A multilayer surface detector for ultracold neutrons (UCNs) is described. Thetop $^{10}$B layer is exposed to the vacuum chamber and directly captures UCNs.The ZnS:Ag layer beneath the $^{10}$B layer is a few microns thick, which issufficient to detect the charged particles from the $^{10}$B(n,$\alpha$)$^7$Lineutron-capture reaction, while thin enough so that ample light due to $\alpha$and $^7$Li escapes for detection by photomultiplier tubes. One-hundred-nm thick$^{10}$B layer gives high UCN detection efficiency, as determined by the meanUCN kinetic energy, detector materials and others. Low background, includingnegligible sensitivity to ambient neutrons, has also been verified throughpulse-shape analysis and comparisons with other existing $^3$He and $^{10}$Bdetectors. This type of detector has been configured in different ways for UCNflux monitoring, development of UCN guides and neutron lifetime research.
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