We discuss here the choice of solid compounds and materials which best suit various types of applications, focusing mainly on the polarized targets. These materials include hydrogen-rich glassy hydrocarbons and simple cubic crystalline ammonia and lithium hydrides. The glassy hydrocarbons can doped by dissolved stable free radicals, while crystalline materials are doped by radiolytic paramagnetic radicals. The leading application of DNP up till now has been the scattering experiments in high-energy and nuclear physics. Other applications include measurements of slow neutron cross-sections, molecular physics using slow neutrons, nuclear magnetism and other solid-state physics experiments, and spin filters. The use of polarized solids in fusion and in magnetic resonance imaging has also been discussed. The material choice evidently depends strongly not only on the application but also on the goal of the experiment or process which is considered. More recently DNP has been used for the signal enhancement in NMR studies of complex chemical and biochemical molecules. In this context DNP and other enhancement techniques are called by the term “hyperpolarization”.

The principles of the continuous-wave (CW) NMR techniques is reviewed, as applied to the measurement of the nuclear polarization in polarized targets. The circuit theory of the series-tuned Q-meter is then described in detail in view of calculating precisely the CW NMR absorption signal and its integral, the signal-to-noise ratio, the probe coupling and sampling, and the signal saturation. Optimization of the series-tuned Q-meter circuit is discussed on the basis of the circuit theory. Improved Q-meter circuits will be reviewed, including the capacitively coupled Q-meter, the crossed-coil NMR circuit, and the introduction of quadrature mixer that enables the measurement of the real and imaginary parts of the RF signal simultaneously. Calibration and measurement of very small NMR signals then follows. We also treat the signal-to-noise issues, the electromagnetic interferences, and the NMR circuit drift issues.

We shall first outline the types of interactions of spins, which are most important for solid polarized targets: the magnetic dipole interaction, the quadrupole interaction, the spin-orbit interaction and the hyperfine interaction. Other direct and indirect spin interactions are described: these give rise to the chemical shift, the Knight shift, molecular spin isomers and to the exchange interaction of electron pairs. These, and in particular the dipolar interaction, are then used in the discussion of the magnetic resonance phenomena, such as the resonance line shape and saturation. The magnetic resonance absorption and the transverse susceptibility are discussed starting from the first principles, and Provotorov's equations are derived. The relaxation of spins, which is phenomenologically introduced already for the saturation, is then overviewed in greater depth, before closing with sudden and adiabatic changes of spin systems in the rotating frame.

Microwave sources and waveguide components are commercially available and therefore we shall limit ourselves to describing their physical principles and main limitations in the polarized target applications. The propagation modes and the complex propagation constants in rectangular and round guideas are derived and described. Simple waveguide components and circuits are discussed in view of control and optimization of the power, frequency and modulation for DNP. The main design principles and criteria are presented for iron core magnets and their cobalt–iron pole pieces. For superconducting solenoid and dipole magnets, the focus is in the winding accuracy. The control and protection of large superconducting magnets is also briefly discussed.

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.