Precision NMR measurement of the magnetic field ratio of the aSPECT spectrometer

The aSPECT retardation spectrometer measures the e- anti ν_e angular correlation coefficient a in free neutron β decay by utilizing a MAC-E filter. This measurement can be used to determine the ratio of (g_A)/(g_V) of the weak coupling constants, as well as to search for physics beyond the Standard Model. In spring/summer 2013 aSPECT had a successful beamtime at the Institut Laue-Langevin, Grenoble (France). The goal of this beamtime is to improve the current uncertainty of a from (Δa)/(a) ∼ 5 % to about 1%. To achieve this goal the systematics of aSPECT have to be understood accordingly. One sensitive parameter to the systematic error of a is the knowledge of the magnetic field ratio, since this directly enters into the transmission function of our spectrometer. To determine the ratio with high precision two NMR probes were designed and implemented inside aSPECT. In this talk we present the design, the implementation inside aSPECT and the first results of the measurements.