Enhanced Performance Neutron Scattering Spectroscopy by Use of Correlation Techniques

Abstract

Neutron correlation spectroscopy can exceed direct spectroscopy in the incoming beam intensity by up to two orders of magnitude at the same energy resolution. However, the propagation of the counting noise in the correlation algorithm of data reduction is disadvantageous for the lowest intensity parts of the observed spectrum. To mitigate this effect at pulsed neutron sources we propose two dimensional time-of-flight recording of each neutron detection event: with respect to both the neutron source pulses and to the rotation phase of the pseudo-random beam modulation statistical chopper. We have identified a formulation of the data reduction algorithm by matching the data processing time channel width to the inherent time resolution of this chopper, which makes the reconstruction of the direct time-of-flight spectra exact and independent of all other contributions to instrumental resolution. Two ways are proposed for most flexible choice of intensity vs. resolution without changing the statistical chopper or its speed: changing the size of the beam window during the experiment or varying intensity and resolution options in the data reduction algorithm after the experiment. This latter is a unique and very promising new potential offered by the correlation method. Furthermore, it displays reduced sensitivity to external background, also due to the much higher signal intensity. This is particularly advantageous for extending the operational range to higher neutron energies. In hot and thermal neutron vibrational spectroscopy, the statistical chopper approach allows us to achieve very significant gains in spectrometer efficiency compared to using conventional choppers. High intensity for the most intense features in the spectra and the reduced sensitivity to sample independent background make correlation spectroscopy a most powerful choice for studying small samples.