Electron-neutrino angular correlation coefficient $a$ measured from free-neutron decay

Measurements of the electron-neutrino angular correlation coefficient $a$ in the free-neutron decay are reported. The method is based on the measurement of the energy spectrum of recoil protons obtained from neutrons inside a highly evacuated tangential reactor beam tube. Spectroscopy was done outside the reactor shielding by means of an electrostatic condenser and an ion-electron converter detector of the coincidence type. The result, based on 30 measured proton-energy spectra, is $a=-0.099\mathrm{}\pm 0.011$, giving $\left|\frac{G_A}{G_V}\right|=1.250\mathrm{}\pm 0.036$. Thus a considerable improvement in accuracy concerning the measurement of $a$ of the free unpolarized neutron has been achieved. The result is in good agreement with the data obtained from polarized neutron decay.