New theories for positrons in insulators

We present a novel path-integral Monte Carlo method for calculating ortho-positronium lifetimes in insulators and illustrate the method with calculations for positronium in hard spheres, solid argon, and a simple zeolite structure. In argon, we find that the positronium wavefunction is compressed, suggesting that the contact potential may be larger than unity. We discuss the difference in physical pictures between our method and the widely used Tao-Eldrup model, and indicate how our approach can address complicated geometries and chemical effects. r 2003 Elsevier Science Ltd. All rights reserved.