International
Tables for
Crystallography
Volume G
Definition and exchange of crystallographic data
Edited by S. R. Hall and B. McMahon

International Tables for Crystallography (2006). Vol. G, ch. 3.3, pp. 119-120

Section 3.3.4.3. Instrument calibration and description

B. H. Tobya*

aNIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562, USA
Correspondence e-mail: brian.toby@nist.gov

3.3.4.3. Instrument calibration and description

| top | pdf |

The data items in these categories are as follows:

(a) PD_CALIB [Scheme scheme4]

(b) PD_CALIBRATION [Scheme scheme5]

(c) PD_INSTR [Scheme scheme6]

(d) Part of PD_DATA [Scheme scheme7]

The bullet ([\bullet]) indicates a category key.

Calibration information can be placed in the PD_CALIB and PD_CALIBRATION categories. The _pd_calibration_* data items are descriptive and will not appear in a loop. The _pd_calib_* items may be looped to describe multiple detectors. Correction values for [2\theta] can be given using the _pd_calib_2theta_offset and _pd_calib_2theta_off_* data items. A calibration equation can be given using _pd_calibration_conversion_eqn. When multiple detectors are used, _pd_calib_detector_response is used to indicate the relative performance of each detector. The detector deadtime is specified using the core data item _diffrn_detector_dtime (which cannot be looped by detector).

If an internal standard is added to the sample for calibration, this information is specified using _pd_calib_std_internal_name to specify the material added and _pd_calib_std_internal_mass_% to specify the amount.

When a set of calibration intensities is measured using an external standard, it is possible to include the measurements and the derived results in a separate CIF block. A data block would then use _pd_calib_std_external_block_id to link to the block containing the calibration information. See Section 3.3.7[link] for a discussion of block pointers and block IDs. Note that the use of a unique name for the block ID allows the calibration information to be stored in a separate file, so that the calibration CIF need not be repeated in every CIF that references it.

The PD_INSTR section of the pdCIF dictionary contains terms that describe the instrument used. For example, the instrument or laboratory location is given using _pd_instr_location. The instrument type can be indicated using _pd_instr_geometry. The instrument geometry can be described in much greater detail using several data items. The geometry is described in terms of four regions of the experiment: radiation source to monochromator (src/mono); monochromator to specimen (mono/spec); specimen to analyser (spec/anal); and analyser to detector (anal/detc). If no monochromator is present, the first two regions are combined into radiation source to specimen (src/spec). If no analyser is present, the last two regions are combined into specimen to detector (spec/detc). Thus two, three or four sets of values describe the dimensions of the instrument and the collimation. For example, _pd_instr_dist_src/mono would be used to specify the distance between the radiation source and the monochromator. Alternatively, _pd_instr_dist_src/spec would be used to specify the distance between the radiation source and the specimen if no monochromator was present.

Two methods may be used to describe the slits limiting the divergence in the equatorial plane. The angular divergence allowed by the slits in degrees can be specified using the _pd_instr_divg_eq_* data items. Alternatively, the dimensions of the slits in the equatorial direction in millimetres may be specified using _pd_instr_slit_eq_*. The dimensions of these slits in the axial direction, i.e. the direction perpendicular to the equatorial plane and containing the incident or diffracted beam as appropriate, are specified in millimetres using _pd_instr_slit_ax_*. The axial slit lengths, along with the _pd_instr_dist_* distances, are useful for estimating the low-angle peak asymmetry (Finger et al., 1994[link]). Note that angular divergence in the axial plane is not a well defined concept for line-focus instruments, but can be specified, where appropriate, using _pd_instr_divg_ax_*.

The axial and equatorial directions are shown schematically in Fig. 3.3.4.1[link]. The equatorial plane contains the equatorial direction vectors, as well as the incident beam, the diffracted beam and the scattering vector. The axial plane is perpendicular to the equatorial plane and contains the sample centre, which is the point where the incident and diffracted beams meet. For area-detection instruments, the designations of axial and equatorial directions may be arbitrary.

[Figure 3.3.4.1]

Figure 3.3.4.1 | top | pdf |

The axial and equatorial directions in a powder-diffraction experiment.

Soller collimators are described using _pd_instr_soller_eq_* data items rather than _pd_instr_divg_eq_* data items. It is common practice to specify the Soller collimation in arc-minutes (e.g. 30′). However, pdCIF defines these items to have units of degrees, so 30′ would be recorded in the CIF as 0.5. It is not usual to limit the axial divergence, except to reduce low-angle asymmetry, but if this is done, the _pd_instr_soller_ax_* data items can be used to define this.

For constant-wavelength instruments, it is common to have a monochromator or filter either before the sample, or after the sample (an analyser), or sometimes both. This is described using _pd_instr_monochr_pre_spec and _pd_instr_monochr_post_spec. It is rare, but possible, to have both a filter and a monochromator in the same location. The BT-1 neutron powder diffractometer at NIST uses both a Cu(311) monochromator and a graphite filter to attenuate the [\lambda/2] component. In this case, the two elements would be placed in a loop: [Scheme scheme8]

Note that the monochromator and analyser takeoff angles are given using _pd_instr_2theta_monochr_pre and _pd_instr_2theta_monochr_post. It is useful to record these values for X-ray studies, as they are needed for proper polarization corrections.

In a conventional Bragg–Brentano diffractometer, the divergence slits limit the illumination area at the sample. However, since the [\varphi] axis (the sample [\theta] axis) is usually set to bisect the [2\theta] angle of the detector, the actual length of the area of the sample that is illuminated changes with [2\theta]. One should choose divergence slits so that the beam does not illuminate areas outside the sample at the lowest diffraction angle used. An alternative method for data collection is to have a divergence slit that opens as [2\theta] increases, so that a constant area of the sample is illuminated. This is known as a [\theta]-compensating slit. Using a [\theta]-compensating slit provides a better signal-to-noise ratio at larger [2\theta] values, but means that the diffraction intensities have to be normalized to compensate for the change in illumination. It also introduces greater optical aberrations with [2\theta]; a flat plate becomes an increasingly worse approximation to the curved sample geometry in the true Bragg–Brentano geometry.

The use of a variable divergence slit can be recorded in the form: [Scheme scheme9] Note that if _pd_instr_cons_illum_flag is not specified, the value is assumed to be no, indicating that a fixed-width divergence slit has been used.

The beam size can be specified in two different ways: as the size at the source, using _pd_instr_source_size_ax and _pd_instr_source_size_eq, or as the size at the sample position, using _pd_instr_beam_size_ax and _pd_instr_beam_size_eq. Note that the size of the beam at the sample differs from the illumination length described above except when the sample is perpendicular to the beam. When a variable-divergence slit is in use, the beam size at the sample changes with [2\theta], so if this size is known directly, the _pd_instr_beam_size_eq data item can be included in the loop containing the diffraction intensities. Similarly, in a constant-divergence instrument, where the illumination length changes with [2\theta], the illumination length can be specified in the loop using _pd_instr_var_illum_len.

There are also several data items in the core CIF dictionary that should be present in the description of the instrument in a pdCIF. Use _diffrn_radiation_probe and _diffrn_radiation_type to specify the type of radiation used and _diffrn_detector_type to specify the detection type.

References

Finger, L. W., Cox, D. E. & Jephcoat, A. P. (1994). A correction for powder diffraction peak asymmetry due to axial divergence. J. Appl. Cryst. 27, 892–900.








































to end of page
to top of page