In X-ray scattering, f' and f'' are the resonant or anomalous X-ray scattering factors,  expressing the non-Thomson part of the scattering form factor.  It is due to the X-ray energy being near (or, strictly speaking, below) any of the binding energies of the core electrons of any of the atoms.

See  https://en.wikipedia.org/wiki/Atomic_form_factor#X-ray_form_factor or https://en.wikipedia.org/wiki/Anomalous_scattering or http://skuld.bmsc.washington.edu/scatter/

I have no idea what the  PANalytical wants for these, but would imagine that the values at http://physics.nist.gov/PhysRefData/FFast/html/form.html would be a good start.