Version 1
R. Ghosh, June 1995
The problems posed in the analysis of data from conventional instruments, IN5, and IN6 are very similar to those seen on backscattering instruments, IN10, and IN16. Some simplification occurs in the second case in the treatment of absorption in the sample, if this is considered to be independent of energy within the limited window measured in the experiment. Detailed analysis including multiple scattering is not attempted in either treatment program discussed here.
The LAMP package can be used to examine raw data from the multiple detector arrays, and perform grouping and subtraction and normalisation tasks. The raw data from the group of instruments is similar, though the backscattering spectrometers have a number of different modes of operation.
The two basic reduction programs are inx for conventional time of flight, and sqw for backscattering measurements. The resultant data files are similar and allow analysis programs to be shared to some degree.
inx sqw
These programs take raw data for sample and background and perform subtractions, taking into account sample absorption for one of several simple geometries. Using a vanadium spectrum as an elastic scattering standard the intensities are normalised for detector efficiency, and the cross-sections resulting can be placed on an absolute scale, approximately. The program inx includes some corrections for the effective variation of the detector efficiency as a function of neutron energy.
The output file contains tabulated values of the Scattering Law, S(Q,E), at constant angle for each angle selected.
A number of interactive fitting programs treat these reduced data typically using convolution of the elastic vanadium spectrum as a resolution function and fit multiple peaks.
e.g. profit convolutes in time of flight, fitting
simple models to TOF data
firr convolutes in energy, fitting simple
models to backscattering data
These programs can be used on an angle by angle basis to
seek systematic Q dependencies for the models
on an angle by angle basis.
Where a more specific model e.g. a plastic crystal is proposed a more comprehensive analysis is possible, including some estimates of multiple scattering, using the whole data set simultaneously.