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Reflectometer D17

D17 is the first ILL dedicated reflectometer and it has been designed to be as flexible as possible in resolution and modes of operation. The instrument is suitable for the analysis of surface structures in solids and solid/liquid interfaces. Horizontal surface experiments, such as free liquids, will suffer from a severe restriction in Q-range and flux and are thus not recommended for this instrument.

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D17 LAMP Book

Data overlap regions

The useable wavelength range on D17 is typically 2 – 20 Å. This means that D17 can simultaneously cover an order of magnitude in Q. 

This is generally not enough to cover a full reflectivity curve, however.  Hence, measurements must be made at at least two angles.  There will be some overlap of data between these angles. 

This page describes the COSMOS procedure for calculating the reflectivity in the overlap region.

 

Step 1:  Calculation of the 'reflectivity' for the separate angles

The 'reflectivity' of the separate angles is calculated.  This calculation finds the specular reflection (within the defined search range of the detector), subtracts background, corrects for detector efficiency (water), and divides the reflected data by the direct beam.

Normally, this gives the reflectivity.  However, particularly for the direct beam at larger angles, the oscillating attenuator is often used to avoid detector saturation.  For this reason, there may be a (constant) multiplicative factor which must be accounted for.

Step 2:  Determination of the overlap region

The Q values for the different angles are calculated and the overlap region between angular data sets is determined.

Step 3:  Interpolation of Q

Within the overlap region, the Q-values for the larger angle are interpolated on to the Q-values for the smaller angle.  There will then be N points spanning the overlap region, where N is the number of Q-values for the smaller angle within the overlap region.

Step 4:  Determination of the scaling factor

The ratio is calculated for the data at each of the matching Q (actual Q for the smaller angle, interpolated Q for the larger angle).

The ratios are then weighted by the reciprocal of the statistical error in each point.

The scaling factor between the higher and the lower angles is then the mean of the weighted ratios. 

Step 5:  Scaling of the larger angle

All of the higher angle data is then multiplied by this factor.

Step 6:  Grouping

The individual angles are then rebinned using the Group Data function (if requested).

Step 7:  Returning the result

The combined data set is then returned to COSMOS and saved.