ComputeIncoherentDOS-v1.rst

Description

Computes the phonon density of states from an inelastic neutron scattering measurement of a powder or polycrystalline sample, assuming that all scattering is incoherent, using the formula (Thermodynamic Properties of Solids, eds. Chaplot, Mittal, Choudhury, Chapter 3) for the 1-phonon incoherent scattering function:

S^{(1)}_{\mathrm{inc}}(Q,E) = \exp\left(-2\bar{W}(Q)\right) \frac{Q^2}{E} \langle n+\frac{1}{2}\pm\frac{1}{2} \rangle \left[ \sum_k \frac{\sigma_k^{\mathrm{scatt}}}{2m_k} g_k(E) \right]

where the term in square brackets is the neutron weighted densiy of states which is calculated by this algorithm, and g_k(E) is the partial density of states for each component (element or isotope) k in the material. m_k is the relative atomic mass of the component.

The average Debye-Waller factor \exp\left(-2\bar{W}(Q)\right) is calculated using an average mean-square displacement \langle u \rangle, using W=Q^2\langle u\rangle/2.

If the data has been normalised to a Vanadium standard measurement, the output of this algorithm is the neutron weighted density of states in milibarns/steradians per formula unit per meV (or per cm^-1). If the sample material has been set and is found to be a pure element, then an additional option will be enabled to calculate the DOS in states per meV (states per cm^-1) by dividing by the scattering cross-section and multiplying by the relative atomic mass.

Restrictions on the Input Workspace

The input workspace must have units of Momentum Transfer and contain histogram data with common binning on all spectra.

Usage

ISIS Example

The following code will run a reduction on a MARI (ISIS) dataset and apply the algorithm to the reduced data. The datafiles (runs 21334, 21335, 21347) and map file 'mari_res2013.map' should be in your path. Run number 21335 is a measurement of a large Aluminium sample from the neutron training course.

from Direct import DirectEnergyConversion
from mantid.simpleapi import *
rd = DirectEnergyConversion.DirectEnergyConversion('MARI')
ws = rd.convert_to_energy(21334, 21335, 60, [-55,0.05,55], 'mari_res2013.map',
    monovan_run=21347, sample_mass=106.4, sample_rmm=27, monovan_mapfile='mari_res2013.map')
ws_sqw = SofQW3(ws, [0,0.1,12], 'Direct', 60)
SetSampleMaterial(ws_sqw,'Al')
ws_dos = ComputeIncoherentDOS(ws_sqw, Temperature=5, StatesPerEnergy=True)

Test Example

This example uses a generated dataset so that it will run on automated tests of the build system where the above datafiles do not exist.

Output