The Physics of Spallation Processes

2.1. THE SCIENCE CASE 72.1.1 Solid state physicsNeutrons are the key to our understandings of solids. Nowadays advances in solid statephysics provide the backbone of many technologies. One example is molecular and organicmagnets i.e. solids built from structurally well defined clusters of magnetic ions in acomplex environment. Such systems are of fundamental importance and could also serveas atomic scale information storage systems. Research on the electron-electron interactionsunderpinning such phenomena as high-temperature superconductivity and “colossal”magneto-resistance are at the cutting edge of solid state physics. The high neutron fluxanticipated at next generation facilities will enable experiments on excitation continua ofmetallic systems, among others, that will yield a wealth of new information. AlternativelyNMR techniques yield valuable local data on the magnetic susceptibility of solids, butinterpretation of these data requires knowledge of the material-specific hyperfine interactions.The interaction parameters are difficult to calculate and to measure independently,especially for complex materials. Even if they could be calculated accurately, NMR wouldremain constrained to energies several orders of magnitude below those of electronic correlationeffects. Synchrotron radiation is another valuable characterization tool for solidstate magnetism. However, the cross section for charge scattering is several orders ofmagnitude larger than that for magnetic scattering of photons. Thus, even magneticstructure determinations of simple single-crystalline solids by magnetic x-ray scatteringare exceedingly difficult. A quantitative determination of the magnetic collective modesand excitation continua of complex electronic materials by inelastic x-ray scattering willnot be feasible in the foreseeable future.2.1.2 Materials science and EngineeringThe interaction of a neutron with the nucleus of an atom is weak, (but not negligible)making the neutron a highly penetrating probe. This allows the investigation of theinterior of materials, rather than the surface layers probed by techniques such as X-rayscattering, electron microscopy or optical methods. This feature also makes the use ofcomplex sample environments such as cryostats, furnaces and pressure cells quite routine,and enables the measurement of bulk processes under realistic conditions. High intensityspallation neutron sources will allow for the first time to investigate materials in real timewith realistic dimensions and under real conditions. One example is the deformation ofmaterials and the understanding of the mechanisms involved. New solid state joiningtechniques require more accurate information about the generation of residual stressesthat will add to in-service stresses and shorten component life. Finite element modelinghas become the main method for the design and assessment of engineering structures.Such models cannot be developed reliably without accurate information to validate them.Neutron diffraction is the only technique that can do this, providing measurements deepinside most engineering materials.2.1.3 Chemical structure, kinetics and dynamicsNeutrons are spin-1/2 particles and therefore have a magnetic moment that can coupledirectly to spatial and temporal variations of the magnetization of materials on an atomic