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The diffusion model for the trapping and detrapping of positrons in grain boundaries

A general solution of the model for positrons which diffuse inside a grain and then can be trapped and detrapped at a grain boundary of arbitrary shape is presented. The closed-form relations for the mean positron lifetime and the positron lifetime spectrum are obtained for a grain of spherical shape. The obtained results slightly differ from those presented in 1996 Phil. Mag. 73 1489 where a similar problem was considered and they extend the consideration given in that paper in J. Phys.: Condens. Matter 10 L547.