DIFRAC

DIFRAC stroring raw intensities

Raw observed intensity data are output to the converted file. In the current version of the program we have dealt only with low-angle background - integrated peak - high-angle background counts, thus leaving the problem of the transfer of profile data to a later date. Conversion of raw counts to net intensities Pobs (i.e. with background subtracted and allowing for differences in effective scan speed) or to |Fobs|**2 values (i.e. applying a Lorentz- polarization correction to the net intensity) is not undertaken by this software. As the raw data are conserved in the converted file, a more detailed modelling and refinement of background than that normally undertaken at present will be feasible, whilst retaining the possibility of a classic data reduction. As manufacturers' diffractometer-control software specifies and records its scan conditions (i.e. scan time, scan width, scan speed and number of scans) in many different forms, we have chosen to standardize this information into four intensity-measurement coefficients cp, cb, cl, and ch most convenient for least-squares calculations.

Let Qobs be the observed integrated-intensity count measured by scanning over the reflection profile, and Lobs and Hobs the observed low- and high-angle background counts respectively. The model relating the raw observations of a diffraction experiment to derived parameters may be written as Qcalc = cp Pcalc (vm) + cb Bcalc ~ Qobs, Lcalc = cl Bcalc ~ Lobs and Hcalc = ch Bcalc ~ Hobs. Pcalc is the calculated net integrated intensity per unit inverse scan speed and is a function of vm adjustable parameters describing the crystal structure, the scale factor, secondary extinction and possibly other physical quantities. Bcalc is the calculated count rate per unit time of the mean background and is itself an adjustable parameter. One can see that in general, cp is equal to the time to count the peak divided by the scan width i.e. the effective scan speed allowing for multiple scans over the same peak, cb to the time to count the peak, cl to the time to count the low-angle background and ch to the time to count the high-angle background. It follows that the scan width is equal to cb / cp.

For a classical treatment, the net intensity Pobs may be calculated from the raw counts by using:

Pobs = [Qobs - 0.5 cb (Lobs / cl + Hobs / ch)] / cp.