Beam crossfire correlation in degrees squared between the
crossfire laboratory X-axis component and the crossfire
laboratory Y-axis component (see AXIS category).
This is a characteristic of the X-ray beam as it illuminates
the sample (or specimen) after all monochromation and
This is the mean of the products of the deviations of the
direction of each photon in XZ plane times the deviations
of the direction of the same photon in the YZ plane
around the mean source beam direction. This will be zero
for uncorrelated crossfire.
Note that for some synchrotrons, this value is specified in
milliradians squared, in which case a conversion is
needed. To convert a value in milliradians squared to a value
in degrees squared, multiply by 0.1802 and divide by \p2.