Characterization of Solid State Ultracold Neutron Detectors

The reflective properties of ultracold neutrons (UCN) enable easy transport and bottling but make neutron detection a technical challenge. Typically, UCN are allowed to accelerate in the Earth's gravitational field to sufficient velocity to penetrate an aluminum entrance window of a $^{3}$He proportional counter. Here we describe the construction and characterization at the ILL of two kinds of prototype solid-state detectors which can be used to monitor the UCN density inside the UCNA spectrometer at LANL without gravitational acceleration, and perhaps more critically, without the danger of $^{3}$He leaks. The first type consists of 300 $\mu$g/cm$^2$ of LiF evaporated onto 200 nm thick Ni foils. The second type consists of $\sim$ 10$^{18}$ $^{10}$B ions implanted in a 200 nm thick V layer, also evaporated onto Ni foils. From monte carlo simulations, we find that LiF has a critical velocity nearly equal to that of aluminum, whereas the boron foils do indeed have a lower cutoff. Because of these cutoffs and the small size of the detectors, our solid-state detectors, thus, equal (for LiF) or outperform (for Boron) aluminum window proportional counters for {\it{in-situ}} density measurements.