Quantitative descriptio n of the e ̃ect of Û-irradiation of PE and PMMA on positroniu m formation on the trapp ed electrons at low temp eratures is suggested. T he considerati on show s that, at least in these substances, the numb er of the trapping centers is probably enough to locali ze all or t ho -p ositroniu m, and theref ore the intensity of the longli ved positroni um comp onent is depend...