Assessing the Silicon Detector Dead Layer with Alpha Particle Spectroscopy

As a test of the Standard Model of Elementary Particle Physics, the Nab experiment at Oak Ridge National Laboratory aims to make a high precision measurement of the neutron beta decay observables, the beta-antineutrino angular correlation coefficient (a) and the Fierz Interference term (b), to test the unitarity of the Cabibbo Kobayashi Maskawa matrix and to search for exotic Scalar and Tensor couplings. Coincident detection of protons and betas from the neutron beta decay with a pair of segmented silicon detectors is a key measurement principle of the Nab experiment. The protons lose ~1/3 of their kinetic energy in the dead layer of the silicon detector, and surface contamination of the detectors increases the non-detectable energy loss, potentially resulting in a biased measurement of the a. For a characterization of the dead layer and surface conditions, an ex-situ source manipulation setup was designed to perform alpha particle spectroscopy at various impact angles, and an in-situ alpha spectroscopy technique was developed to monitor additional surface contaminations. In this talk, I will present the application of the alpha particle spectroscopy to assess the dead layer of the silicon detectors used in the Nab experiment.