SetSampleMaterial v1¶

SetSampleMaterial dialog.

Table of Contents

Summary
Properties
Description
References
Source

Summary ¶

Sets the neutrons information in the sample.

Properties ¶

Name	Direction	Type	Default	Description
InputWorkspace	InOut	Workspace	Mandatory	The workspace with which to associate the sample
ChemicalFormula	Input	string		The chemical formula, see examples in documentation
AtomicNumber	Input	number	0	The atomic number
MassNumber	Input	number	0	Mass number if ion (use 0 for default mass sensity)
SampleNumberDensity	Input	number	Optional	This number density of the sample in number of atoms per cubic angstrom will be used instead of calculated
ZParameter	Input	number	Optional	Number of formula units in unit cell
UnitCellVolume	Input	number	Optional	Unit cell volume in Angstoms^3. Will be calculated from the OrientedLattice if not supplied.
CoherentXSection	Input	number	Optional	Optional: This coherent cross-section for the sample material in barns will be used instead of tabulated
IncoherentXSection	Input	number	Optional	Optional: This incoherent cross-section for the sample material in barns will be used instead of tabulated
AttenuationXSection	Input	number	Optional	Optional: This absorption cross-section for the sample material in barns will be used instead of tabulated
ScatteringXSection	Input	number	Optional	Optional: This total scattering cross-section (coherent + incoherent) for the sample material in barns will be used instead of tabulated
SampleMassDensity	Input	number	Optional	Measured mass density in g/cubic cm of the sample to be used to calculate the number density.

Sets the neutrons information in the sample. You can either enter details about the chemical formula or atomic number, or you can provide specific values for the attenuation and scattering cross sections and the sample number density. If you decide to provide specific values you must give values for all three (attenuation and scattering cross sections and the sample number density), and any formula information will be ignored. If you miss any of the three specific values then the other will be ignored.

Neutron scattering lengths and cross sections of the elements and their isotopes have been taken from NIST.

Chemical Composition with Examples ¶

H2 O - Isotopically averaged Hydrogen
(H2)2 O - Heavy water
D2 O - Another way to specify heavy water

Enter a composition as a molecular formula of elements or isotopes. For example, basic elements might be H, Fe or Si, etc. A molecular formula of elements might be H4-N2-C3, which corresponds to a molecule with 4 Hydrogen atoms, 2 Nitrogen atoms and 3 Carbon atoms. Each element in a molecular formula is followed by the number of the atoms for that element, specified without a hyphen, because each element is separated from other elements using a hyphen.

The number of atoms can be integer or float, but must start with a digit, e.g. 0.6 is fine but .6 is not. This can be used to set elemental ratios within a chemical composition. For example 95.1% Vanadium 4.9% Niobium can be expressed as V0.951 Nb0.049. Warning: Using this representation will calculate all properties except for SampleNumberDensity which must be set manually if required

Isotopes may also be included in a material composition, and can be specified alone (as in Li7), or in a molecular formula (as in (Li7)2-C-H4-N-Cl6). Note, however, that No Spaces or Hyphens are allowed in an isotope symbol specification. Also Note that for isotopes specified in a molecular expression, the isotope must be enclosed by parenthesis, except for two special cases, D and T, which stand for H2 and H3, respectively.

Cross Section Calculations ¶

Each of the cross sections ( $\sigma$ ) are calculated according to

$\sigma = \frac{1}{N_{atoms}}\sum_{i}\sigma_{i}n_{i}$

where $N_{atoms} = \sum_{i}n_{i}$ . A concrete example for the total cross section of D2 O

$\sigma = \frac{1}{2+1}\left( 7.64*2 + 4.232*1\right) = 6.504\ barns$

Number Density ¶

The number density is defined as

$\rho_n = \frac{N_{atoms}ZParameter}{UnitCellVolume}$

It can can be generated in one of three ways:

Specifying it directly with SampleNumberDensity.
Specifying the ZParameter and the UnitCellVolume (or letting the algorithm calculate it from the OrientedLattice on the InputWorkspace).
Specifying the mass density. In this case the number density is calculated as

$\rho_n = \frac{N_{atoms} \rho_m N_A}{M_r}$

where $\rho_m$ is the mass density, $N_A$ is the Avogadro constant, and $M_r$ the relative molecular mass.

Linear Absorption Coefficients ¶

$\mu_s = \rho_n \frac{1}{N_{atoms}}\sum_{i}s_{i}n_{i} \text{ units of 1/cm}$

$s = \sigma_{total scattering}$

$\mu_a = \rho_n \frac{1}{N_{atoms}}\sum_{i}a_{i}n_{i} \text{ units of 1/cm}$

$a = \sigma_{absorption} (\lambda=1.8)$

A detailed version of this is found in [2].

Normalized Laue ¶

The low- $Q$ limit of $S(Q)$ is $-L$ where $L$ is called the normalized Laue term

$bAverage = <b_{coh}> = \frac{1}{N_{atoms}}\sum_{i}b_{coh,i}$

$bSquaredAverage = <b_{tot}^2> = \frac{1}{N_{atoms}}\sum_{i}b_{tot,i}^2$

$L = \frac{<b_{tot}^2>-<b_{coh}>^2}{<b_{coh}>^2}$

References ¶

The data used in this algorithm comes from the following paper.

Varley F. Sears, Neutron scattering lengths and cross sections, Neutron News 3:3 (1992) 26 doi: 10.1080/10448639208218770
J. A. K. Howard, O. Johnson, A. J. Schultz and A. M. Stringer, Determination of the neutron absorption cross section for hydrogen as a function of wavelength with a pulsed neutron source, J. Appl. Cryst. (1987). 20, 120-122 doi: 10.1107/S0021889887087028

Categories: Algorithms | Sample

Source ¶

C++ source: SetSampleMaterial.cpp (last modified: 2018-01-19)

C++ header: SetSampleMaterial.h (last modified: 2018-02-22)