Simulation of Low Energy Bremsstrahlung in the Nab experiment using Kohn-Ebert Method

The Nab experiment is a precision study of correlations in unpolarized neutron decay. Cold neutrons decay in flight inside a custom spectrometer. The goal of Nab is to measure a, the electron-neutrino coefficient, and b, the Fierz interference term that vanishes in the standard model (SM). A measured value of a at the Nab goal precision of △a/a ≈ 10^-3 will provide an independent competitive evaluation of the V_ud element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. The Nab precision goal for the measurement of b is Δb ≈ 0.003. In this way, Nab will provide a critical test CKM unitarity, and, combining a and b, sensitive new tests of physics beyond the SM.

Nab measures and detects protons and electrons in coincidence with two pixelated silicon detectors placed asymmetrically apart from the decay region. In measuring the energy of decay electrons, it is imperative to properly account for physics processes, such as bremsstrahlung, that affect the electron energy deposited in the Si detector. In the pursuit of reaching Nab's precision goals, there must be less than 1% of the events in the tail of the detector's electron energy response, and the tail must be determined to ±10%. In an attempt to better understand the effect of bremsstrahlung on electrons in Nab's spectrometer, Geant4 was modified to utilize the Kohn-Ebert method of calculating bremsstrahlung for electrons between 10 keV and 1MeV. The results of this simulation will be compared to standard Geant4 E&M packages, as well as source data from the Nab spectrometer.