The two-dimensional angular correlation of annihilation radiation (2D-ACAR) setup at the Positron Centre Delft has been coupled to the reactor-based variable-energy slow-positron beam POSH. Depth-selective 2D-ACAR experiments have been performed in crystalline Si containing a subsurface layer of nanocavities produced by ion implantation. Positronium (Ps) atoms trapped in large nanocavities (similar to 10 nm) are present in excited kinetic states (hot Ps). Refilling of the nanocavities by means of low dose He implantation cools down the Ps and reduces Ps formation.