B₄C thin films for neutron detection

Due to the very limited availability of ³He, new kinds of neutron detectors, not based on ³He, are urgently needed. Here, we present a method to produce thin films of ¹⁰B₄C, with maximized detection efficiency, intended to be part of a new generation of large area neutron detectors. B₄C thin films have been deposited onto Al-blade and Si wafer substrates by dc magnetron sputtering from ^natB₄C and ¹⁰B₄C targets in an Ar discharge, using an industrial deposition system. The films were characterized with scanning electron microscopy, elastic recoil detection analysis, x-ray reflectivity, and neutron radiography. We show that the film-substrate adhesion and film purity are improved by increased substrate temperature and deposition rate. A deposition rate of 3.8 Å/s and substrate temperature of 400 °C result in films with a density close to bulk values and good adhesion to film thickness above 3 μm. Boron-10 contents of almost 80 at. % are obtained in 6.3 m² of 1 μm thick ¹⁰B₄C thin films coated on Al-blades. Initial neutron absorption measurements agree with Monte Carlo simulations and show that the layer thickness, number of layers, neutron wavelength, and amount of impurities are determining factors. The study also shows the importance of having uniform layer thicknesses over large areas, which for a full-scale detector could be in total ∼1000 m² of two-side coated Al-blades with ∼1 μm thick ¹⁰B₄C films.