ess.reduce.unwrap.lut.chopper_distance_along_beam#
- ess.reduce.unwrap.lut.chopper_distance_along_beam(axle_position, source_position)[source]#
Flight distance from the source to a chopper along the incident beam.
A disk chopper’s axle is offset from the beam (the disk sits above, below, or to the side of it), so the straight-line distance
norm(axle_position - source_position)overestimates the flight distance, and for closely-spaced choppers can even place them in the wrong order along the cascade. The neutron crosses the disk where the incident beam intersects it, so the flight distance is the projection of the axle position onto the incident-beam direction. Assuming the incident beam runs along the z axis, this is the z-component of the offset from the source. This matches the straight-lineLtotalused for detectors and monitors, where the component lies on the beam and the projection reduces to the norm.