Scattering depth correction of evanescent waves in inelastic neutron scattering using a neutron prism

Grazing Incidence Neutron Spin Echo Spectroscopy (GINSES) has recently been applied to measure the dynamics of surfactant membranes close to a hydrophilic silicon wall. The scattering depth of the evanescent wave inside the microemulsion depends strongly on the angle of incidence and the wavelength. The inherently low scattering intensity of GINSES measurements, however, requires the integration over a rather broad wavelength band. In particular, at a pulsed source the instrument operates with a broad wavelength band covering all neutrons within one frame between two pulses. In order to yield viable counting statistics it is highly desirable to integrate data corresponding to significant fractions of the wavelength band. Therefore, in a normal reflectometry setup the penetration length would be smeared and blur the depth dependence of the experimental results. Here we describe a new method to strongly mitigate this effect and show its application in a GINSES experiment at the neutron spin echo instrument at the Spallation Neutron Source (SNS). A prism in front of the sample was introduced in order to adapt the angle of the incoming beam according to the wavelength by this optical component. As an example an experiment on a bicontinuous microemulsion using these neutron optics is presented.