Characterization of thin-foil ultracold neutron detectors

We have fabricated ultracold neutron detectors that consist of silicon charged particle detectors coupled with thin nickel foils coated with either natural LiF or 10B implanted into vanadium. The foils convert neutrons into energetic, readily detectable, charged particles which are in turn detected by silicon detectors. The detectors were tested at the Institut Laue-Langevin with a gravitational spectrometer. From a rigorous Monte Carlo simulation of the experiment, the minimum detection cutoff velocities (effective potentials) were determined to be 309±17cm/s (49.8±2.7neV) for LiF and 367±39cm/s (70.3±7.5neV) for 10B/V. Although the result for LiF is consistent with expectations, the result for 10B/V is significantly higher. We interpret this discrepancy as due to contamination. We also show that while a thicker foil is more efficient for ultracold neutron detection, a thinner foil is more ideal for determining the cutoff velocity.