hollow_rectangular_prism

Hollow rectangular parallelepiped with uniform scattering length density.

Parameter	Description	Units	Default value
scale	Scale factor or Volume fraction	None	1
background	Source background	cm^-1	0.001
sld	Parallelepiped scattering length density	10^-6Å^-2	6.3
sld_solvent	Solvent scattering length density	10^-6Å^-2	1
length_a	Shortest, external, size of the parallelepiped	Å	35
b2a_ratio	Ratio sides b/a	Å	1
c2a_ratio	Ratio sides c/a	Å	1
thickness	Thickness of parallelepiped	Å	1
theta	c axis to beam angle	degree	0
phi	rotation about beam	degree	0
psi	rotation about c axis	degree	0

The returned value is scaled to units of cm^-1 sr^-1, absolute scale.

Definition

This model provides the form factor, \(P(q)\), for a hollow rectangular parallelepiped with a wall of thickness \(\Delta\). The 1D scattering intensity for this model is calculated by forming the difference of the amplitudes of two massive parallelepipeds differing in their outermost dimensions in each direction by the same length increment \(2\Delta\) ([1] Nayuk, 2012).

As in the case of the massive parallelepiped model (rectangular_prism), the scattering amplitude is computed for a particular orientation of the parallelepiped with respect to the scattering vector and then averaged over all possible orientations, giving

\[P(q) = \frac{1}{V^2} \frac{2}{\pi} \times \, \int_0^{\frac{\pi}{2}} \, \int_0^{\frac{\pi}{2}} A_{P\Delta}^2(q) \, \sin\theta \, d\theta \, d\phi\]

where \(\theta\) is the angle between the \(z\) axis and the longest axis of the parallelepiped, \(\phi\) is the angle between the scattering vector (lying in the \(xy\) plane) and the \(y\) axis, and

\begin{align*} A_{P\Delta}(q) & = A B C \left[\frac{\sin \bigl( q \frac{C}{2} \cos\theta \bigr)} {\left( q \frac{C}{2} \cos\theta \right)} \right] \left[\frac{\sin \bigl( q \frac{A}{2} \sin\theta \sin\phi \bigr)} {\left( q \frac{A}{2} \sin\theta \sin\phi \right)}\right] \left[\frac{\sin \bigl( q \frac{B}{2} \sin\theta \cos\phi \bigr)} {\left( q \frac{B}{2} \sin\theta \cos\phi \right)}\right] \\ & - 8 \left(\frac{A}{2}-\Delta\right) \left(\frac{B}{2}-\Delta\right) \left(\frac{C}{2}-\Delta\right) \left[ \frac{\sin \bigl[ q \bigl(\frac{C}{2}-\Delta\bigr) \cos\theta \bigr]} {q \bigl(\frac{C}{2}-\Delta\bigr) \cos\theta} \right] \left[ \frac{\sin \bigl[ q \bigl(\frac{A}{2}-\Delta\bigr) \sin\theta \sin\phi \bigr]} {q \bigl(\frac{A}{2}-\Delta\bigr) \sin\theta \sin\phi} \right] \left[ \frac{\sin \bigl[ q \bigl(\frac{B}{2}-\Delta\bigr) \sin\theta \cos\phi \bigr]} {q \bigl(\frac{B}{2}-\Delta\bigr) \sin\theta \cos\phi} \right] \end{align*}

where \(A\), \(B\) and \(C\) are the external sides of the parallelepiped fulfilling \(A \le B \le C\), and the volume \(V\) of the parallelepiped is

\[V = A B C \, - \, (A - 2\Delta) (B - 2\Delta) (C - 2\Delta)\]

The 1D scattering intensity is then calculated as

\[I(q) = \text{scale} \times V \times (\rho_\text{p} - \rho_\text{solvent})^2 \times P(q) + \text{background}\]

where \(\rho_\text{p}\) is the scattering length density of the parallelepiped, \(\rho_\text{solvent}\) is the scattering length density of the solvent, and (if the data are in absolute units) scale represents the volume fraction (which is unitless) of the rectangular shell of material (i.e. not including the volume of the solvent filled core).

For 2d data the orientation of the particle is required, described using angles \(\theta\), \(\phi\) and \(\Psi\) as in the diagrams below, for further details of the calculation and angular dispersions see Oriented particles . The angle \(\Psi\) is the rotational angle around the long C axis. For example, \(\Psi = 0\) when the B axis is parallel to the x-axis of the detector.

For 2d, constraints must be applied during fitting to ensure that the inequality \(A < B < C\) is not violated, and hence the correct definition of angles is preserved. The calculation will not report an error if the inequality is not preserved, but the results may be not correct.

../../_images/parallelepiped_angle_definition.png

Fig. 64 Definition of the angles for oriented hollow rectangular prism. Note that rotation \(\theta\), initially in the \(xz\) plane, is carried out first, then rotation \(\phi\) about the \(z\) axis, finally rotation \(\Psi\) is now around the axis of the prism. The neutron or X-ray beam is along the \(z\) axis.