorts of calculations (eg
dosimetry) but it depends on three things: the energy of the photon,
the atomic number of the absorbing material (eg tissue, air etc) and
the density of this material. 

3. We often make life a little easier by normalizing this to density
i.e. we divide it by the density of the absorbing medium and report
uen/p  where p is the density in g/cubic cm. This is the mass energy
absorption coefficient. It units are cm squared/g. Unlike the energy
absorption coefficient, this only depends on two things: the photon
energy and the absorbing material's atomic number.

4. Dave has provided a version of the ICRU definitions for the mass
energy absorption coefficient. My gut definition is not nearly as
accurate but might be somewhat more satisfying: the fraction of the
energy absorbed by a chunk of whatever material we are considering.

Thoughts about Dales post: 

>Good catch Alan.  Not too picky at all since any energy escaping
>from the volume under consideration should probably be included

Perhaps the key to the problem that follows is related to "the volume
under cosideration"

>at least one reference, Tsoulfanidis "Measurement and Detection of
>Radiation", ignores the flourescence correction to the photoelectric
>effect and the pair production correction for the escaping
>anihilation photons.  The [photoelectric] flourescence is probably
>ignored because the low energy photons are locally absorbed
>Tsoulfanidis comments that the correction for the 511 photons is
>sometimes made and gives a reference, Hubbell,
>J.H., NSRDS-NBS 29 August 1969, that makes this correction.  Since
>Hubbell is one of the most common sources of Mass Energy Absorption
>Coefficients, it seems to me to be questionable why anyone would
>leave that correction out of their definition for the coefficient.

One problem is that there are all sorts of names for coefficients
floating around that might or might not mean the same thing: true
absorption, real absorption, energy absorption, dose absorption,
energy transfer etc (see the beautiful section in Attix's "Radiation
Dosimetry Vol 1" by Evans). 

My poor understanding would have me believe that the mass energy
absorption coefficient and the mass energy transfer coefficient
usually apply to an infinitimally small region. As such the 511 keV
annihilation photons from pair production should not be considered to
be absorbed -  nevertheless: 

Why might Tsoulfanidis assume the 511 stuff (along with the
photoelectric effect x rays) were absorbed? Because his "volume under
consideration" is large. It could be said he is interested in the
real world rather than an infinitesimal point and is not  cocerned
with what Hubbell does.  Evans comments "For larger absorbers it has
been a common approximation to ignore this correction term for
annihilation radiation." In other words, how many 511 photons
produced in someones body by pair production would escape? Not many.

Best wishes, and Dale, it was a great pleasure meeting you in Boston.
Ellsworth, my daughter loved the "switchback?" Thanks.

Paul Frame
Professional Training Programs
Oak Ridge Institute for Science and Education
framep@orau.gov