Editing Neutron scattering (section)

== Neutron-matter interaction ==
Because neutrons are electrically neutral, they penetrate more deeply into matter than electrically charged particles of comparable kinetic energy, and thus are valuable as probes of bulk properties.

Neutrons interact with atomic nuclei and with magnetic fields from unpaired electrons, causing pronounced [[Interference (wave propagation)|interference]] and [[energy transfer]] effects in neutron scattering experiments. Unlike an [[x-ray]] [[photon]] with a similar wavelength, which interacts with the [[electron cloud]] surrounding the [[atomic nucleus|nucleus]], neutrons interact primarily with the nucleus itself, as described by [[Fermi's pseudopotential]]. Neutron scattering and absorption [[Neutron cross-section|cross section]]s vary widely from [[isotope]] to isotope.

Neutron scattering can be incoherent or coherent, also depending on isotope. Among all isotopes, hydrogen has the highest scattering cross section. Important elements like carbon and oxygen are quite visible in neutron scattering—this is in marked contrast to [[X-ray scattering]] where cross sections systematically increase with atomic number. Thus neutrons can be used to analyze materials with low atomic numbers, including 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, as isotropic potential varies randomly from isotope to isotope, which makes it possible to tune the (scattering) contrast to suit the experiment.

Scattering almost always presents both elastic and inelastic components. The fraction of elastic scattering is determined by the [[Debye-Waller factor]] or the [[Mössbauer-Lamb factor]]. Depending on the research question, most measurements concentrate on either elastic or inelastic scattering.

Achieving a precise velocity, i.e. a precise energy and [[de Broglie wavelength]], of a neutron beam is important. Such single-energy beams are termed 'monochromatic', and monochromaticity is achieved either with a crystal monochromator or with a [[time of flight|time of flight (TOF)]] [[spectrometer]]. In the time-of-flight technique, neutrons are sent through a sequence of two rotating slits such that only neutrons of a particular velocity are selected. Spallation sources have been developed that can create a rapid pulse of neutrons. The pulse contains neutrons of many different velocities or de Broglie wavelengths, but separate velocities of the scattered neutrons can be determined ''afterwards'' by measuring the time of flight of the neutrons between the sample and neutron detector.

=== Magnetic scattering ===

The neutron has a net electric charge of zero, but has a significant [[Nucleon magnetic moment|magnetic moment]], although only about 0.1% of that of the [[electron]]. Nevertheless, it is large enough to scatter from local magnetic fields inside condensed matter, providing a weakly interacting and hence penetrating probe of ordered magnetic structures and electron spin fluctuations.<ref name="Zaliznyak">{{Citation|last1=Zaliznyak|first1=Igor A.|title=Magnetic Neutron Scattering|date=2004|url=https://inis.iaea.org/search/search.aspx?orig_q=RN:36002750|last2=Lee|first2=Seung-Hun}}</ref>