Editing Neutron diffraction (section)

== Principle ==

=== Processes ===
Neutrons are produced through three major processes, fission, spallation, and Low energy nuclear reactions.{{cn|date=February 2025}}

==== Fission ====
In research reactors, fission takes place when a fissile nucleus, such as [[uranium-235]] (<sup>235</sup>U), absorbs a neutron and subsequently splits into two smaller fragments. This process releases energy along with additional neutrons. On average, each [[Nuclear fission|fission]] event produces about 2.5 neutrons. While one neutron is required to maintain the [[chain reaction]], the surplus neutrons can be utilized for various experimental applications.<ref>{{Cite book |last=LEMBO |first=MARY FRANCES |title=Nuclear engineering |year=2006 |isbn=9780429224515 |pages=15}}</ref>

==== Spallation ====
In spallation sources, high-energy protons (on the order of 1 [[Electronvolt|GeV]]) bombard a heavy metal target (e.g., [[uranium]] (U), [[tungsten]] (W), [[tantalum]] (Ta), [[lead]] (Pb), or [[Mercury (element)|mercury]] (Hg)). This interaction causes the nuclei to spit out neutrons. Proton interactions result in around ten to thirty neutrons per event, of which the bulk are known as "evaporation neutrons"(~2 MeV), while a minority are identified as "cascade neutrons" with energies reaching up to the GeV range. Although spallation is a very efficient technique of neutron production, the technique generates high energy particles, therefore requiring shielding for safety.<ref name="Carpenter-2015">{{Cite book |last=Carpenter |first=John M. |title=Elements of slow-neutron scattering: basics, techniques, and applications |date=2015 |publisher=Cambridge University Press |isbn=978-1-139-02931-5 |location=Cambridge}}</ref>
[[File:Three_major_process_for_neutron_production.png|thumb|Illustration of three major fundamental processes generating neutrons for scattering experiments:   Nuclear fission (Top), Spallation (middle),  Low energy reaction (bottom).<ref>{{Cite journal |last=Dronskowski |first=Richard |last2=Brückel |first2=Thomas |last3=Kohlmann |first3=Holger |last4=Avdeev |first4=Maxim |last5=Houben |first5=Andreas |last6=Meven |first6=Martin |last7=Hofmann |first7=Michael |last8=Kamiyama |first8=Takashi |last9=Zobel |first9=Mirijam |last10=Schweika |first10=Werner |last11=Hermann |first11=Raphaël P. |last12=Sano-Furukawa |first12=Asami |date=2024-06-25 |title=Neutron diffraction: a primer |url=https://www.degruyter.com/document/doi/10.1515/zkri-2024-0001/html |journal=Zeitschrift für Kristallographie - Crystalline Materials |language=en |volume=239 |issue=5-6 |pages=139–166 |doi=10.1515/zkri-2024-0001 |issn=2194-4946}}</ref>]]

==== Low energy nuclear reactions ====
Low-energy nuclear reactions are the basis of neutron production in accelerator-driven sources. The selected target materials are based on the energy levels; lighter metals such as [[lithium]] (Li) and [[beryllium]] (Be) can be used toachieve their maximum possible reaction rate under 30 MeV, while heavier elements such as tungsten (W) and [[carbon]] (C) provide better performance above 312 MeV. These Compact Accelerator-driven Neutron Sources (CANS) have matured and are now approaching the performance of fission and spallation sources.<ref>{{Cite journal |last1=Ashkar |first1=Rana |last2=Bilheux |first2=Hassina Z. |last3=Bordallo |first3=Heliosa |last4=Briber |first4=Robert |last5=Callaway |first5=David J. E. |last6=Cheng |first6=Xiaolin |last7=Chu |first7=Xiang-Qiang |last8=Curtis |first8=Joseph E. |last9=Dadmun |first9=Mark |last10=Fenimore |first10=Paul |last11=Fushman |first11=David |last12=Gabel |first12=Frank |last13=Gupta |first13=Kushol |last14=Herberle |first14=Frederick |last15=Heinrich |first15=Frank |date=2018-12-01 |title=Neutron scattering in the biological sciences: progress and prospects |url=https://journals.iucr.org/paper?S2059798318017503 |journal=Acta Crystallographica Section D Structural Biology |volume=74 |issue=12 |pages=1129–1168 |doi=10.1107/S2059798318017503 |pmid=30605130 |issn=2059-7983|hdl=2381/45684 |hdl-access=free }}</ref>

=== De-Broglie relation ===
Neutron scattering relies on the wave-particle dual nature of neutrons. The [[De Broglie relation|De-Broglie relation]] links the [[wavelength]] (''λ'') of a neutron to its energy (E)<ref name="Carpenter-2015" />

<math>\lambda=h/mv</math>

where: ''h'' is Planck's constant, ''p'' is the [[momentum]] of the neutron, ''m'' is the mass of the neutron, ''v'' is the [[velocity]] of the neutron.