Fe-55 and soft photon emitters

[scherer@uiuc.edu (David Scherer)]

It seems to me that the exposure rate constant approach is not very useful
at these very low energies.  Except for monatomic layers, self absorption
cannot be ignored.  In fact, the atomic cross section for Fe at 6 keV is
8230 barn/atom, so if an Fe atom emits one photon, the probability of
surviving only one neighboring atom is 6 percent (assuming an atomic radius
of 1 A).  Put that atom is a solid and the escape probability is nil.  (For
air, the cross section is about 960 barn/molecule, still quite high.)

In cases of low energy x-ray emitters, some kind of transport analysis is
needed, even if it is just a point kernal (line-of-sight attenuation) approach.

Cross section data are from the program "mucal" produced at IIT.  Visit
their interactive Web site at http://www.csrri.iit.edu/mucal.html

Dave Scherer
scherer@uiuc.edu