Shaping the neutron beam profile at the Nab experiment

The Nab experiment, currently being installed at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source at Oak Ridge National Laboratory, will make precise measurements of unpolarized neutron beta decays, in particular the electron-neutrino correlation coefficient, a, and the Fierz interference term, b. In Nab, the neutron beam passes through the decay volume, from which decay electrons and protons are guided by a long electromagnetic spectrometer to two silicon detectors. These detectors record the electron energy and proton time-of-flight. A known source of error in the determination of a is the “edge effect” caused by non-uniformity in the beam profile as it passes through the decay volume and the gyration of the decay products in the magnetic field. In order to achieve the desired level of precision for the measurement of the neutrino-electron correlation coefficient, the neutron beam must therefore be sufficiently uniform in the decay volume. I will discuss efforts to shape and understand the beam profile, including the fabrication of the internal collimation system, simulations conducted in McStas, and direct measurements of the beam profile via copper foil activation.