Section 3.2.2.2.2. Apparatus and instrumentation before the crystal

The data items in these categories are as follows:

(a) DIFFRN_ATTENUATOR

(b) DIFFRN_RADIATION

(c) DIFFRN_RADIATION_WAVELENGTH

(d) DIFFRN_SOURCE

The bullet () indicates a category key. The dagger () indicates a deprecated item, which should not be used in the creation of new CIFs.

Attenuator properties are described by data items in the DIFFRN_ATTENUATOR category. Where an attenuator is used to reduce the intensity of an X-ray beam, this category may be used to describe the attenuator and its scaling factor. Details of multiple attenuator settings or materials can be included and each is identified by a code. A matching code value ( _diffrn_refln_attenuator_code) appears in the list of intensities against each reflection that must be scaled by the appropriate attenuation factor. In Example 3.2.2.1, the intensity of the second reflection has been reduced using a zirconium attenuator and must be multiplied by 16.976 to place it on the same scale as the first (and other unattenuated intensities).

The DIFFRN_RADIATION category describes the radiation used in the diffraction experiment and its experimental handling by collimation and monochromatization before it interacts with the sample. [Post-sample treatment of the radiation beam after diffraction (including passage through any analyser or collimator) is described by data items in the complementary DIFFRN_DETECTOR category.] Many of the data items in this category are descriptive. Additional information about the generation of the radiation is also found in the DIFFRN_SOURCE category.

The use of _diffrn_radiation_probe is strongly recommended as an unambiguous indicator of the probing radiation or particle type (its permitted values are x-ray, neutron, electron and gamma). The similar-sounding data name _diffrn_radiation_type allows for a more detailed description of the radiation type, such as white-beam or (using the CIF code for the Greek character α, \a) 'Cu K\a' for copper Kα radiation. In the case of monochromatic (or near-monochromatic) X-radiation, a better representation is given by the use of _diffrn_radiation_xray_symbol, which can have one of a limited number of values expressing the X-ray wavelength according to IUPAC conventions (e.g. K-L₃, corresponding to the older Siegbahn notation Kα₁). If this data item is used, the element used as the X-ray generator target must also be specified using the data item _diffrn_source_target. Software for reading CIFs should be aware of these two alternative representations.

If the radiation beam is monochromatic, the wavelength can be provided using _diffrn_radiation_wavelength. For a polychromatic beam, the other data items in the DIFFRN_RADIATION_WAVELENGTH category allow different wavelength components and an associated weighting factor for each component to be listed. In the list of experimental intensity measurements from a polychromatic beam (the DIFFRN_REFLN category, discussed below), each reflection has an associated _diffrn_refln_wavelength_id that must match the corresponding _diffrn_radiation_wavelength_id in this list.

The DIFFRN_SOURCE category specifies the characteristics of the radiation source in the experiment and is closely related to the DIFFRN_RADIATION category, which is concerned with the handling of the radiation beam before it reaches the specimen. (The now-deprecated data name _diffrn_radiation_source shows that there was no formal separation of the descriptions of the radiation generator and the radiation in the first release of the core dictionary.)

The general class of radiation is specified by the data name _diffrn_source, which is a free-text field. Typical entries would be 'sealed X-ray tube', 'nuclear reactor', 'synchrotron', 'spallation source', 'rotating-anode X-ray tube' or 'electron microscope'. It is clear that the category could describe non-X-ray experiments, but several of the data names within the category (e.g. _diffrn_source_target) have meanings that are specific to an X-ray experiment. New data names might be introduced if experiments using other radiation types become more common. For now, details that a user wishes to record that are not properly described by the existing data names may be stored in the _diffrn_source_details field.