Hollow rectangular parallelepiped with uniform scattering length density.

Parameter Description Units Default value
scale Source intensity None 1
background Source background cm-1 0.001
sld Parallelepiped scattering length density 10-6-2 6.3
sld_solvent Solvent scattering length density 10-6-2 1
length_a Shorter side of the parallelepiped 35
b2a_ratio Ratio sides b/a 1
c2a_ratio Ratio sides c/a 1
thickness Thickness of parallelepiped 1

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

This model provides the form factor, \(P(q)\), for a hollow rectangular parallelepiped with a wall of thickness \(\Delta\). It computes only the 1D scattering, not the 2D.


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 \(2\Delta\) (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 of the parallelepiped, \(\rho_\text{solvent}\) is the scattering length of the solvent, and (if the data are in absolute units) scale represents the volume fraction (which is unitless).

The 2D scattering intensity is not computed by this model.


Validation of the code was conducted by qualitatively comparing the output of the 1D model to the curves shown in (Nayuk, 2012).


Fig. 55 1D plot corresponding to the default parameters of the model.


R Nayuk and K Huber, Z. Phys. Chem., 226 (2012) 837-854