THE SCATTERING AT ZERO ANGLE IN CONTRAST VARIATION

The scattering of neutrons, x-rays, or light by a polydisperse system of particles is studied. As is well known, for fixed wave vector q the scattering intensity I(q) is a quadratic function of the scattering length density of the external medium. The minimum of the parabola is the minimal scattering intensity I(min) (q). It is shown that I(min) (0) is, for a certain large class of particles, directly proportional to the width of the size distribution [so that I(min) (0) = 0 for strictly monodisperse particles]. Explicit formulas are presented which permit the measurement of the size distribution for spheres, disks, rods, and similar particles.