Positronium in large voids. Silicagel

The lifetime spectra of positrons annihilating in a porous medium (silicagel) were measured a function of temperature. In the low temperature limit the lifetime of o-Ps in the pores agrees rather well with the popular Tao-Eldrup formula. With the rise of temperature the second long-lived component grows; that component was identified as o-Ps annihilation from the 1p-like state in the potential well. The lifetimes of o-Ps in various states of the spherical well have been calculated. The experimental value of the lifetime for the 1p state is in good agreement withe model calculation.