Beta-decay asymmetry from the decays of orientedMn52andCo

Abstract

LBL-3488The beta decay of 52 Mn has been investigated by measuring the angular distribution asymmetry of the positrons emitted from 52 Mn oriented at low temperatures. The 52 Mn nuclei were polarized in an iron lattice and cooled by thermal contact w.ith an adiabatically demagnetized paramagnetic salt. The positron asymmetry was determined by means of two independent techniques: the positrons were detected directly using highpurity germanium detectors and indirectly using Nal detectors to observe the annihilation gamma rays. These two techniques yielded a consistent result for the asymmetry, corresponding for this allowed decay to a ratio of the Fermi to Gamow-Teller matrix elements of y = C'f1F/CAMGT = -0.144 ± 0.006. An auxiliary experiment on 60 co gave a beta asymmetry A = -0.971 ± 0.034, in excellent agreement with the theoretical value of -1 expected for maximal parity violation. 0 0 0 0 4 2 0 6 -1-

14 Particle Decay Study of Oriented Nuclei

“…Different corrections for backscattering, positron annihilation and background were accurately taken into account. A few years earlier, the Berkeley group already had confirmed this value with their measurement A 1 /{vlc) = +0.615(21) [33], Their result was obtained with a Ge-detector operated at about 20 Κ and with 60 Co implanted into an iron foil.…”

Tests of the standard electroweak model in nuclear beta decay

“…Only a limited number of measurements of this type were carried out till now (Table V) and several of these results have a poor precision (Vanneste, 1986). For 60 Co two rather precise results were reported (i.e A = −1.01(2) (Chirovsky et al, 1980) and A = 0.972(34) (Hung et al, 1976)), but as logf t = 7.5 for this transition the effect of recoil effects like weak magnetism may Ar, compared to more recent measurements. Only results with a precision better than 10% are included while, in addition, for a given isotope only the most precise result is shown.…”

Tests of P-violation in neutron and nuclear beta decay

“…This was shown when the experiment was later repeated with a thin source and in a very low external magnetic field, yielding for the β asymmetry parameter A = −1.01 (2) [19,20]. Another experiment yielded A = −0.972(34) [21]. Both are in agreement with the Standard Model value of A SM = −0.987(9) for maximal parity violation (this value takes into account the small contributions from so-called recoil order terms and was calculated [22] with the formalism of [23] and nuclear matrix elements provided by I.S.…”