The Physics of Spallation Processes

Chapter 2Research with NeutronsA neutron is an uncharged (electrically neutral) subatomic particle with mass 1,839 timesthat of the electron. Neutrons are stable when bound in an atomic nucleus, whilst havinga mean lifetime of approximately 900 seconds as a free particle, decaying through the weakinteraction into a proton, an electron, and an antineutrino. The neutron (like the proton)is the ground state of a three-quark system and consists of one “up” and two “down”quarks with spin 1/2 and baryon number 1. QCD allows the calculation of particle massesand magnetic moments; predictions, 939 MeV and -1.86 nuclear magneton, compare wellwith measured values, 939.6 MeV and -1.913 nuclear magneton. The neutron and theproton form nearly the entire mass of atomic nuclei, so they are both called nucleons.They interact through the nuclear, weak, electromagnetic and gravitational forces.At the end of the second World War researchers in the USA gained access to the largeneutron fluxes that even relatively modest nuclear reactors were capable of delivering.For more than a decade (Nobel Prize in physics to Sir James Chadwick in 1935 for theneutron discovery in 1932) neutrons had then been known as building blocks in the atomicnucleus. Enrico Fermi showed in 1942 that neutrons from fission of the uranium nucleuscould support a controlled chain reaction. He had earlier made the important discoverythat slowed-down or thermal neutrons show a much greater inclination to react than fastones do (Nobel Prize for this discovery, among others, to Fermi in 1938). The specialproperties of these slow neutrons make them suitable for detecting the positions andmovements of atoms. Even before the entry of the nuclear reactors into the researcharena, results of using simple neutron sources had indicated that neutron beams could beused for studying solid bodies and liquids (condensed matter). However, there were manydifficulties to overcome before these possibilities could be realized. The 1994 Nobel Prizein Physics was awarded to Bertram Brockhouse and Clifford Shull for their pioneeringcontributions to the development of neutron scattering techniques (neutron diffractionand neutron spectroscopy) for studies of condensed matter. In simple terms, they helpedanswer the questions of where atoms ”are” and of what atoms ”do”.Neutrons are an ideal probe for investigation of the structural and dynamical propertiesof matter. Their electrical neutrality enables them to penetrate deep into matter anddue to the low energy the matter can be studied without being destroyed. The magneticmoment enables neutrons to explore microscopic magnetic structures and study magneticfluctuations. The energies of thermal neutrons are similar to the energies of elementaryexcitations in solids and consequently molecular vibrations, lattice modes and the dynamicsof atomic motions can be probed. Very much the same is true for the wavelengths of5