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y = ff_sphere(p, x, [y]) : Sphere form factor [Guinier]


function y=ff_sphere(varargin)


 y = ff_sphere(p, x, [y]) : Sphere form factor [Guinier]

   iFunc/ff_sphere monodisperse spherical particle form factor P(q) with uniform 
       scattering length density. 
     The 'x' wave-vector/momentum axis is usually in nm-1 or Angs-1.
     The parameter R is given in inverse unit of the axis (that is nm or Angs).
     Typical values for parameters are R=10-100 Angs, eta=1e-6.
     The value at q=0 is (4/3*pi*eta*R^3)^2

     Ref: Guinier, A. and G. Fournet, "Small-Angle Scattering of X-Rays", 
            John Wiley and Sons, New York, (1955).
          Extracted from sasfit/sasfit_ff/sasfit_ff_sphere.c
     I. Bressler, et al, Journal of Applied Crystallography, 2015, 48 (5), 1587-1598

 input:  p: sphere model parameters (double)
            p = [ R=Sphere_Radius eta=SLD particle/matrix ]
          or 'guess'
         x: wave-vector/momentum axis (double, e.g. nm-1 or Angs-1)
         y: when values are given and p='guess', a guess of the parameters is performed (double)
 output: y: model value (intensity)
 ex:     y=ff_sphere([10 2e-6], 0:0.01:1); or plot(ff_sphere,[10 1e-6],0:0.01:1)

 Version: Nov. 26, 2018
 See also iFunc, iFunc/fits, iFunc/plot, ff_core_shell
 (c) E.Farhi, ILL. License: EUPL.


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