The Physics of Spallation Processes

2.1. THE SCIENCE CASE 9of present day neutron instrumentation. The reliability of models crucially depends onthe knowledge of the physical and chemical properties of the materials involved (oceaniccrust, upper mantle, continental crust). Mineral structures and material behaviors underextreme temperature and pressure conditions simulating the real conditions deep in earthwould be in the field of in-situ studies when more intense neutron sources would beavailable.2.1.7 Fundamental neutron physicsThe features of neutrons appearing as both composite particles and quantum waves havebeen investigated with thermal, cold and ultra cold neutrons at many sources. Accuratemeasurements of the neutron β-decay confirmed the number of particle families predictedin the Standard Model at three. Neutron experiments have made substantial contributionsto our understanding of strong, electroweak and gravitational interactions. A significantlyhigher intensity and pulse structure of future generation neutron sources would providenew possibilities for fundamental neutron physics experiments. One of several questionsconcerns the handedness of the universe. The grand unified theory assumes a left-rightsymmetric universe and explains the evident left handedness of nature through a spontaneoussymmetry breaking caused by a phase transition of the vacuum, a scenario whichwould entail a small right handed component for the neutrinos. Looking at the decay ofa neutron into a hydrogen atom could provide a definite answer, since one of the four hydrogenhyperfine states cannot be populated at all if neutrinos are completely left handed.The best way to check whether deviations in the singlet scattering lengths signal a breakdownof isospin invariance, is a direct scattering measurement of the neutron-neutronscattering at very low energy as available at the ESS. Ultra cold neutrons could be usedto study elastic and inelastic surface reflections and quantum gravitational states.2.1.8 Muons as probes for condensed matterSpallation neutron sources as discussed in the following chapter represent at the sametime an intense resource of muons. Muons provide an alternative to the neutron as aprobe of condensed matter and are frequently used in complementary experiments. Themuon can be implanted into virtually any material and its spin polarization monitoredto determine its site in crystal lattices or molecules, giving information about the localatomic structure and dynamics. Resulting from the decay of positive or negative pionsinto a muon and a neutrino, muons have spin 1/2, carry one elementary electric charge,and have a mass about 207 times the rest mass of the electron or 1/9th of the proton restmass. Thus, from a particle-physics point of view they are ”heavy electrons”, whereasfrom a solid-state-physics or chemistry point of view they are ”light protons”. In the restframe of the pion, the muon magnetic moment is antiparallel to the muon momentum,allowing muon beams with a very high degree of spin polarization (nearly 100% whenthe muons are collected from pions decaying at rest) to be produced. Free muons havea mean lifetime of 2.2 µs, decaying into a positron and two neutrinos, with the positronemitted preferentially in the direction of the muon spin, allowing the time evolution of