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 μm thick nickel phosphorus coated on stainless steel, the spin-flip probability per bounce was found to be β_NiPonSS = (3 .3_-5.6^+1.8) ×10^-6 . For 50 μm thick nickel phosphorus coated on aluminum, the spin-flip probability per bounce was found to be β_NiPonAl = (3 .6_-5.9^+2.1) ×10^-6 . For the copper guide used as reference, the spin flip probability per bounce was found to be β_Cu = (6 .7_-2.5^+5.0) ×10^-6 . The results on the nickel phosphorus-coated surfaces may be interpreted as upper limits, yielding β_NiPonSS < 6.2 ×10^-6 (90% C.L.) and β_NiPonAl < 7.0 ×10^-6 (90% C.L.) for 50 μm thick nickel phosphorus coated on stainless steel and 50 μm 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.