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: Aug. 22, 2017
See also iFunc, iFunc/fits, iFunc/plot, ff_core_shell
(c) E.Farhi, ILL. License: EUPL.