Measurements of the electron-neutrino angular correlation coefficient u in the freeneutron 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*0.011, giving jGA/GVI=1.250* 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.The precise determination of weak-interaction p a r a m e t e r s (especially of the ratio of the coupling constants G,/G,) without a possible interference f r o m nuclear effects requires the study of the decay of f r e e neutrons with an accuracy better than previously reported. The most accurate value of GA/G, presently available f r o m a single experiment i s based on a measurement by Christensen et a1.l of the electron-asymmetry coefficient A in the decay of polarized neutrons, giving / G,/G, I = 1.26 k0.02. Measurements of the electron-neutrino angular correlation coefficient a of comparable accuracy have not been performed so f a r . The best experimental value of a is that of Grigor'ev et a1.' They measured coincidences between the electrons and the recoil protons from an unpolarized neutron beam and reported a = -0.091 i 0.039 and G~/ G , = -1.22 k0.08.The determination of a reported h e r e was done by a high-precision measurement of the energy spectrum of the recoil protons in free-neutron decay314 in an evacuated tangential through-tube near the c o r e of the ASTRA reactor. The theoretical shape of this spectrum had been computed by Riehs5 and by ~a c h t m a n n~ for various values of a. The shape to be expected in this experiment due to the finite source volume and to the neutron motion had been calculated by Nachtmann and P a~l .~ The spectrometer used (Fig. 1) i s an electrostatic spherical condenser8 with an ion-electron converter detector of the coincidence type.''1° Between c o r e and spectrometer a number of collimators and baffles w e r e used t o reduce the direct reactor radiation. The source aperture which restricted the effective source volume near the core of the reactor had a diameter of 4.0 c m and w a s situated 297.5 c m f r o m the spectrometer entrance aperture (3.8-cm diameter).After energy selection by the spherical condenser (90" deflection angle, 30.0-cm radius, 6.0-cm electrode distance) the protons were accelerated towards a self-supporting aluminum foil of 10-mm diameter (usable range of acceleration voltage -20 to -35 kV, foil thickness approximately 30 pg/cm2). The secondary electrons ejected f r o m both sides of the foil w e r e accelerated to ground potential (by the electron lenses shown) and were measured by means of scintillation detecto...
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