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Since MSA version 0.70b relative radial sensitivity profiles can be set via the detector parameter file. The sensitivity profile specified for a detector definition is identified by file name. The expected format of detector sensitivity files is described below.

A relative radial sensitivity profile is a radial function, which determines the transfer of intensity from the absolute square of the wave function in a diffraction plane to the recorded detector signal depending on the diffraction angle. The sensitivity of real annular dark-field detectors is not uniform and often shows a strong decay of response close to its the inner radius. The consideration of this detector transfer characteristics is critical for simulating STEM image intensities quantitatively, e.g. in A. Rosenauer, et al. [Ultramicroscopy 109 (2009) 1171-1182].

Relative radial detector profile measured for a typical HAADF detector (left) and the first lines of a respective detector sensitivity file.

Detector sensitivity files are expected to be text files with a header line consisting of two numbers, and a sequence of lines containing one sensitivity value per line.

The first of two numbers in the header line denote the total number of radial sensitivity samples in the profile (number of lines following, respectively). The second number identifies the reference sample number iref of the profile corresponding to the inner detector radius beta1. In the example shown above, this is sample number 129 of 960. The simulation assumes that the first sample (i=1) corresponds to the center of the annular detector (theta=0) and calculate the sensitivity s at scattering angle theta with the formula

i = 1 + theta · (iref-1) / beta1
s = profile( i )

where i refers to a sample of the provided profile corresponding to theta.

For matching experimental and simulated images on the same absolute intensity scale, re-scaling of both images is required.

1. Determine the average detector response for the same settings controlling the probe current, the detector readout, and the scanning (dwell time) as used for the imaging.
2. Subtract the vacuum intensity level (counts) from the detector image and from the experimental image.
3. Calculate the average response of the detector over the whole active area, and divide the detector response image and the experimental image, both, after subtraction of the vacuum count level.
4. Finally, extract the radial average of the normalized detector image. This should give a curve such as shown above.

 

Line	Parameters	Description (MSA version 0.70+)
01	<number>, <float>	NR = number of radial samples, iref = sample number corresponding to the inner radius of the annular detector. At least NR lines of sensitivity data must follow this header line.
02	<float>	First sensitivity sample, This sample is assigned to the center of the annular detector.
03	<float>	Second sensitivity sample
...	<float>	(more data)
NR + 1	<float>	Last sensitivity sample

 

Last update: Jan 26, 2019    contact    disclaimer(de)