Time of Flight- or Larmor Method for Neutron Reflectometry at a Steady State Reactor

Abstract

Two methods to measure neutron specular reflectivity at a steady state reactor are considered: the time of flight (TOF) method, where the time of flight of neutrons from a chopper to a detector is used to determine the wavelength and the LARMOR method, where the precession of the polarization of a neutron beam around an applied magnetic induction is used to determine the wavelength. Using computer simulations Poisson distributed ‘experimental data’ are generated taking into account the wavelength dependent flux, transmissions and detector efficiencies. Statistical accuracies for the ‘experimental’ neutron reflectivities are estimated from ‘experimental’ numbers of counts. Resolution effects due to finite widths of time channels for TOF and due to the Fourier Transform algorithm for LARMOR are taken into account properly. Numerical results show that, although both methods are feasible, the TOF method has advantages over the LARMOR method.