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High Energy Neutron Scattering from Hydrogen

Roger Cowley (University of Oxford)

The scattering from hydrogen in polythene has been measured with the direct time-of flight spectrometer, MARI, at the ISIS facility of the Rutherford Appleton Laboratory with incident neutron energies between 0.5eV and 600 eV. These experiments are interesting because the results of experiments from the indirect time-of-flight spectrometer, VESUVIO, gave intensities in this energy range for most hydrogen containing materials that were about 60% of the intensity expected from hydrogen and this was initially interpreted as due to entanglement. Since VESUVIO is the only instrument in the world that routinely operates with incident neutron energies in the eV range we considered that it would be worthwhile to measure the scattering from hydrogen at high incident neutron energies on a different type of instrument. The MARI direct time-of-flight instrument was chosen and we have studied the scattering for several different incident neutron energies. After several problems we obtained good data which after background subtraction, calibration of the incident energy and conversion to an energy spectra gave the intensity of the hydrogen scattering as within error independent of the wave vector transfer for scattering angles from about 1 degree up to 70 degrees for 3 incident neutron energies between 10 eV and 100 eV. When the data was put on an absolute scale, by measuring the scattering from 5 metal foils of known thicknesses under the same conditions we found that the absolute intensity of the scattering from the hydrogen was in agreement with that expected by conventional theory to an accuracy of . 3.0% over a wide range of wave-vector transfers. We consider that these measurements and some recent measurements with a VESUVIO time-of-flight instrument in Argentina show that the many results on hydrogen using the VESUVIO spectrometer were incorrect and that there is no need for any novel explanation. We have also used high energy neutron scattering to study electronic excitations and some recent results on YBCO and NiO will be discussed and compared with the RIXS results.

I am grateful to my colleagues: C. Stock, S. Bennington and N. Gidopoulos. Financial support was provided by the Lindemann Trust and by STFC.

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