⁷¹Ga Has A High Spin State And Collective Rotation

The nuclei with Z = 28^-40 recently has attracted much theoretical and experimental attentions.The low-and medium-spin states in the nuclei Sr,Y,and Zr near by the sub-shell closure can be well interpreted in terms of the spherical shell model.Meanwhile,collective states are observed in the light Br and Kr isotopes by the energy gap in the Nilsson diagram at N,Z = 34,36 for oblate deformation and N,Z = 38 for prolate deformation.These features can be interpreted as the dynamical interplay between single-particle and collective degrees of freedom,and also are attributed to the presence of deformed shell gaps at N = Z = 34,36,and 38,which the shell gaps favor competing oblate and prolate structures.Recently structure of neutron-rich Ga isotopes,in the transitional region between these two domains,are considered as interplay between single-particle and collective excitations.The odd-A Ga nuclei has three protons above the Z = 28 shell closure,the structure is expected to be even more complex,owing to the inclusion of a broken proton pair in collective effects.The experiment was carried out with 7Li beam at the HI-13 tandem Accelerator of China Institute of Atomic Energy?CIAE?in Beijing at bombarding energies of 30 and 35 MeV.The high^-spin states of ⁷¹Ga were populated via the ⁷⁰Zn?7Li,2n2p?⁷¹Ga fusion^-evaporation reaction.In the first run with 30 MeV beam energy,the target thickness was 2.15 mg/cm² backing on 0.93 mg/cm² Au.And in the second run with 35 MeV beam energy,the target thickness was 3.48 mg/cm² with 15.75 mg/cm² Pb backing.An array consisting of 12 Compton-suppressed HPGe-BGO spectrometers and two planar HPGe detectors ware used to detect online ? rays emitted by the reaction products.The level scheme has been extended to a spin of I? = 29/2⁺ and an excitation energy of 6.5 MeV.A total of 23 new levels and 34 new transitions have been assigned to ⁷¹Ga.There into,two bands and a new negative parity sequence built on top of the 9/2⁺,5/2^-and 3/2^-levels were identified in 71 Ga.The 9/2⁺,5/2^-and 3/2^-states were determined in previous transfer reaction to contain significant components from the 99/2,f_5/2 and p?3/2?proton or-bitals,respectively.The 9/2⁺ level was marked as the band head of positive-parity cascade in the Ga nuclei and with all probability originate from the promotion of the unpaired proton into the g_9/2 orbital.Positive-parity decoupled rotational bands have been also identified in the 63,65,67,69Ga,odd-A As and Br isotopes.The decoupled nature of these bands is interpreted to arise from configurations involving particles in the g_9/2 proton subshell,which decouples from the core due to the strong Coriolis force.Therefore,when excite the Ga nuclei to higher energies,the break^-up of the neutron pairs for g_9/2 orbital must be taken into account.The observed sequence reflect that the level structures of the ⁷¹Ga exhibit a typical particle-core coupling pattern,which can be explained by single particle excitations and their coupling to the positive-parity states of the even-even core of the 6828Zn40.The band crossing frequencies in the positive parity band structures are discussed in terms of Total-Routhian-Surface?TRS?and Cranked Shell model?CSM?cal-culations.The observed alignment frequencies are in good agreement with the predicted ones from the single-particle Routhians calculated for both neutrons and protons.Large-basis shell model calculations have been performed to interpret the experimental level structures of 71 Ga.The ground state 3/2^-is dominated by the ????configuration and the main configuration of the I? = 5/2^-,9/2^-,13/2^-,17/2^-and 21/2^-states and the I? = 7/2^-,11/2^-,15/2^-,19/2^-and 23/2^-states are dominated by???onfigurations,respectively.These configurations indicate that in order to gain higher spins for negative^-parity states,we would require the promotion of one of the p?3/2?unpaired proton into the f_5/2 orbital and coupling of a broken pair of g_9/2 neutrons,meanwhile exciting a neutron pair into the g_9/2 orbital from f_5/2 orbital.For positive parity band,the alignment of one g_9/2 neutron pair leads the dominant configuration has change significantly,that one proton is excited to the f_5/2 orbit from the p?3/2?orbits and two neutrons are excited to the g_9/2 orbit from the f_5/2 and p1/2 orbits.