Study of irradiation effect on positronium formation in polypropylene

60Co γ-rays and positron irradiation effect on positronium (Ps) formation in the bulk and near surface region of polypropylene (PP) was studied. It is revealed that the reduction of Ps formation probability in PP bulk as a function of elapsed time may not be due to enhanced concentration of oxygen groups, and should not be ascribed to a simple accumulation of free radicals. Results indicate that Ps in PP is not formed through only one branch. That branch sensitive to the irradiation as well as the existence of sample surface seems related to relatively mobile positrons.