Absorption and Multiple Scattering Corrections¶

Total cross section (\(\sigma_t\))¶

First, we define the total cross section per atom, which for the majority of elements, is equal to the sum of the element’s scattering and absorption cross sections:

where \(\lambda\) is the neutron wavelength, \(\sigma_s = \int_{\Omega} \frac{d\sigma}{d\Omega} \left( \lambda, 2\theta \right) d\Omega\), and \(\frac{d\sigma}{d\Omega} \left( \lambda, 2\theta \right)\) is the differential cross-section which is the cross-section (\(\sigma\)) per solid angle (\(d\Omega\)).

Scattering cross section (\(\sigma_s\) )¶

Typically, the scattering cross section is directly found from tabulated single atom values [1] and with good approximation, assumed independent of wavelength. If we observe the equation above for \(\sigma_s\), we have that it is equal to the integration of the differential cross section. Yet, for the typical diffraction experiment, the differential cross-section is exactly the quantity we wish to measure. Thus, to proceed one could chose from the following:

1. Measure the sample and integrate over all \(d\Omega\) (very difficult to do exactly since instruments typically cannot cover all \(4 \pi\) of \(\Omega\)).
2. Perform a transmission measurement to directly measure \(\sigma_t\) and subtract a known value for \(\sigma_a\) (also not typical).
3. Use tabulated single atom values for \(\sigma_s\) [1] (bingo!).

For the energies used to measure structure of materials, \(\sigma_s\) is to reasonable approximation independent of \(\lambda\). However, exceptions include nuclear resonances, strong Bragg scattering as in beryllium, and significant inelastic effects. [2]

Absorption cross section (\(\sigma_a\))¶

For the absorption cross section, again typically the cross section is used from the same tabulated single atom values [1] but is only valid for the reference wavelength, 1.7982 \(\AA\), at which it was measured at. Thus, we can look up the value of \(\sigma_a (1.7982 \AA)\). Yet, the absorption cross section is a linear function of wavelength (away from nuclear resonances). Thus, we can calculate the absorption cross section at other wavelengths from:

NOTE: In Mantid, the reference wavelength is defined as a variable ReferenceLambda in NeutronAtom.cpp [3]. This variable should be used for consistency in all sample correction algorithms that calculate \(\sigma_a (\lambda)\) from this reference wavelength.

Attenuation Length (\(\mu\))¶

The attenuation length is defined as:

where \(\rho\) is the atomic number density of the sample. Note that \(\rho\) is not the crystallographic or microscopic density of a unit cell but the macroscopic density of the bulk sample. If one were to measure a powder sample, the powder would not pack perfectly and thus there would be a given packing fraction, \(f\). Then, if the microscopic density is given as \(\rho_{micro}\), it is related to \(\rho\) via:

The definition of \(\rho\) is used throughout the rest of this documentation.

Techniques for Calculating Corrections¶

Methods for calculating the absorption corrections (and also the multiple scattering) generally fall into these categories:

1. Analytical solutions. For some simple situations it is possible to do an exact, analytical integration to produce a formula for the absorption correction based on parameters of the sample

• Very fast.
• Results are often stored in look up tables.
• Use often involves interpolating between tabulated results.
• Implemented in Mantid

2. Numerical integration

• Generally provides quicker solutions than Monte Carlo methods.
• Some assumptions are included: sample geometries and the scattering processes.
• Less flexible than the Monte Carlo integration or ray-tracing for tackling different problems.
• Implemented in Mantid

3. Monte Carlo integration

• Generally a more computationally demanding calculation and slower to solution. Monte Carlo is used for the numerical integration technique.
• Relaxation of most assumptions needed by analytical solutions.
• More flexible than the analytical techniques for shapes, beam profiles, and mixed number of scattering processes.
• Implemented in Mantid

4. Monte Carlo ray tracing

• Most general solution in that it is a virtual neutron experiment with all neutron histories kept. Slowest to solution.
• Relatively no assumptions needed. Can simulate mixed numbers of scattering, complex scattering processes (ie scattering sample to sample environment back to sample then to detector), moderator and guides included.
• Most flexible but mainly a tool for designing new instruments than for calculating sample corrections.
• Typically calculated in another program specific to ray tracing and then imported into Mantid.

The analytical and numerical integration methods generally provide a quicker solution, but at the expense of having to make assumptions about sample geometries and scattering processes that make them less flexible than the Monte Carlo techniques (integration and ray-tracing). However, in many cases analytical and numerical integration solutions are satisfactory and allow much more efficient analysis of results.

