.. _core-shell-parallelepiped: core_shell_parallelepiped ======================================================= Rectangular solid with a core-shell structure. =========== ==================================================== ============ ============= Parameter Description Units Default value =========== ==================================================== ============ ============= scale Source intensity None 1 background Source background |cm^-1| 0.001 sld_core Parallelepiped core scattering length density |1e-6Ang^-2| 1 sld_a Parallelepiped A rim scattering length density |1e-6Ang^-2| 2 sld_b Parallelepiped B rim scattering length density |1e-6Ang^-2| 4 sld_c Parallelepiped C rim scattering length density |1e-6Ang^-2| 2 sld_solvent Solvent scattering length density |1e-6Ang^-2| 6 length_a Shorter side of the parallelepiped |Ang| 35 length_b Second side of the parallelepiped |Ang| 75 length_c Larger side of the parallelepiped |Ang| 400 thick_rim_a Thickness of A rim |Ang| 10 thick_rim_b Thickness of B rim |Ang| 10 thick_rim_c Thickness of C rim |Ang| 10 theta In plane angle degree 0 phi Out of plane angle degree 0 psi Rotation angle around its own c axis against q plane degree 0 =========== ==================================================== ============ ============= The returned value is scaled to units of |cm^-1| |sr^-1|, absolute scale. **Definition** Calculates the form factor for a rectangular solid with a core-shell structure. The thickness and the scattering length density of the shell or "rim" can be different on each (pair) of faces. However at this time the model does **NOT** actually calculate a c face rim despite the presence of the parameter. .. note:: This model was originally ported from NIST IGOR macros. However,t is not yet fully understood by the SasView developers and is currently review. The form factor is normalized by the particle volume $V$ such that .. math:: I(q) = \text{scale}\frac{\langle f^2 \rangle}{V} + \text{background} where $\langle \ldots \rangle$ is an average over all possible orientations of the rectangular solid. The function calculated is the form factor of the rectangular solid below. The core of the solid is defined by the dimensions $A$, $B$, $C$ such that $A < B < C$. .. image:: img/core_shell_parallelepiped_geometry.jpg There are rectangular "slabs" of thickness $t_A$ that add to the $A$ dimension (on the $BC$ faces). There are similar slabs on the $AC$ $(=t_B)$ and $AB$ $(=t_C)$ faces. The projection in the $AB$ plane is then .. image:: img/core_shell_parallelepiped_projection.jpg The volume of the solid is .. math:: V = ABC + 2t_ABC + 2t_BAC + 2t_CAB **meaning that there are "gaps" at the corners of the solid.** Again note that $t_C = 0$ currently. The intensity calculated follows the :ref:`parallelepiped` model, with the core-shell intensity being calculated as the square of the sum of the amplitudes of the core and shell, in the same manner as a core-shell model. .. note:: For the calculation of the form factor to be valid, the sides of the solid MUST be chosen such that** $A < B < C$. If this inequality is not satisfied, the model will not report an error, but the calculation will not be correct and thus the result wrong. FITTING NOTES If the scale is set equal to the particle volume fraction, |phi|, the returned value is the scattered intensity per unit volume, $I(q) = \phi P(q)$. However, **no interparticle interference effects are included in this calculation.** There are many parameters in this model. Hold as many fixed as possible with known values, or you will certainly end up at a solution that is unphysical. Constraints must be applied during fitting to ensure that the inequality $A < B < C$ is not violated. The calculation will not report an error, but the results will not be correct. The returned value is in units of |cm^-1|, on absolute scale. NB: The 2nd virial coefficient of the core_shell_parallelepiped is calculated based on the the averaged effective radius $(=\sqrt{(A+2t_A)(B+2t_B)/\pi})$ and length $(C+2t_C)$ values, and used as the effective radius for $S(Q)$ when $P(Q) * S(Q)$ is applied. To provide easy access to the orientation of the parallelepiped, we define the axis of the cylinder using three angles $\theta$, $\phi$ and $\Psi$. (see :ref:`cylinder orientation `). The angle $\Psi$ is the rotational angle around the *long_c* axis against the $q$ plane. For example, $\Psi = 0$ when the *short_b* axis is parallel to the *x*-axis of the detector. .. figure:: img/parallelepiped_angle_definition.jpg Definition of the angles for oriented core-shell parallelepipeds. .. figure:: img/parallelepiped_angle_projection.jpg Examples of the angles for oriented core-shell parallelepipeds against the detector plane. .. figure:: img/core_shell_parallelepiped_autogenfig.png 1D and 2D plots corresponding to the default parameters of the model. **References** .. [#] P Mittelbach and G Porod, *Acta Physica Austriaca*, 14 (1961) 185-211 Equations (1), (13-14). (in German) .. [#] D Singh (2009). *Small angle scattering studies of self assembly in lipid mixtures*, John's Hopkins University Thesis (2009) 223-225. `Available from Proquest `_ **Authorship and Verification** * **Author:** NIST IGOR/DANSE **Date:** pre 2010 * **Converted to sasmodels by:** Miguel Gonzales **Date:** February 26, 2016 * **Last Modified by:** Wojciech Potrzebowski **Date:** January 11, 2017 * **Currently Under review by:** Paul Butler