.. _barbell: barbell ======================================================= Cylinder with spherical end caps =========== ================================= ============ ============= Parameter Description Units Default value =========== ================================= ============ ============= scale Scale factor or Volume fraction None 1 background Source background |cm^-1| 0.001 sld Barbell scattering length density |1e-6Ang^-2| 4 sld_solvent Solvent scattering length density |1e-6Ang^-2| 1 radius_bell Spherical bell radius |Ang| 40 radius Cylindrical bar radius |Ang| 20 length Cylinder bar length |Ang| 400 theta Barbell axis to beam angle degree 60 phi Rotation about beam degree 60 =========== ================================= ============ ============= The returned value is scaled to units of |cm^-1| |sr^-1|, absolute scale. **Definition** Calculates the scattering from a barbell-shaped cylinder. Like :ref:`capped-cylinder`, this is a spherocylinder with spherical end caps that have a radius larger than that of the cylinder, but with the center of the end cap radius lying outside of the cylinder. See the diagram for the details of the geometry and restrictions on parameter values. .. figure:: img/barbell_geometry.jpg Barbell geometry, where $r$ is *radius*, $R$ is *radius_bell* and $L$ is *length*. Since the end cap radius $R \geq r$ and by definition for this geometry $h \ge 0$, $h$ is then defined by $r$ and $R$ as $h = \sqrt{R^2 - r^2}$ The scattered intensity $I(q)$ is calculated as .. math:: I(q) = \frac{\Delta \rho^2}{V} \left where the amplitude $A(q,\alpha)$ with the rod axis at angle $\alpha$ to $q$ is given as .. math:: A(q) =&\ \pi r^2L \frac{\sin\left(\tfrac12 qL\cos\alpha\right)} {\tfrac12 qL\cos\alpha} \frac{2 J_1(qr\sin\alpha)}{qr\sin\alpha} \\ &\ + 4 \pi R^3 \int_{-h/R}^1 dt \cos\left[ q\cos\alpha \left(Rt + h + {\tfrac12} L\right)\right] \times (1-t^2) \frac{J_1\left[qR\sin\alpha \left(1-t^2\right)^{1/2}\right]} {qR\sin\alpha \left(1-t^2\right)^{1/2}} The $\left<\ldots\right>$ brackets denote an average of the structure over all orientations. $\left$ is then the form factor, $P(q)$. The scale factor is equivalent to the volume fraction of cylinders, each of volume, $V$. Contrast $\Delta\rho$ is the difference of scattering length densities of the cylinder and the surrounding solvent. The volume of the barbell is .. math:: V = \pi r_c^2 L + 2\pi\left(\tfrac23R^3 + R^2h-\tfrac13h^3\right) and its radius of gyration is .. math:: R_g^2 =&\ \left[ \tfrac{12}{5}R^4 + R^3\left(3L + \tfrac{18}{5} h\right) + R^2\left(L^2 + Lh + \tfrac25 h^2\right) + R\left(\tfrac14 L^3 + \tfrac12 L^2h - Lh^2\right) \right. \\ &\ \left. + Lh^4 - \tfrac12 L^2h^3 - \tfrac14 L^3h + \tfrac25 h^4\right] \left( 4R^2 + 3LR + 2Rh - 3Lh - 2h^2\right)^{-1} .. note:: The requirement that $R \geq r$ is not enforced in the model! It is up to you to restrict this during analysis. The 2D scattering intensity is calculated similar to the 2D cylinder model. .. figure:: img/cylinder_angle_definition.png Definition of the angles for oriented 2D barbells. .. figure:: img/barbell_autogenfig.png 1D and 2D plots corresponding to the default parameters of the model. **Source** :download:`barbell.py ` $\ \star\ $ :download:`barbell.c ` $\ \star\ $ :download:`gauss76.c ` $\ \star\ $ :download:`sas_J1.c ` $\ \star\ $ :download:`polevl.c ` **References** #. H Kaya, *J. Appl. Cryst.*, 37 (2004) 223-230 #. H Kaya and N R deSouza, *J. Appl. Cryst.*, 37 (2004) 508-509 (addenda and errata) #. L. Onsager, *Ann. New York Acad. Sci.*, 51 (1949) 627-659 **Authorship and Verification** * **Author:** NIST IGOR/DANSE **Date:** pre 2010 * **Last Modified by:** Paul Butler **Date:** March 20, 2016 * **Last Reviewed by:** Richard Heenan **Date:** January 4, 2017