Simulation of Beta Decay and Silicon Detector Performance for the Nab Experiment

The Nab Experiment's goal is to use Oak Ridge National Laboratory's (ORNL) Spallation Neutron Source (SNS) to record high precision measurements of two constants related to unpolarized neutron beta decay: the electron-neutrino correlation coefficient, a, and the Fierz interference term, b. Nab's stated goal is to achieve an uncertainty of 10-3 for both constants. The experiment uses two silicon detectors covered with a thin aluminum wire grid over the active area. The grid wire's presence helps maintain a uniform electrostatic potential across the surface of the detectors. A powerful electromagnetic spectrometer guides the decay products, protons and electrons, to their respective detectors. We used a monte carlo simulation, constructed with CERN's Geant4 simulation package, to investigate what effect the grid wires would have and any potential uncertainty they might introduce to the determination of the parameter a. At present, the Nab experiment has taken preliminary data for calibration and equipment testing. The full run for data collection should begin within the year after maintenance to the SNS is completed.