Lifetimes and magnetic moments in odd-odd 70 As

The extensive experimental and theoretical work on the structure of N∼Z, A = 60-80 nuclei revealed many interesting features: large prolate deformations (β = 0.4), strong shape variations as a function of particle number, excitation energy, spin and shape coexistence effects. They are related with drastic changes of properties observed in nuclei with Z≥33 when going from spherical nuclei with N = 50 to neutron deficient nuclei with N = 38 or 40. Both the rapid changes in structure and the shape coexistence appear to reflect the competition between the shell gaps which occur at large oblate and prolate deformations near nucleon numbers 36 and 38 for both protons and neutrons. For N∼Z nuclei the same shell gaps appear simultaneously for both protons and neutrons and reinforce each other. The microscopic structure of the nuclei in the mass region A = 60-80 is essentially determined by the 1g9/2, 2p1/2, 1f5/2 and 2p3/2 orbitals. Because no unique interpretation of the unusual features discovered in these nuclei exists, the systematic experimental study of structure of these nuclei is still an interesting subject. As part of a systematic experimental study undertaken to investigate the structure of neutron deficient, odd-odd As nuclei, 68,70,72 As, the level scheme of 70 As was investigated using heavy ion induced reactions and in-beam γ-ray spectroscopy techniques. At energies between 500 and 900 keV a multiplet of negative parity levels has been observed. At higher energies a high-spin positive parity sequence of levels starting with a Ex = 1676 keV, Jπ 8+ level is strongly populated. Additional information is required in order to establish the structure of low and high-spin levels of both parities. The magnetic moments of the 8+ and 9+ levels have been measured using the time-integral perturbed angular distribution technique and the lifetimes of four levels have been determined using the recoil-distance method. From the measured lifetime for the 9+ level a B(M1) ∼ 1μ02 was extracted for the 9+ → 8+ transition. The measured magnetic moments for both 8+ and 9+ levels are in definite disagreement with the presumed two quasi-particle πg9/2xνg9/2 configuration, pointing on a more complex configuration for these levels. To explain the measured values for the magnetic moments of the 8+ state a {πf5/2, ν[(f5/2)5/2-3(g9/2)8+2]21/2-}8+ configuration was proposed. A value close to the measured value for the magnetic moment was obtained. The fast M1 transition between 9+ and 8+ levels gives a further support for this complex structure proposed for positive parity levels. (authors)