Re: Scattered X-Rays

["David Scherer" <scherer@mirlink.wustl.edu>]

A good reference for scatter x-ray beams is found in NCRP 49.  In the section
on secondary barriers (p.59) there is a table showing the  scattered/primary
ratio at various angles for several tube voltages.  The data indicate that the
exposure is greatest in the backscatter direction (toward the source), not the
forward (II) direction.  I looked at Dale Trout's 1972 paper in Radiology, from
which the table was extracted, and he has very nice polar plots of the
secondary fields.  Does this agree with everyone's experience?

This intrigues me a little, since the Compton cross section is slightly forward
peaked at diagnostic energies.  My guess is that the lower energy  (more highly
scattered) photons have much higher mass absorbtion coefficients in air, so the
overall exposure is higher.  The f-factor is just about flat in this energy
range, so the backscattered radiation should result in a higher absorbed dose,
too.

David Scherer