Introduction¶

Determination of the structure and/or dynamics of samples depends on the analysis of single scattering data. Overall, the absorption correction is a factor, \(A\), such that 0 < \(A\) < 1 . It is a factor that accounts for the loss of intensity from single scattering in the sample (or other component in the instrument). due to both scattering and capture events in the sample. The factor \(A\) is divided by the measured intensity. Thus, the absorption correction has an overall multiplicative enhancement of the measured intensity.

Basic Sample Absorption Theory¶

The figure shows how a general single scattering process might occur. The neutron travels a certain distance \(l_1\) through the sample before a single scattering event occurs in the volume element \(dV\) of the sample. Then, the neutron travels a final length \(l_2\) before leaving the sample and being picked up by a detector.

To formulate the absorption sample correction, first, we assume we have a homogeneous sample of a given shape that is fully illuminated by the incident beam. Then, the number of neutrons per unit solid angle scattered once by a volume element \(dV\) of the sample and seen by a detector is given by:

where \(J_0\) is the incident flux, \(\rho\) is the atomic number density, \(\frac{d\sigma}{d\Omega} \left( \theta \right)\) is the differential cross-section, \(l_1\) and \(l_2\) are the path lengths for incident neutrons to \(dV\) and from \(dV\) to the detector, respectively, and \(\lambda_1\) and \(\lambda_2\) are the incident and scattered wavelength, respectively.

Yet, this is only the contribution from a single volume element, or voxel, of the sample volume that contributes to a detector. Thus, if we integrate over the entire sample volume (all the voxels), we arrive at the total intensity of neutrons scattered once through an angle \(\theta\) and then leaving the sample without further scattering, given as:

In the last part of the equation for \(I_1(\theta)\), we have introduced the term \(A\), given as:

This is the basic absorption correction for a single sample volume (i.e. no container included, no partial correction factors, no partial illumination, etc.). NOTE: In some references, this term \(A\) is instead defined as the inverse \(A^{-1}\). Some references use the notation here: [2] [4] [5] [7] [8] and [10], while others use the inverse notation: [6].

For elastic scattering, \(\lambda_1\) = \(\lambda_2\) = \(\lambda\) and we can simplify to:

Partial Absorption Correction Factors: Paalman and Pings Formalism¶

When the scattering of a sample (liquid, powder, gas, etc.) is measured, the sample is often held in a thick sample container. This container contributes significantly to the measured neutron beam. Often the empty container is measured and the signal from the container (\(C\)) subtracted from the signal of sample plus container (\(S+C\)).

The Paalman-Pings formalism (PPF) [4] provides a framework for correcting for individual component absorption contributions, or the partial absorption correction factors, when a sample is measured in a container and, possibly, one or more sample environments. PPF builds on the earlier work of Ritter [5], who described a graphical approach of accounting for partial absorption correction factors. The PPF goes beyond the work of Ritter in two important ways

• The graphical approach is formulated instead using numerical integration.
• The contribution from the sample/container correlation region, or the interface, where the density of each is affected due to their inter-material interactions.

In their analysis, Paalman and Pings show that the latter point is not generally of consequence since this region only exists for a few angstroms in most materials, but the ability to account for it is re-assuring. The sample/container interaction could be of significant importance in cases where the container and sample are single crystal or poly-crystalline.

Generally, the container measurement (neglecting multiple scattering and inelastic effects) is written:

• \(I^E_C\) experimental intensity from the isolated container.
• \(I_C\) theoretical intensity from the isolated container.
• \(A_{C,C}\) is the absorption factor for scattering in the container region and absorption in the container.

The full PPF for the sample and container measurement (neglecting multiple scattering and inelastic effects) is written:

• \(I^E_{S+C}\) experimentally measured intensity from \(S+C\).
• \(I_S\) theoretical intensity from the isolated sample.
• \(I_C\) theoretical intensity from the isolated container.
• \(I_{SC}\) theoretical intensity from the correlated sample and container interface.
• \(A_{S,SC}\) is the absorption factor for scattering in the sample region and absorption by the sample and container.
• \(A_{C,SC}\) is the absorption factor for scattering in the container region and absorption by the sample and container.
• \(A_{SC,SC}\) is the absorption factor for scattering in the correlated sample and container interface and absorption by the sample and container.

As discussed above, the final term in this expression is generally neglected.

