Highly overpressurised helium bubbles in Al were obtained by 3-45 MeV alpha implantation and isochronal annealing upto 900K. Positron lifetimes in the bubbles, measured at temperatures between 20K and 300K, and the parameters deduced from them indicated for the first time the transformation of dense helium bubbles into the solid phase, as expected from the Simon-Glatzel equation.