Characterization of electroless nickel-phosphorus plating for ultracold-neutron storage

TL;DR

This study characterizes electroless nickel-phosphorus coatings for ultracold neutron storage, demonstrating their suitability and identifying key surface considerations to optimize UCN transport and storage efficiency.

Contribution

It provides a detailed evaluation of electroless nickel plating's effectiveness for UCN storage, including loss probabilities and surface roughness effects, guiding material choices for neutron sources.

Findings

UCN loss probability per wall bounce is approximately 2.8 to 4.1 x 10^-4.

Surface roughness has minimal impact on UCN storage performance.

A vendor was selected to plate the UCN-production vessel with acceptable storage properties.

Abstract

Electroless nickel plating is an established industrial process that provides a robust and relatively low-cost coating suitable for transporting and storing ultracold neutrons (UCN). Using roughness measurements and UCN-storage experiments we characterized UCN guides made from polished aluminum or stainless-steel tubes plated by several vendors. All electroless nickel platings were similarly suited for UCN storage with an average loss probability per wall bounce of $2.8\cdot10^{-4}$ to $4.1\cdot10^{-4}$ for energies between 90 neV and 190 neV, or a ratio of imaginary to real Fermi potential $\eta$ of $1.7\cdot10^{-4}$ to $3.3\cdot10^{-4}$. Measurements at different elevations indicate that the energy dependence of UCN losses is well described by the imaginary Fermi potential. Some special considerations are required to avoid an increase in surface roughness during the plating process and hence a reduction in UCN transmission. Increased roughness had only a minor impact on storage properties. Based on these findings we chose a vendor to plate the UCN-production vessel that will contain the superfluid-helium converter for the new TRIUMF UltraCold Advanced Neutron (TUCAN) source, achieving acceptable UCN-storage properties with ${\eta=3.5(5)\cdot10^{-4}}$.