Hyperpolarized gases have found a steadily increasing range of applications in nuclear magnetic resonance (NMR) and NMR imaging (MRI). They can be regarded as a new class of MR contrast agent or as a way of greatly enhancing the temporal resolution of the measurement of processes relevant to areas as diverse as materials science and biomedicine. We concentrate on the properties and applications of hyperpolarized xenon. This review discusses the physics of producing hyperpolarization, the NMR-relevant properties of 129 Xe, specific MRI methods for hyperpolarized gases, applications of xenon to biology and medicine, polarization transfer to other nuclear species and low-field imaging.

The systematics of the even light P o isotopes (N126) are studied in the framework of the Particle-Core Model. The strong perturbation of the systematics in the very light isotopes is interpreted as arising from the interaction between regular and intruder structures. Results of Potential Energy Surface (PES) calculations and predictions of the Pairing Vibration Model support this interpretation. The mixing between the regular and intruder structures is studied within the IBM-2 and in a simple two-state mixing picture. Matrix elements of the interaction and their spin-dependence are extracted. The 'reconstructed systematics' show the coexistence of a spherical structure, which v aries little with the neutron number, with an intruder band, strongly lowered in energy as the neutron numberapproaches midshell. The crossing of the two congurations takes place over a few isotopes; the intruder band becomes the ground-state conguration in 192 Po.

The structure of the low-lying levels in the mirror nuclei 57 Ni and 57 Cu is described within the extended unified model. The problem of single-particle energies in 56 Ni is treated in detail. ''Bare'' single-particle energies are extracted from existing experimental data for the energy levels in 57 Ni and 57 Cu by carefully considering the influence of the coupling to excitations of the core. Important contributions arise, influencing especially the results on the spin-orbit splitting. The differences between the Coulomb energy shifts of various orbitals in 56 Ni are discussed and compared with those resulting from Hartree-Fock calculations carried out using a broad range of Skyrme interactions. The parameters of the Woods-Saxon potential reproducing these neutron ''bare'' single-particle energies and the charge root-mean-square radius of 56 Ni are extracted. It is demonstrated that the contributions associated with the Thomas-Ehrman effect and the electromagnetic spinorbit interaction are important and large enough to account for the differences between the Coulomb energy shifts of the single-particle levels in 56 Ni. ͓S0556-2813͑96͒03711-9͔

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The level structure of 132,134 Ba was investigated in ͑p,t͒ transfer reactions. High resolution spectra ͑7-8 keV full width at half maximum͒ with targets from a mass separator allow us to resolve levels up to 4.0 MeV. New 0 ϩ states are identified. The monopole transfer strength shows distributions similar to those observed for 194,196 Pt. This is consistent with the O͑6͒ limit of the interacting boson approximation ͑IBA-1͒ model, without ruling out other, more microscopically based interpretations for the low-lying structure of these nuclei. We also compare with the IBA-2 model and geometrical model calculations. ͓S0556-2813͑96͒04209-4͔PACS number͑s͒: 21.60. Fw, 21.60.Ev, 25.40.Hs, 27.60.ϩj The low-lying excitations of even Pt nuclei of mass 194 and 196 are of interest as a realization of a ␥-soft collective structure ͑weak restoring force in the ␥ degree of freedom͒. Similar properties were proposed for the even Ba isotopes near mass 132 ͓1͔. In the interacting boson approximation ͑IBA-1͒ model ͓2͔ these features are described in the limit of the O͑6͒ dynamical symmetry. The resemblance of these Ba and Pt isotopes is expected due to their location with respect to the closed shells.The similarity of 134 Ba (Nϭ5) and 196 Pt (Nϭ6), stated by Casten and von Brentano ͓1͔, is based on the energy spectra and electromagnetic transition probabilities ͓3͔. For 132 132 Ba, 135 Ba, and 136 Ba with intensities of 0.08%, 0.08%, and 0.01% relative to the transition from 134 Ba, respectively.