Department Microstructure and Residual Stress Analysis

nxs - a library for neutron cross section calculations

nxsPlotter: a graphical user interface for nxs.

A collection/library of routines, written in C, for the computation of neutron scattering and absorption cross sections for polycrystalline/powder materials based on the composition of a crystallographic unit cell.

Author: Mirko Boin, boin@helmholtz-berlin.de

p.s.: nxs is not nexus, here! ;-)

Introduction

The nxs library for computing neutron scattering and absorption cross sections provides a number of C structs and functions to calculate wavelength-dependent cross section values for polycrystalline/powder-like materials. The definition of a material is represented by the composition of a unit cell (NXS_UnitCell). A unit cell is created from the specification of a space group and its unit cell parameters. The SgInfo routines from Ralf W. Grosse-Kunstleve is included here for such purposes. Monoatomic materials as well as multi-atomic compounds are created by adding NXS_AtomInfo atom information/properties. The library also provides a reading and saving routines to compose unit cells from nxs parameter files.

Howto

The below example shows howto quickly use the library routines to initialise a unit cell and calculate some cross sections.

NXS_UnitCell uc = nxs_newUnitCell();

int numAtoms = nxs_readParameterFile( nxsFileName, &uc, &atomInfoList);
if( numAtoms > 0 )
{
int i=0;
nxs_initUnitCell(&uc);
for( i=0; i<numAtoms; i++ )
nxs_addAtomInfo( &uc, atomInfoList[i] );
nxs_initHKL( &uc );
double lambda=0.1;
for( lambda=0.1; lambda<4.0; lambda+=0.1 )
{
printf("%f\n",nxs_Absorption(lambda, &uc ) );
}
}

Parameterdateien

Gegenwärtig bietet nxs Routinen zum Lesen und Speichern von nxs-Parameterdateien einer bestimmten Art. Es handelt sich dabei um ein menschenlesbares, INI-ähnliches Dateiformat zur Speicherung der notwendigen Informationen für die Zusammensetzung einer kristallographischen Einheitszelle. Ein Beispiel für NaCl ist unten angegeben:

#
# This is an nxs parameter file
#

# define the unit cell parameters:
#   space_group                      - the space group number or Hermann or Hall symbol [string]
#   lattice_a, ...b, ...c            - the lattice spacings a,b,c [angstrom]
#   lattice_alpha, ...beta, ...gamma - the lattice angles alpha,beta,gamma [degree]
#   debye_temp                       - the Debye temperature [K]
space_group=225
lattice_a=5.64
lattice_c=4.95
lattice_alpha=90
debye_temp=320

# add atoms to the unit cell:
# notation is "atom_number = name b_coh sigma_inc sigma_abs_2200 molar_mass x y z"
#   name           - labels the current atom/isotope [string]
#   b_coh          - the coherent scattering length [fm]
#   sigma_inc      - the incoherent scattering cross section [barns]
#   sigma_abs_2200 - the absorption cross sect. at 2200 m/s [barns]
#   molar_mass     - the Molar mass [g/mol]
#   x y z          - the Wyckoff postion of the atom inside the unit cell
#
# e.g.: add_atom = Fe 9.45 0.4 2.56 55.85 0.0 0.0 0.0

[atoms]
add_atom=Na 3.63 1.62 0.53 22.99 0.0 0.0 0.0
add_atom=Cl 9.577 5.3 33.5 35.45 0.5 0.5 0.5

Copyright

The nxs library includes the SgInfo library, whose free usage is granted by the following notice:

Space Group Info (c) 1994-96 Ralf W. Grosse-Kunstleve Permission to use and distribute this software and its documentation for noncommercial use and without fee is hereby granted, provided that the above copyright notice appears in all copies and that both that copyright notice and this permission notice appear in the supporting documentation. It is not allowed to sell this software in any way. This software is not in the public domain.

References

Main reference:

"nxs: a program library for neutron cross section calculations"
Boin, M. (2012). J. Appl. Cryst. 45, 603-607, doi: 10.1107/S0021889812016056

Applications of nxs:

The nxs library has been used in a variety of applications, e.g. neutron simulation for instrument development, experiment planning and data analysis. Some examples are listed below:

A virtual sample calculating nxs has been implemented for the neutron simulation software VITESS.

Neutron Bragg edge measurements and simulation (using nxs within McStas) of a texured Al sample.

"Validation of Bragg edge experiments by Monte Carlo simulations for quantitative texture analysis"
Boin, M.; Hilger, A.; Kardjilov, N.; Zhang, S.Y., Oliver, E.C.; James, J.A., Randau, C. & Wimpory, R.C. (2011). J. Appl. Cryst. 44, 1040-1046, doi: 10.1107/S0021889811025970
"Time-of-flight neutron imaging at a continuous source: Proof of principle using a scintillator CCD imaging detector"
Strobl, M.; Hilger, A.; Boin, M.; Kardjilov, N.; Wimpory, R.; Clemens, D.; Mühlbauer, M.; Schillinger, B.; Wilpert, T.; Schulz, C.; Rolfs, K.; Davies, C. M.; O'Dowd, N.; Tiernan, P. & Manke, I. (2011). Nucl. Instrum. Methods Phys. Res., Sect. A 651, 149-155, doi: 10.1016/j.nima.2010.12.121
"Monte Carlo simulations for the analysis of texture and strain measured with Bragg edge neutron transmission"
Boin, M.; Wimpory, R.C.; Hilger, A.; Kardjilov, N.; Zhang, S.Y. & Strobl, M. (2011). J. Phys.: Conf. Ser. 340, 012022/1-12, doi: 10.1088/1742-6596/340/1/012022
"Geant4 based simulations for novel neutron detector development"
Kittelmann, T.; Stefanescu, I.; Kanaki, K.; Boin, M.; Hall-Wilton, R. & Zeitelhack, K. (2014). J. Phys.: Conf. Ser. 513, 022017, doi: 10.1088/1742-6596/513/2/022017
"A new transmission based monochromator for energy-selective neutron imaging at the ICON beamline"
Peetermans, S.; Tamaki, M.; Hartmann, S.; Kaestner, A.; Morgano, M. & Lehmann, E. H. (2014). Nucl. Instrum. Methods Phys. Res., Sect. A 757, 28-32, doi: 10.1016/j.nima.2014.04.033