Polarisation modulated crosscorrelation spectroscopy on a pulsed neutron source

Abstract

We introduce a crosscorrelation technique by which a total scattering polarisation analysis spectrometer on a pulsed neutron source can be modified to give full neutron polarisation and energy analysis without changing the physical configuration of the instrument. The method relies on pseudorandom modulation of the polarisation direction of the incident beam. Subsequent crosscorrelation of the detected, spin analysed, countrate with the pseudorandom modulating sequence yields a two dimensional spectrum corresponding to the difference between the spin flip and non spin flip cross sections of the sample measured at all incident and final energies for which the polarising and analysing devices are effective.As an illustration of the technique its implementation on the proposed POLARIS spectrometer at the Rutherford Appleton Laboratory Spallation Neutron Source is described, and the expected dynamic (Q, ω) range and resolution evaluated.