TIME COMPONENT OF THE MIRROR BETA-DECAYS OF B-12 AND N-12

Beta-ray angular distributions from aligned B-12 and N-12 have been observed in order to study the meson-exchange effect in a nucleus. Alignment-correlation coefficients alpha-/+ for B-12 and N-12 were determined to be alpha- = -(0.0174 +/- 0.0059)%/MeV and alpha+ = -(0.2774 +/- 0.0086)%/MeV, respectively. These values are fairly consistent with the previous values, while the reliability was greatly improved. From these coefficients, the nuclear parameter y, i.e. the ratio of the time component of the axial vector currents of the Gamow-Teller matrix element, was determined to be y = 4.13 +/- 0.15. The value is enhanced by 33% from that calculated by use of the impulse approximation, which clearly shows a considerable effect due to meson-exchange currents.