4.2

Materials

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:

  1. Specifying it directly with SampleNumberDensity.
  2. Specifying the ZParameter and the UnitCellVolume (or letting the algorithm calculate it from the OrientedLattice on the InputWorkspace).
  3. 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.

  1. Varley F. Sears, Neutron scattering lengths and cross sections, Neutron News 3:3 (1992) 26 doi: 10.1080/10448639208218770
  2. 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

Category: Concepts