  The so-called roll bar measurement uses a heavy metal material, optically
thick to x-rays, to form a shadow of the x-ray origination spot. This spot is
where an energetic electron beam interacts with a high Z target. The material
(the "roll bar") is slightly curved to avoid alignment problems. The roll bar
is constructed and positioned so that the x-rays are shadowed in the horizontal
and vertical directions, so information is obtained in two dimensions. If a
beam profile is assumed (or measured by other means), the equivalent x-ray spot
size can be calculated from the x-ray shadow cast by the roll bar. Thus the
ellipticity of the beam can be calculated, assuming the ellipse of the x-ray
spot is aligned with the roll bar. The data is recorded using a scintillator
and gated camera. Data will be presented from measurements using the ETA II
induction LINAC. The accuracy of the measurement is checked using small
elliptical targets.
