Theoretical models are presented for the enhancement of the electron density at a positron in a semiconductor or insulator host. The model better suited for typical semiconductors is based on the many-body theory for the screening of a positron in electron gas. The starting point of the model for insulators is the atomic polarizability. The common parameter in both models is the highfrequency dielectric constant. Moreover, the enhancement depends on the ambient electron density in the semiconductor model and on the unit-cell volume in the insulator model. With use of the models developed, positron lifetimes in perfect semiconductor and insulator crystals have been calculated. In the calculations, three-dimensional electron densities and electrostatic potentials are obtained by atomic superposition and the fully three-dimensional positron wave functions are solved by a relaxation method. The calculated positron lifetimes agree with the experimental ones within a few picoseconds. Moreover, we have used the model to predict lifetimes of positrons trapped by lattice defects such as vacancies and vacancy clusters.

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Nd2Fe14B permanent magnets with different load lines were irradiated with 20 MeV protons at temperatures between 300 and 15 K, and the flux loss was measured as a function of the irradiation temperature. The results show that the magnetic flux loss depends drastically on temperature and on the shape of the sample. A new theoretical model which explains the observed temperature dependence of irradiation effects is introduced.

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