.. _hollow-rectangular-prism-thin-walls:

hollow_rectangular_prism_thin_walls
=======================================================

Hollow rectangular parallelepiped with thin walls.

=========== ======================================== ============ =============
Parameter   Description                              Units        Default value
=========== ======================================== ============ =============
scale       Source intensity                         None                     1
background  Source background                        |cm^-1|              0.001
sld         Parallelepiped scattering length density |1e-6Ang^-2|           6.3
sld_solvent Solvent scattering length density        |1e-6Ang^-2|             1
length_a    Shorter side of the parallelepiped       |Ang|                   35
b2a_ratio   Ratio sides b/a                          |Ang|                    1
c2a_ratio   Ratio sides c/a                          |Ang|                    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
prism with infinitely thin walls. It computes only the 1D scattering, not the 2D.


**Definition**

The 1D scattering intensity for this model is calculated according to the
equations given by Nayuk and Huber (Nayuk, 2012).

Assuming a hollow parallelepiped with infinitely thin walls, edge lengths
$A \le B \le C$ and presenting an orientation with respect to the
scattering vector given by $\theta$ and $\phi$, where $\theta$ is the angle
between the $z$ axis and the longest axis of the parallelepiped $C$, and
$\phi$ is the angle between the scattering vector (lying in the $xy$ plane)
and the $y$ axis, the form factor is given by

.. math::

    P(q) = \frac{1}{V^2} \frac{2}{\pi} \int_0^{\frac{\pi}{2}}
           \int_0^{\frac{\pi}{2}} [A_L(q)+A_T(q)]^2 \sin\theta\,d\theta\,d\phi

where

.. math::

    V &= 2AB + 2AC + 2BC \\
    A_L(q) &=  8 \times \frac{
            \sin \left( \tfrac{1}{2} q A \sin\phi \sin\theta \right)
            \sin \left( \tfrac{1}{2} q B \cos\phi \sin\theta \right)
            \cos \left( \tfrac{1}{2} q C \cos\theta \right)
        }{q^2 \, \sin^2\theta \, \sin\phi \cos\phi} \\
    A_T(q) &=  A_F(q) \times
      \frac{2\,\sin \left( \tfrac{1}{2} q C \cos\theta \right)}{q\,\cos\theta}

and

.. math::

  A_F(q) =  4 \frac{ \cos \left( \tfrac{1}{2} q A \sin\phi \sin\theta \right)
                       \sin \left( \tfrac{1}{2} q B \cos\phi \sin\theta \right) }
                     {q \, \cos\phi \, \sin\theta} +
              4 \frac{ \sin \left( \tfrac{1}{2} q A \sin\phi \sin\theta \right)
                       \cos \left( \tfrac{1}{2} q B \cos\phi \sin\theta \right) }
                     {q \, \sin\phi \, \sin\theta}

The 1D scattering intensity is then calculated as

.. math::

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

where $V$ is the volume of the rectangular prism, $\rho_\text{p}$
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**

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



.. figure:: img/hollow_rectangular_prism_thin_walls_autogenfig.png

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

**References**

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