Solid State Ultra-Cold Neutron Detectors

The reflective properties of Ultra-Cold 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 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 (natural isotopic abundance) evaporated on $\sim 1 \mu$m~thick nickel foils. The second type consists of $\sim 10^{18}$ $^{10}$B ions implanted in a 2000\AA\ thick vanadium layer, also evaporated onto nickel foils. We find that both types have a lower critical velocity than aluminum, and thus, outperform aluminum window proportional counters for \emph{in-situ} density measurements.