Measurements were made of the doubly differential cross sections for three inclusive pion reactions on 3 He: Ϫ double charge exchange ͑DCX͒, and ϩ and Ϫ inelastic scattering. The cross sections for DCX were measured at incident pion energies of 120, 180, and 240 MeV, and at angles of 25°, 50°, 80°, 105°, and 130°, while inelastic scattering cross sections were measured at 120, 180, and 240 MeV and scattering angles of 50°, 80°, 105°, and 130°. In each case the outgoing pion energy spectra were measured from 10 MeV up to the kinematic limit. The DCX spectra exhibit a double-peaked structure at forward angles that can be understood as a consequence of a sequential single charge exchange mechanism. Model calculations based on this mechanism are in rough agreement with the measured spectra. The doubly differential cross sections measured for the inelastic scattering reactions exhibit a prominent quasielastic peak. A distorted-wave impulseapproximation calculation of the quasielastic cross sections has been performed and a comparison made with the measurements. ͓S0556-2813͑97͒01204-1͔PACS number͑s͒: 25.80.Ek, 25.80.Gn, 25.80.Ls, 27.10.ϩh The earliest inclusive DCX measurements were made at the JINR Synchro-cyclotron in Dubna by Batusov et al. ͓1͔ using nuclear emulsions. Later experiments by Batusov et al. ͓2͔ and Gilly et al. ͓3͔ were performed in which total DCX cross sections on a range of nuclei from He to Pb were measured.

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Second-neighbour defect clusters, described in the Hartree-Fock approximation with Kunz-Klein localising potentials out to fourth neighbours, embedded in an infinite shell model lattice are analysed with consistent distortion and polarisation for F+, F (H-)+, and (H2-)0 defects in MgO. Basis sets for the defects are optimised, and are improved by recontraction for the first and second neighbours. Questions of total energy, nearest-neighbour displacements and localisation are addressed for each defect. For the F+ centre, its ground state in relation to the valence band is discussed, as is the localisation of its unrelaxed excited state. For the F+ and (H2-)0 centres, spin densities are evaluated at each step of the calculation, and compared with experiment. Calculations that completely neglect the ion-size effect of all ions except at the defect centre give some qualitatively plausible results. The full-cluster results are used to derive short-range shell model parameters for H-and H2- in MgO. The method and results are critically reviewed.

Barium fluoride is an example of a high-density scintillator for detecting high-energy radiation. In use, its luminescent transmission is seriously degraded by radiation damage. This effect is associated with oxygen, among other impurities. At one time it was suspected that oxygen O Ϫ , having been dissociated from a defect complex by radiation damage, absorbed some of the luminescent energy of the crystal. This explanation has now been abandoned, and the present work shows quantitatively that it is not correct. Specifically, a detailed study of the optical absorption of O Ϫ shows that its excitation energy, split by spin polarization, is ϳ50% higher than the luminescent frequencies of the crystal. Instead, color centers, such as F centers, have come to be suspected. One origin of the color center is shown here to be the dissociation of a defect complex made up of an O 2Ϫ ion bound to a fluoride vacancy, accompanied by electron transfer from oxygen to vacancy, forming an F center. The study of the optical excitation of O Ϫ is used to assess the qualitative and quantitative importance of the main elements of the physical model and computational method in such a simulation. These elements include the ion-size effect of Ba 2ϩ ions, spin-polarization effects in ground and excited states, electric quadrupole moment consistency between the O Ϫ ion and the embedding BaF 2 crystal, basis set augmentation and optimization in the treatment of a quantum molecular cluster that includes the impurity for both ground and excited states, correlation correction, and projection of excited states onto spin eigenstates.