Data items in the DIFFRN_DETECTOR_ELEMENT category record
              the details about spatial layout and other characteristics
              of each element of a detector which may have multiple elements.
              In most cases, giving more detailed information
              in ARRAY_STRUCTURE_LIST and ARRAY_STRUCTURE_LIST_AXIS
              is preferable to simply providing the centre of the
              detector element.
 
        loop_
        _diffrn_detector_element.detector_id
        _diffrn_detector_element.id
        _diffrn_detector_element.reference_center_fast
        _diffrn_detector_element.reference_center_slow
        _diffrn_detector_element.reference_center_units
        d1     d1_ccd_1  201.5 201.5  mm
        d1     d1_ccd_2  -1.8  201.5  mm
        d1     d1_ccd_3  201.6  -1.4  mm
        d1     d1_ccd_4  -1.7   -1.5  mm