Muon diffusion in copper bolow 2K

Transverse field SR experiments were performed on several different samples of copper in the temperature range below 2K, including isotope separated copper, Fe and Si doped polycrystalline copper and monocrystalline copper. No strong isotope dependent effect was observed. A⁶³Cu and a natural copper sample of identical purity both yield 0.16 s^–1 for the low-temperature plateau, while an increased linewidth inthe⁶⁵Cu case may be related to the strong effects of Fe impurities. Careful transverse field measurements on large single crystals at 0.08K reveal non-Gaussian lineshapes in accordance with the picture of diffusing muons at this temperature. This allows us to reject several of the existing models for muon behaviour in copper below 2K.