 For NSE spin incoherent scattering has the disadvantage that it flips the neutron spins during scattering with a probability of 2/3.Thus converting 2/3 of the scattering intensity into "non-polarized" background and putting a factor of -1/3 in front of the cos-Fourier integral 
contribution pertaining the incoherent intensity.This signal subtracts from the coherent echo signal.