Nuclear Structure TowardsN=40Ca60: In-Beam

Abstract

Excited states in the neutron-rich N ¼ 38, 36 nuclei 60 Ti and 58 Ti were populated in nucleon-removal reactions from 61 V projectiles at 90 MeV=nucleon. The γ-ray transitions from such states in these Ti isotopes were detected with the advanced γ-ray tracking array GRETINA and were corrected event by event for large Doppler shifts (v=c ∼ 0.4) using the γ-ray interaction points deduced from online signal decomposition. The new data indicate that a steep decrease in quadrupole collectivity occurs when moving from neutron-rich N ¼ 36, 38 Fe and Cr toward the Ti and Ca isotones. In fact, 58;60 Ti provide some of the most neutron-rich benchmarks accessible today for calculations attempting to determine the structure of the potentially doubly magic nucleus 60 Ca. DOI: 10.1103/PhysRevLett.112.112503 PACS numbers: 23.20.Lv, 21.60.Cs, 27.30.+t, 29.38.Db One of the main goals of nuclear physics is the development of a predictive model for the properties of all nuclei, including the shortest-lived species in as yet unexplored regions of the nuclear chart. This is important, for example, in the quest to understand the origin of the elements in the Universe since many nucleosynthesis processes involve nuclei far removed from the valley of β stability. One of the cornerstones in the description of nuclear properties is nuclear shell structure-whereby discrete nucleon singleparticle orbitals are clustered in energy, resulting in stabilizing energy gaps occurring for certain "magic" proton or neutron numbers. Doubly magic nuclei, with both proton and neutron magic numbers, are particularly important for the development of nuclear models as they serve as essentially inert cores, reducing the many-body problem to that of the set of "valence nucleons" outside this core. However, modifications of shell structure have already been observed in short-lived nuclei with extreme neutron-to-proton ratios, with new shell gaps developing and some of the canonical magic numbers disappearing [1][2][3][4]. Considerable experimental and theoretical efforts are aimed at describing the physics driving such changes which are revealed most clearly on the neutron-rich side of the nuclear chart.Data for chains of proton-magic isotopes and regions of rapid shell evolution offer (complementary) challenging tests of nuclear models, allowing changes in nuclear structure to be tracked as a function of isospin and providing demanding benchmarks for calculations incorporating new physics effects. The chain of Ca isotopes (with magic proton number Z ¼ 20) and the region of neutron-rich nuclei near N ¼ 40, which are subject to rapid shell and shape changes [5][6][7][8][9], coincide at 60 Ca. In addition to the first spin-orbit driven neutron subshell closure at N ¼ 28 48 Ca, the neutron-rich Ca isotopes exhibit two additional subshell gaps at N ¼ 32 [10] and N ¼ 34 [11], attributed in part to the action of the monopole parts of the proton-neutron tensor force in the regime of large neutron excess [12,13].Nothing is known experimentally about the properti...

Study of the deformation-drivingνd5/2orbital in 67 28 Ni 39 using one-neutron transfer reactions

“…On the other hand, the profile corresponding to the d 5/2 orbital appears to be much more fragmented in agreement with the experimental findings. The actual value of the effective single-particle energy gap in 68 Ni from the Hamiltonian [42] at N = 40 is in agreement with the one extracted experimentally and the one extracted in the recent d( 68 Ni, p) 69 Ni reaction study [43], although the calculated strengths appear to be shifted to higher energies by 1 MeV. The missing correlation mechanism to lower these strengths is still not identified and further theoretical investigations need to be implemented in order to understand this trend.…”

First spectroscopy of 61Ti and the transition to the Island of Inversion at N = 40

“…The location of such pure configurations in the most neutron-rich nuclei is paramount to test the predictions of shell model calculations for the most exotic N = 40 nuclei 60 Ca and 59 K which are still beyond the reach of presently available radioactive beam facilities for spectroscopic studies. First experimental hints have been obtained from the first spectroscopy of 60 Ti [9] and the discovery of particle-bound 60 Ca [10].…”

β -decay properties of neutron-rich Ca, Ti, and Cr isotopes

“…Indeed, the spectroscopy of high spin states in Ti isotopes suggests that these orbitals are practically degenerate [8]. With increasing neutron number in the vicinity of N = 40 a * Electronic address: p.sarriguren@csic.es new island of inversion is predicted and deformation sets in [9][10][11][12].…”