General Notes¶

Legends for Absorption Correction Algorithms in Mantid Table¶

Indicates the energy modes that the algorithm can accommodate:

Indicates the technique used for calculating the absorption correction:

Options that describe what functions the algorithm is capable of and the output types:

Absorption Correction Algorithms in Mantid Table¶

Introduction¶

Determination of the structure and/or dynamics of samples depends on the analysis of single scattering data. Small but unwanted higher-order scattering is always present although in many typical experiments multiple scattering effects are negligible. However, in some cases the data may contain a significant contribution from multiple scattering. In neutron scattering, the absorption cross-section is often much smaller than the scattering cross-section. For this reason it is necessary to account for multiple scattering events. Using the PPF notation from previously, the measured beam from \(S+C\) (neglecting multiple scattering and inelastic effects) is given as:

Thus, the multiple scattering is a parasitic signal that needs to be subtracted from the experimentally measured \(I^E\) intensity. To get an idea of when and why multiple scattering corrections are needed, let us define \(\sigma_n\) as the likelihood of a neutron being scattered \(n\) times. Then it is possible to show [6] that:

Where practical, the shape and thickness of a sample are carefully chosen to minimize as much unwanted multiple. This may be achieved by using a sample that is either [7]

• Small in comparison with its mean free path.
• Strongly absorbing (the absorption cross section is much greater than the scattering cross section. Usually this means the dimensions of a sample are chosen to ensure that between 10% and 20% of incident neutrons end up being scattered [8] ).

Increasing the absorption cross section is not always attainable - due to the type of material in question - or desirable, due to the accompanying intensity losses becoming overly prohibitive.

Theory¶

The figure shows how a general double scattering process might occur. The neutron travels a certain distance \(l_1\) through the sample before the first scattering event in the volume element \(dV_1\). The second scattering occurs in another volume element \(dV_2\) after a distance \(l_{12}\) has been traversed following which the neutron travels a final length \(l_2\) before leaving the sample and being picked up by a detector.

We define the multiple scattering intensity, \(I_m\), in terms of the total scattering intensity, \(I_{total}\), from \(n\) number of orders of scattering intensity, \(I_n\), as:

Then, we see that to compute the multiple scattering, we must compute \(n-1\) scattering intensity terms to subtract from the total, \(I_{total}\).

Let us first just consider the secondary scattering term, \(I_2\). We again assume we have a homogeneous sample of a given shape that is fully illuminated by the incident beam. Then, extending from Eq. (5), we have that the number of neutrons per unit solid angle scattered once by a volume element \(dV_1\) and then a second time by a volume element \(dV_2\) of the sample and seen by a detector is given by:

where \(\theta_1\) is the angle between the incident path and scatter path from \(dV_1\), \(\theta_2\) is the angle between scatter path from \(dV_1\) and scatter path from dV_2, \(\theta_s\) is the angle between the incident path and scatter path from \(dV_2\) to the detector, \(\mu_{12}\) is the scattered wavelength from volume element \(dV_1\), and the \(l_12\) term is due to the inverse square law (that as the distance increases between \(dV_1\) and \(dV_2\), the solid angle subtended by \(dV_2\) at \(dV_1\) decreases as the inverse square of \(l_{12}\)).

And the total secondary scattering intensity seen by a detector is:

We can generalize this for \(i^{th}\) order of scatter terms as:

and

Which then the multiple scattering up to the \(n\) order scattering term is given as:

Thus, some of the difficulties in correcting multiple scattering arises from:

1. For each \(i^{th}\) order of scattering we must perform \(i\) volume integrals \(dV^{i}\) over the sample (although these terms tend to zero as explained in the introduction).
2. Without the elastic scattering assumption, we need to know each emerging \(\lambda_i\) due to energy transfer of the \(i^{th}\) scattering event.
3. Without the isotropic approximation, we need to know each \(\frac{d\sigma}{d\Omega} \left( \theta_i \right)\) of the \(i^{th}\) scattering event. This requires knowledge of \(\frac{d\sigma}{d\Omega}\), which is exactly what we are usually trying to measure (for diffraction)!
4. It is arbitrary at which \(n^{th}\) order of scatter should the correction be cutoff.
5. This kind of calculation is difficult for all but the simplest of geometries (i.e. cylindrical, planar and spherical) although Monte Carlo integration methods may be utilized for the multiple scattering calculations of more general shapes.

Assumptions¶

General Notes¶

Legends for Multiple Scattering Correction Algorithms in Mantid Table¶

Indicates the energy modes that the algorithm can accommodate:

Indicates the technique used for calculating the absorption correction:

Options that describe what functions the algorithm is capable of, assumptions, and the output types:

Multiple Scattering Correction Algorithms in Mantid Table¶