Determination of the electron-antineutrino angular correlation coefficient a0 and the parameter |λ|=|GA/GV| in free neutron β-decay from measurements of the integrated energy spectrum of recoil protons stored in an ion trap

Recoil protons from the,B-decay of unpolarized neutrons are stored in a cryogenic ion trap formed by the superposition of an axisymmetric electrostatic field on a coaxial magnetic field, which varies uniformly in space between regions of high (similar to4.30 T) and low (similar to0.61 T) homogeneous field. Protons generated in the high field region are adiabatically focused onto a 'mirror' electrode in the low field region. The integrated energy spectrum is determined by counting the number of stored protons as a function of mirror potential. Violation of the exact adiabatic conditions is corrected for by carrying out alternate measurements at trapping times which differ by a factor of 2. The results: a(0) = -0.1054 +/- 0.0055, \alpha\ = 1.271 +/- 0.018 are in agreement with previous measurements of these quantities.