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Date added: Mar 20, 2009

Neutron Compton Scattering. Seminar on 2 April 2009

Seminar Title

Neutron Compton Scattering from hydrogenous systems

Dr Jerry Mayers

ISIS, Rutherford Appleton Laboratory

2.00pm to 3.00pm

Thursday 2 April 2009

Newton G13

Abstract

Neutron Compton Scattering is a technique made possible by the development of intense pulsed neutron sources such as the ISIS source at the Rutherford Appleton Laboratory. It measures the momentum distribution n(p) of atoms in liquids and solids. The theoretical basis of the technique is similar to the measurement of electron momentum distributions by Compton scattering of gamma rays from electrons (hence the name). At sufficiently high incident energies the impulse approximation is valid and the neutron scatters from a single atom with conservation of momentum and kinetic energy of the neutron+atom. By measuring the energy and momentum change of the neutron, the momentum of the atom can be determined. The technique can give useful results on any light atoms [1]. However it is particularly useful for the study of hydrogen atoms in condensed matter systems. The proton signal is well separated from the signal from other masses and n(p) can be measured with good resolution. The momentum distribution is related to the wave function of the proton via the standard relation of quantum mechanics

Proton momentum distribution

In favourable circumstances the wave function can be reconstructed from n(p) in a model independent way. In principle this wave function contains all the information which can be known about the proton behaviour on very short time scales. In particular it can be used to reconstruct the potential well in which the proton sits. Recent developments in detector technology for eV neutrons on the VESUVIO spectrometer at ISIS have given an order of magnitude improvement in the accuracy of measurements on protons and deuterons. The talk will outline the experimental technique in simple terms and give examples of recent scientific results on systems such as supersolid helium [1] bulk water [2,3], water in interaction with proteins [4], water in confined geometry [5] and protons in proton glasses.

1. Measurement of the kinetic energy and lattice constant in hcp solid helium at temperatures 0.07-0.4 K. Phys Rev Lett 98 085301 (2007).

2. Excess of proton mean kinetic energy in supercooled water. Phys Rev Lett 12 127802 (2008).

3. Proton Momentum Distribution of Liquid Water from Room Temperature to the Supercritical Phase . Phys Rev Lett 100 177801 (2008).

4. Proton momentum distribution in a protein hydration shell. Phys Rev Lett 98 138102 (2007).

5. Anomalous behavior of proton zero point motion in water confined in carbon nanotubes Phys Rev Lett 97 247801 (2006).

6. Quantum and classical relaxation in the proton glass. Phys Rev Lett 97 145501 (2008).

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