Thirty-minute coherence in free induction decay signals in superfluid 3He-B

The persistent induction signal (PIS) is an extremely long lived NMR mode in, the B-phase of superfluid He-3. The first signals observed lived for zip to 25s following a moderate NMR tipping pulse, but were found to be highly irreproducible. Here we present new measurements obtained in, a smooth sapphire cell designed to achieve the lowest possible temperatures. The signals observed in this cell are quite reproducible but are only excited over a restricted range of magnetic field gradients. The free induction signals exceed 30 minutes at the lowest temperatures.