Neutron Scattering - Neutron-matter Interaction

Neutron-matter Interaction

Since neutrons are electrically neutral, they penetrate matter more deeply than electrically charged particles of comparable kinetic energy; therefore they are valuable probes of bulk properties.

Neutrons interact with atomic nuclei and magnetic fields from unpaired electrons. The neutrons cause pronounced interference and energy transfer effects in scattering experiments. Unlike an x-ray photon with a similar wavelength, which interacts with the electron cloud surrounding the nucleus, neutrons primarily interact with the nucleus itself. The interaction is described by Fermi's pseudopotential. Neutron scattering and absorption cross sections vary widely from isotope to isotope.

Also depending on isotope, the scattering can be incoherent or coherent. Among all isotopes, hydrogen has the highest neutron scattering cross section. Also, important elements like carbon and oxygen are well visible in neutron scattering. This is marked contrast to X-ray scattering where cross sections systematically increase with atomic number. Thus neutrons can be used to analyse materials with low atomic numbers like proteins and surfactants. This can be done at synchrotron sources but very high intensities are needed which may cause the structures to change. The nucleus provides a very short range, isotropic potential varying randomly from isotope to isotope, making it possible to tune the nuclear scattering contrast to suit the experiment.

The scattering almost always has an elastic and an inelastic component. The fraction of elastic scattering is given by the Debye-Waller factor or the Mössbauer-Lamb factor. Depending on the research question, most measurements concentrate on either the elastic or the inelastic scattering.

Achieving a precise velocity, ie. a precise energy & de Broglie wavelength, of the neutron beam is important. Such single-energy beams are termed 'monochromatic'. Monochromaticity is achieved either with a crystal monochromator or a time of flight spectrometer. In the time-of-flight technique, neutrons are sent through a sequence of two rotating slits, so that only neutrons of a particular velocity are selected. Recently there has been the development of spallation sources which can create a rapid pulse of neutrons. In this method, the pulse contains neutrons of many different velocities / de Broglie wavelengths. However, the velocities of the scattered neutrons can be determined afterwards by measuring the time of flight of the neutrons between the sample and neutron detector.