Measurement of spin-flip probabilities for ultracold neutrons interacting with nickel phosphorus coated surfaces

We report a measurement of the spin-flip probabilities for ultracold neutrons interacting with surfaces coated with nickel phosphorus. For 50~$\mu$m thick nickel phosphorus coated on stainless steel, the spin-flip probability per bounce was found to be $\beta_{\rm NiP;on;SS} = (3.3^{+1.8}{-5.6}) \times 10^{-6}$\mathrm{.}For50$\mu$m thick nickel phosphorus coated on aluminum, the spin-flip probability per bounce was found to be $\beta{\rm NiP;on;Al} = (3.6^{+2.1}{-5.9}) \times 10^{-6}$. For the copper guide used as reference, the spin flip probability per bounce was found to be $\beta{\rm Cu} = (6.7^{+5.0}{-2.5}) \times 10^{-6}$. The results on the nickel phosphorus-coated surfaces may be interpreted as upper limits, yielding ${\beta }_{\mathrm{N}\mathrm{i}\mathrm{P}\mathrm{o}\mathrm{n}\mathrm{S}\mathrm{S}}<6.2\times {10}^{-6}$ (90% C.L.) and $\beta{\rm NiP;on;Al} < 7.0 \times 10^{-6}$(90\mathrm{\%}C\mathrm{.}L\mathrm{.})for50$\mu$m thick nickel phosphorus coated on stainless steel and 50~$\mu$m thick nickel phosphorus coated on aluminum, respectively. Nickel phosphorus coated stainless steel or aluminum provides a solution when low-cost, mechanically robust, and non-depolarizing UCN guides with a high-Fermi-potential are needed.