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  βNiPonSS=(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  βNiPonAl=(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  βCu=(6.7−2.5+5.0)×10−6 . The results on the nickel phosphorus-coated surfaces may be interpreted as upper limits, yielding  βNiPonSS<6.2×10−6  (90% C.L.) and  βNiPonAl<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.