Time component of the mirror beta decays of12B and12N

Beta-ray angular distributions from aligned¹²B and¹²N have been observed in order to study the meson-exchange effect in a nucleus. Alignment-correlation coefficients for¹²B and¹²N were determined to be _–=–(0.0174±0.0059)%/MeV and ₊=–(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 parametery, i.e. the ratio of the time component of the axial vector currents of the Gamow-Teller matrix element, was determined to bey=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.