Positronium formation in a polymer blend of polyethylene and chlorinated polyethylene

We discuss positronium (Ps) formation in a polymer blend system consisting of low-density polyethylene (PE) and chlorinated polyethylene (CPE, random copolymer with a chlorine weight content of 35%). Positron lifetime measurements were performed as a function of CPE composition (0--100%), electric field $(F=0--40\mathrm{k}\mathrm{V}/\mathrm{c}\mathrm{m}),$ temperature $(T=30--300\mathrm{K}),$ and positron irradiation time $(t=0--110\mathrm{h})$ at $T=75\mathrm{K},$ and room temperature. We note that Ps formation occurs by recombination of electron-positron pairs with both small (\ensuremath{\lesssim}3 nm) and large (\ensuremath{\gtrsim}3 nm) initial separations. At low temperatures, pairs with large separations are formed not only in the positron spur (blob) but also as a result of coupling of a thermalized positron with one of the localized electrons produced by positron irradiation. For PE, the contribution of the latter process to Ps formation appears as a gradual increase in relative intensity of the long-lived ortho-positronium $(o\ensuremath{-}\mathrm{Ps})$ component as a function of positron irradiation time. The addition of CPE appreciably reduces the contribution from pairs with large separations, and Ps formation in pure CPE occurs exclusively from pairs with small initial separations, much less influenced by external parameters such as the temperature and electric field.