Methods other than DNP may also produce high nuclear spin polarization, either in thermal equilibrium with the solid lattice, or in dynamic equilibrium in a rotating frame. Optical pumping methods create a very high enhancement of the nuclear spin polarization based on spin exchange collisions with atoms whose outer electron is polarized by circular polarized light. Some methods are also based on creating high non-equilibrium polarization that is then frozen in by increasing the spin–lattice relaxation time. Chemical and biomedical research teams use the term “hyperpolarization” to describe the general methods of spin polarization enhancement beyond thermal equilibrium; DNP methods belong clearly to these. Other methods include optical pumping and chemical polarization methods such as Chemical Induced Dynamic Nuclear Polarization (CIDNP) and Parahydrogen Induced Polarization (PHIP).

The concepts of angular momentum, spin and magnetic moment are worked out using standard quantum mechanical formalism. The concepts of intrinsic spin of a pointlike particle is contrasted with the intrinsic angular momentum of composite particles. The Larmor frequency and the magnetic resonance of non-interacting spins are introduced. The quantum statistics of a system of spins is overviewed, before introducing the thermodynamics of a spin system in a static frame of reference. Nuclear magnetic phase transitions are briefly reviewed.

Polarized targets need continuous cooling of large heat load during DNP at temperatures around or below 1 K. This can be achieved by continuous-flow refrigerators based on the evaporation of liquid 4He or 3He, or on the dilution of 3He by 4He. The refrigerator components have unusual requirements due to the large helium mass flow rates and to the demand of long uninterrupted runs of operation. We describe first the heat transfer mechanisms from the solid target material to the coolant fluid, and then evaluate the various cooling cycles in detail. The heat loads, ranging from some W/cm3 to some tens of μW/cm3, and the choice of the cooling method, are evaluated, before discussing the design of other cryogenic parts of the apparatus, including the precooling heat exchangers, thermometry and other instrumentation, and the pump and gas purification systems.

The DNP phenomenoma are first overviewed basing on magnetic spin transitions and on thermal reservoirs, before turning to the microscopic and quantum statistical descriptions using the high-temperature approximation. The dynamic cooling of dipolar interactions is then extended to low temperatures and the stationary solution of Borghini is developed. The physical limits of the equal spin temperature model are discussed, focusing on the electron spin concentration, cross relaxation and hyperfine interactions, before treating the limitations arising from the heat transport, diffusion barrier, leakage factor and phonon bottleneck . The resolved and differential solid effect mechanisms are then presented before turning to the cross effect, Overhauser effect and DNP of hyperfine nuclei. The microwave frequency modulation effects are discussed in view of the “hole burning” due to limited cross relaxation and due to uneven power absorption cause by the magnetic dispersion and by inhomogeneity of the magnetic field.

The DNP phenomenoma are first overviewed basing on magnetic spin transitions and on thermal reservoirs, before turning to the microscopic and quantum statistical descriptions using the high-temperature approximation. The dynamic cooling of dipolar interactions is then extended to low temperatures and the stationary solution of Borghini is developed. The physical limits of the equal spin temperature model are discussed, focusing on the electron spin concentration, cross relaxation and hyperfine interactions, before treating the limitations arising from the heat transport, diffusion barrier, leakage factor and phonon bottleneck. The resolved and differential solid effect mechanisms are then presented before turning to the cross effect, Overhauser effect and DNP of hyperfine nuclei. The microwave frequency modulation effects are discussed in view of the “hole burning” due to limited cross relaxation and due to uneven power absorption cause by the magnetic dispersion and by inhomogeneity of the magnetic field.

We shall first discuss the origin of the spins and magnetic dipole moments of the nucleons and nuclei. The nuclear magnetic resonance (NMR) lineshape will then be reviewed in general theoretical terms first, before turning to the microscopic sources of line broadening and frequency shifts that are valid for solid materials only. The relaxation mechanisms of nuclear spins will then be described, focusing on relaxation via paramagnetic electrons. During frozen spin operation the polarization loss is different for positive and negative polarization, which is explained by the polarization-dependent heat transfer from the nuclear spins to the liquid helium coolant.

The figure of merit is defined for some scattering applications; this figure permits the objective comparison of the various target types and polarization methods. The optimization of the polarized target operation in particle physics experiments is briefly discussed before treating the sources of possible false asymmetries due to the target. Finally a series of uses of polarized target techniques beyond particle and nuclear physics experiments is presented. These include notably the coherent small-angle neutron scattering (SANS) used in the studies of biological macromolecules, time–resolved SANS, pseudomagnetism, nuclear magnetic ordering, DNP enhancement of high-resolution NMR spectroscopy, particularly in solid state using the magic angle spinning techniques. The sensitivity and contrast enhancement are briefly discussed for magnetic resonance imaging (MRI) techniques. These use various DNP techniques and radical-free injectable polarized fluid methods, as well as the dissolution DNP techniques.