β+decay of61Ga

Abstract

The mirror β + -decay of 61 Ga was investigated by means of β-and γ-ray spectrometry at the ISOLDE-PSB facility using laser ionisation and mass-separation. The results for the 61 Ga half-life and the Q EC value are 168(3) ms and 9255(50) keV, respectively. The β-decay strength to the ground and low-lying excited states of the daughter 61 Zn was deduced. The experimental results are in a good agreement with large-scale shell model calculations. (Submitted to: Phys. Rev. C, November, 2001) IntroductionAccording to the experimentally proven Conserved Vector Current ( CVC ) hypothesis the Fermi (F) β decay is expected to have a constant strength for analog transitions for a given isospin multiplet, independent on the nuclear structure. The situation is quite different for the Gamow-Teller strength, B(GT), which depends on a product of the GTmatrix element < στ > and the axial-vector constant g a . The presence of the nuclear environment results in renormalisation of the free-nucleon g a axial-vector constant, determined by the neutron β decay. The GT-matrix element also varies from nucleus to nucleus. It is thus a challenge to calculate the strengths for the GT transitions with the same high accuracy as of for the F decay [1], as well as to disentangle the matrix element from the axial-vector constant from the experimentally measured B(GT). The measurement of β decays of T=1/2 mirror nuclei, characterised by a mixture of fast F and allowed GT transitions gives an unique opportunity to study the Gamow-Teller β-decay between isobaric analog states.Since the strength of the F transitions is constant for mirror transitions, the B(GT) strength can be evaluated very accurately and compared with theoretical calculations. The similarity of the wave functions for members of mirror doublets simplify theoretical calculations. Studies of mirror nuclei can therefore be used to test various theoretical models related to e.g. spin-isospin excitation modes. Moreover, the accurate measurements of the β-decay strength and the magnetic moments of the mirror states would allow to evaluate the effective axial-vector constant in a given nucleus [2].In this context the expansion of our knowledge on mirror decays for the heaviest available mirror nuclei in the fp shell is of significant importance. These studies are also becoming increasingly important in the context of recent progress in large-scale shellmodel calculations, which at present are extended up to the A=65 mass region [4]. The GT-matrix elements of mirror decays have been measured with high-accuracy in the fp shell from 41 Sc up to 55 Ni [5,6,7,8,9,10,11,12]. Information on the GT strength was obtained for heavier nuclei in the fp shell [13,14,15,16]. This paper reports on a new measurement of the mirror β + -decay of the 61 Ga nucleus (Z=31, N=30). Excited levels of the daughter nucleus 61 Zn are well known from reaction studies [17,18] and in-beam spectroscopy experiments [19,20,21,22]. The half-life of 61 Ga, produced in fragmentation reactions, was measured wi...

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