Strong supercooling and stimulation of theA-Btransition in superfluidHe3

We report the first studies of the A-B transition in superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ confined by microscopically smooth surfaces. The transition exhibited dramatic supercooling, down to 0.15${\mathit{T}}_{\mathit{c}}$. The lifetime τ of the metastable A phase has a temperature dependence consistent with Leggett’s ‘‘baked Alaska’’ model based on nucleation by ionizing radiation. Radiation sources near the cryostat strongly reduce the lifetime, but do not alter its temperature dependence, and τ increases with increasing magnetic field in accord with Leggett’s model.