Module SANS-Evaluation

# VITESS Module SANS-Evaluation

The module eval_sans delivers the function S(Q) or F(Q) of a SANS simulation. This has to be done in two steps: In a first run, an isotropic scatterer is used; then the simulation with the sample is started. The use of the isotropic scatterer is necessary to eliminate influences of the intensity distribution from moderator und instrument. This reference file from the run with the isotropic scatter can be generated as S(Q) file by the module eval_sans using normalisation factor=1.0, or by the module eval_elast using the option evaluation parameter=momentum transfer. The reference spectrum needs the same binning as the S(Q) file.
The procedure is like that: Assuming the SANS instrument is finished (including sample, detector and eval_sans), you change the parameter scattering object in the parameter file of sample_sans to isotropic scattering. (You might save this as an instrument of its own.) Now you add eval_elast or change or add eval_sans as just described and start this simulation. The output spectrum file of eval_elast or S(Q) file of eval_sans now contains the reference spectrum (of the isotropic scatterer). If needs to be assured that this file will not be changed by the simulation using the sample, either by copying and renaming it or by using two instruments. This file can now be used as the reference file in the eval_sans module. eval_sans will calculate I(Q) of the sample and deliver S(Q) as the ratio

S(Q) = F_norm * I_smpl(Q)/I_ref(Q)

F_norm considers the different scattering probabilities of the SANS samples and the isotropic scatterer into forward direction (θ=0). For the isotropic scatterer, the form factor F(Q) and the product (ρp-ρs)2 fp Vp are set to 1 (cf. 'sample_sans.html'.) Therefore:

F_norm = 1 / {(ρp-ρs)2 fp Vp F(Q)}

It is of the order of magnitude 10-4 to 10-3.

### Parameters for module 'eval_sans'

Parameter Unit | Description | Range or Values | Command Option
S(Q) file | The spectrum file: it contains the S(Q) result | - | -S
reference file | The reference file contains the intensity distribution I(Q) at the detector for isotropic scattering | - | -F
number of bins | The number of bins determines the segmentation of the momentum transfer and therewith the number of values written to the S(Q) file. | >= 1 | -n
minimum, maximum [1/Å] | The bounds of the interval for the evaluation parameter Q. | >=0 max>min | -m -M
normalization parameter | The ratio of the intensity of the isotropic scatterer to that of the SANS sample in forward direction (θ=0) The theoretical value is: F_norm = 1 / ((ρp-ρs)2 fp Vp F(Q)) | >0 typical 1e-4 | -p
increase to next bin [%] | For a logarithmic binning the percentage has to be given by which the upper limit of each bin exceeds its lower limit. If this value is set, the parameter 'number of bins' is ignored | >0 | -R
dead-spot [deg] | (optional) All neutrons with a scattering angle(2 θ) between 0 and dead-spot will not be considered in the evaluation. | >= 0 | -d
time of flight | yes: the wavelength is determined from the given flight path and the time-of-flight calculated for the trajectory no : the wavelength of all trajectories is supposed to be identical with the reference wavelength | 'yes', 'no' | -w
flight path [cm] | (for option TOF): Length of the neutron flight path from point of t=0 setting (source or pulse shaping chopper) | > 0 | -l
time offset [ms] | (for option TOF) global shift of the neutron time t -> t - t_offset. It is used to shift the reference point for the time of flight analysis (e.g. by half the length of a long pulse). | >= 0 | -T
reference wavelength [Å] | (for option: 'No TOF') The (average) wavelength set by velocity selector or monchromator This module does not use the wavelenghth of the trajectory for data evaluation, but (as in real life) the wavelength given here. | > 0 | -r
time interval begin/end [ms] | (optional) The bounds of the time interval for the evaluation. Only neutrons arriving in that time interval are considered in the evaluation. | >=0 end>begin | -e -E
colour | (optional) Only trajectories of the given color are evaluated. Color -1 or 'nothing' means: all trajectories are evaluated | >= -1 | -C

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Last modified: Tue May 8 17:08:06 MET DST 2001