Electron-neutrino angular correlation coefficientameasured from free-neutron decay

Abstract

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...

Standard solar models and the uncertainties in predicted capture rates of solar neutrinos

“…In addition to making use of the neutron lifetime, there are two other ways of determining the square of the effective Gamow-Teller coupling constant. They involve (1) a measurement of the electron-nuclear spin correlation in the decay of polarized neutrons (see, for example, Krohn and Ringo, 1975) and (2) the electronneutrino correlation determined from the energy spectrum of the recoil proton (see, for example, Dobrozemsky et a/. , 1975).…”