Measuring Proton Energy Loss through Gold Films

The NIST neutron-lifetime measurement counts protons resulting from neutron beta decay by accelerating the resulting protons to around 30keV and then detecting them in silicon charged particle detectors. To account for proton losses in dead layers, backscattering, or energies below the detection threshold, a variety of energies and silicon detectors are used which have entrance windows consisting of silicon or various thicknesses of gold. Our project aims to better understand proton energy loss in detector entrance windows and compare our measurements with known stopping powers and simulations of spectra using SRIM. We create these thin (10s of nm) gold layers directly on the silicon surface of PIPS detectors, measure the thickness of the layer, and vary the proton beam’s energy to obtain the detected energy spectra for protons traveling through the gold-plus-silicon dead layer and through just the bare silicon dead layer as function of incident proton energy and gold thickness. To deposit the gold, a thin wire is thermally evaporated under a vacuum. We have used different techniques to determine the thickness, including UV-Vis spectroscopy and an Atomic Force Microscope. We discuss our experimental techniques, challenges, results, and future improvements.