Neutron flux measurement for beam polarization study at Fundamental neutron Physics Beamline of SNS*

    The goal of the Nab experiment at Spallation Neutron Source (SNS) in Oak Ridge National Lab is to measure the beta neutrino correlation co-efficient a_βν and Fierz interference term (b) in unpolarized free neutron decay with the relative uncertainty of ∆a/a < 10^-3 and ∆b/b < 3 × 10^-3 respectively. One of the major sources of systematic uncertainty in such a measurement is the degree of polarization of the neutron beam. Other properties of the neutron beam such as time of flight spectrum, beam profile, and beam stability impact our ability to measure the polarization, as well as the expected counting rate and fiducial area for the decay product detectors.

    In the current setup ¹⁴N based counting-mode and ³He based current-mode detectors are installed upstream and downstream of the Nab spectrometer to monitor the neutron beam flux, but our scheme for the beam polarization measurement requires that the flux be measured as an instantaneous current in both the detector. We have developed a novel approach using a transimpedance amplifier as a pulse charge integrator, thus making it a ratemeter with a high dynamic range to be used with a neutron counting detector. In this talk, I will present an overview of the neutron monitor circuit along with its performance for beam flux measurement, and the results of studies of the properties of the neutron beam used in Nab.