Section 3.7.4.4. The radiation source

Data items in this category are as follows:

DIFFRN_RADIATION

The bullet () indicates a category key. The arrow () is a reference to a parent data item. Items in italics are defined in the mmCIF dictionary.

The DIFFRN_RADIATION category is defined in the mmCIF dictionary (Section 3.6.5.2 ; see the detailed discussion in Section 3.2.2.2.2 ). The CBF/imgCIF dictionary adds the items _diffrn_radiation.div_x_source, *.div_y_source and *.div_x_y_source to specify beam crossfire, and the items _diffrn_radiation.polarizn_source_norm and *.polarizn_source_ratio to provide a definition of polarization relative to the laboratory coordinate system rather than relative to the diffraction plane. The value of the beam crossfire component _diffrn_radiation.div_x_source is the mean deviation in degrees of the X-ray beam from being parallel to the X axis as it illuminates the sample. The value of the beam crossfire component _diffrn_radiation.div_y_source is the mean deviation in degrees of the X-ray beam from being parallel to the Y axis as it illuminates the sample. The value of the beam crossfire component _diffrn_radiation.div_x_y_source is the correlation of the X and Y components. The value of the normal component of the polarization _diffrn_radiation.polarizn_source_norm is the angle in degrees, as viewed from the specimen, between the normal to the polarization plane and the laboratory Y axis as defined in the AXIS category. The dimensionless value of _diffrn_radiation.polarisn_ratio is the ratio , where is the intensity (amplitude squared) of the electric vector of the illumination of the sample normal to the polarization and is the intensity of the electric vector of the illumination of the sample in the plane of polarization. With suitable choices of laboratory axes, the definitions conform to synchrotron conventions. See Chapter 4.6 for a detailed description of these items.