2. Discussion of theories

We utilize a coordinate system in which the photon momentum defines the *z* axis, and the *x*-*z* plane contains the initial
electron momentum
and .
For convenience we define
as the angle between
and the initial electron
momentum ,
as the angle between
and the
final electron momentum ,
and
as the angle between
the plane containing
and
and the plane containing
and .
This coordinate system is shown in Figure
6.1. We define the energies of the initial and
final electron and the photon as *E*_{1},*E*_{2} and *k*, respectively. We
also define the momentum transfered to the nucleus
.
The quantity of interest in our discussion is
the triply differential cross section
for radiation in
scattering of an unpolarized electron from a neutral atom of atomic
number *Z*, where the polarization properties of the radiated photon
and scattering electron are not observed. Here ()
is the finite solid angle subtended by the photon
momentum (outgoing electron momentum). (Some discussion of the
polarization correlations associated with the triply differential
cross section was given in [4].)