and diamagnetic states. Note that many diamagnetic muon states really behave like paramagnetic centers, according to the standard definition of a paramagnet. For example, in most metallic samples, which are Pauli paramagnets, the muon's positive charge is collectively screened by a cloud of conduction electrons. Thus, in metals, the muon is not bound to a single electron, hence it is in the so-called diamagnetic state and behaves like a free muon. In insulators or semiconductors a collective screening cannot take place and the muon will usually pick up one electron and form a so-called muonium (Mu=μ++e−), which has similar size (Bohr radius), reduced mass, and ionization energy to the hydrogen atom. This is the prototype of the so-called paramagnetic state. Detection of muon polarization The decay of the positive muon into a positron and two neutrinos occurs via the weak interaction process after a mean lifetime of τμ = 2.197034(21) μs: Parity violation in the weak interaction leads in this more complicated case (three body decay) to an anisotropic distribution of the positron emission with respect to the spin direction of the μ+ at the decay time. The positron emission probability is given by where is the angle between the positron trajectory and the μ+-spin, and is an intrinsic asymmetry parameter determined by the weak decay mechanism. This anisotropic emission constitutes in fact the basics for the μSR technique. The average asymmetry is measured over a statistical ensemble of implanted muons and it depends on further experimental parameters, such as the beam spin polarization , close to one, as already mentioned. Theoretically =1/3 is obtained if all emitted positrons are detected with the same efficiency, irrespective of their energy. Practically, values of ≈ 0.25 are routinely obtained. The muon spin motion may be measured over a time scale dictated by the muon decay, i.e. a few times τμ, roughly 10 µs. The asymmetry in the muon decay correlates the positron emission and the muon spin directions. The simplest example is when the spin direction of all muons remains constant in time after implantation (no motion). In this case the asymmetry shows up as an imbalance between the positron counts in two equivalent detectors placed in front and behind the sample, along the beam axis. Each of them records an exponentially decaying rate as a function of the time t elapsed from implantation, according to with for the detector looking towards