For the first time positronium is investigated in fine powders of MgO and Al_2O_3 using age-momentum correlation technique based on a relativistic positron beam. The application of this technique for investigating the interaction of positronium with the grain surfaces is discussed and compared with other techniques. The previously reported interaction of the positronium with paramagnetic centres is further studied. A qualitative interpretation of the spectra is given. In the Al_2O_3 samples we found some unexpected behaviour of the conversion quenching of ortho-positronium at irradiation induced paramagnetic surface defect.
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