7.3.1} The FLUKA geometry reference frame

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 The FLUKA geometry, described in Chap. 8}, and all particle space coordinates
 (position x, y, z, and corresponding direction cosines) are based on a
 right-handed Cartesian reference frame. The origin of the frame (0, 0, 0) and
 the direction of the three perpendicular axes can be chosen arbitrarily by the
 user, but by default a particle beam and its space characteristics (angular
 divergence, transversal profile, polarisation) are referred to a beam direction
 coincident with the z-axis of the geometry frame of reference. A different beam
 direction can be specified by means of command BEAMAXES: see a description of
 the command and especially the corresponding Notes.

 Also some scoring structures (binnings, see command USRBIN) are defined as
 meshes parallel to the reference axes, and in the case of cylindrical binnings
 the basic axis is again the z-axis. In a similar way, some geometrical bodies
 (planes, parallelepipeds, infinite cylinders) are described by their
 orientation with respect to the axes. But rototranslation transformations,
 defined by commands ROT-DEFIni and ROTPRBIN, allow the user to input binnings
 and geometrical bodies with arbitrary orientation with respect to axes.

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