Measuring the proton spectrum in neutron decay—Latest results with aSPECT

The retardation spectrometer aSPECT was built to measure the shape of the proton spectrum in free neutron decay with high precision. This allows us to determine the antineutrino electron angular correlation coefficient a  . We aim for a precision more than one order of magnitude better than the present best value, which is Δa/a=5%Δa/a=5%.In a recent beam time performed at the Institut Laue-Langevin during April/May 2008 we reached a statistical accuracy of about 2% per 24 h measurement time. Several systematic effects were investigated experimentally. We expect the total relative uncertainty to be well below 5%.