Input file formats

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To generate the SQW file from which Horace reads S(Q,w), neutron scattering data for each individual run and needs to be provided in one of two formats: the legacy ASCII format SPE file, together with an ASCII detector parameter file (the PAR file), or their replacements the HDF5 (Hierarchical Data Format) NXSPE file. Two functions are available as part of Horace to create SQW files from these input files, namely gen_sqw (creates a new SQW file) and accumulate_sqw (accumulates data to an existing SQW file). The functions to generate SQW files are described in detail here.

SPE file and PAR file

The ASCII format SQW file stores S(w) and associated error bars as a function of energy transfer, ħw, for each detector in turn. In addition to the set of .spe files, Horace requires an accompanying ASCII file which contains information about the location of the detectors in the spectrometer’s reference frame, the PAR file. Although these ASCII format files have largely been superseded in favour of the NXSPE format described below, such files are ubiquitous as the format in which historic data is saved, and are recognised by several other neutron visualisation and analysis programs. Some programs can also write their own output as SPE files, and consequently the SPE file is sometimes used as a transportable format data file for time-of-flight neutron spectrometers. The format of these two files is described here, however, it is not recommended to create new .spe files as this is now an obsolete format.


SPE file format

The SPE file contains the intensity and estimated standard deviation on those intensities for each detector element in turn, with header blocks that give the number of detectors and energy bins, and the scattering angle and energy transfer bin boundaries. These blocks are all separated by character strings that begin with '###'. The format in ful is as follows:

ndet ne
### Phi Grid
phi(1) phi(2) phi(3) phi(4) phi(5) phi(6) phi(7) phi(8)
phi(9) phi(10) phi(11) phi(12) phi(13) phi(14) phi(15) phi(16)
... phi(ndet+1)
### Energy Grid
en(1) en(2) en(3) en(4) en(5) en(6) en(7) en(8)
en(9) en(10) en(11) en(12) en(13) en(14) en(15) en(16)
... en(ne+1)
L2(2) phi(2) azim(2) width(2) length(2)
 :  :  :  :  :
L2(ndet) phi(ndet) azim(ndet) width(ndet) length(ndet)


PAR file format

The PAR file contains the position information of the detectors and their sizes. The format is:

ndet
L2(1) phi(1) azim(1) width(1) length(1)
L2(2) phi(2) azim(2) width(2) length(2)
 :  :  :  :  :
L2(ndet) phi(ndet) azim(ndet) width(ndet) length(ndet)

where

ndet: Total number of detector elements.

L2: Distance (m) from the sample to detector.

phi: Scattering angle (degrees) i.e. angle between the incident beam direction and a line connecting the sample to the detector. In spherical polar coordinates where the conventional z-axis points in the direction of the incident beam, this angle is the conventional polar angle theta.

azim: Azimuthal angle (degrees). In the spherical polar frame defined above, and with the conventional y-axis pointing vertically upwards, this angle is the conventional azimuthal angle phi.

width: Width (m) of the detector perpendicular to the Debye-Scherrer ring.

length: Length (m) of the detector tangential to the Debye-Scherrer ring.

The width and length of the detector are not actually used by Horace, but dummy values at least need to be present in the file.


NXSPE file

The recommended input data file is the NXSPE file, which holds both the S(w) data and errors for each detector and detector position and size information, together with crystal orientation anglepsi and the incident neutron energy Ei. The .nxspe file stores the information in a NeXus format file [Ref NeXus], which is a common data exchange format for neutron, X-ray and muon data that is built on top of the HDF5 (Hierarchical Data Format) scientific data format [Ref HDF]. Data files in the .nxspe format are produced by the Mantid data analysis software [Ref Mantid, Mantid web page]. They are directly produced by the data reduction algorithms within Mantid for the direct and indirect geometry spectrometers at both the ISIS spallation neutron source at the Rutherford Appleton Laboratory in the UK and the SNS spallation neutron source at Oak Ridge National Laboratory in the USA. Mantid is an open source data manipulation and analysis framework for neutron and muon data analysis. If Mantid is used to perform the data corrections for a neutron spectrometer, then the algorithm SaveNXSPE in Mantid can be used to output .nxspe files. Full details of how to use Mantid and the input/output for each algorithm are available at the Mantid web site. Alternatively, if the user can read corrected scattering data, associated estimated errors, and detector parameters into Matlab arrays, .nxspe files can be written to file using the Horace utility function gen_nxspe. Full details of the input argument definitions and array formats for this function are available on the Horace web site {ref Horace].