Primary and secondary nucleation of the transition between the A and B phases of superfluid He-3

We have studied nucleation in superfluid He-3 across the A-B phase transiti on driven by a magnetic field, in a controllable environment at very low te mperatures. Both B --> A and A --> B secondary nucleation appear to be gove rned by the survival of pockets of the new phase trapped at surfaces. We fi nd that, at fields near B-AB, primary A --> B nucleation cannot be triggere d by ionizing or neutron irradiation even at very high intensities; In our cell primary A --> B nucleation can only be triggered externally by mild me chanical shock.