Observation of positronium spin rotation with silica aerogel

Abstract

Positronium (Ps) atoms in a magnetic field have their spins precessing about the magnetic field direction with precessional frequencies given by the energy difference between the various eigenstates of positronium in a magnetic field. Since positrons emitted from beta decay are polarized along the direction of their velocities, the density matrix describing positronium atoms at the instant of positronium formation in a magnetic field has nonzero coherence terms. These nonzero off-diagonal elements of the density matrix give rise to time oscillations of the annihilation amplitude, and hence the cross section for 3 γ decay of positronium at the Bohr frequencies [1]. In this experimental study, we describe a simple system for observing these oscillations with the aim of gaining a better understanding of the phenomenon of positronium spin rotation and to verify the results, as reported in [2], with silica aerogel in vacuum as a source of positronium atoms.