II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Evolution of collectivity in Ti isotopes towards the N = 40 island ofinversion
T. Koiwai∗1,∗2 and K. Wimmer∗3,∗2 for the RIBF142R1 Collaboration
Over the past decade, signiﬁcant eﬀorts on both theexperimental and theoretical fronts have focused on
the development of nuclear collectivity in exotic isotopes around N = 40. It was revealed that the newisland of inversion, where the intruder conﬁguration isstrongly favored in the ground state, at N = 40 is centered around 64 Cr.1,2) It is then interesting to question
whether this enhanced collectivity established in theregion around the Fe and Cr isotopes presents itself in
exotic systems with fewer protons, namely, Ti isotopes.
Several theoretical calculations2–6) show the diﬀerencein the reduced transition rates, B(E2; 0+ → 2+ ) values, for neutron-rich Ti isotopes beyond 56 Ti when
considering diﬀerent model spaces. Although the energies for the ﬁrst 2+ state are consistent and agree withexperimental data, the B(E2) values diﬀer, dependingon whether the gd shell is included in the calculations.
This suggests that the boundary of the island of inversion is located between 56 Ti and 58 Ti. To answer
these questions, the Coulomb excitation of 56, 58 Ti wasmeasured.
The experiment was conducted at the RIBF as apart of the HiCARI campaign. A 70 Zn primary beamwith a maximum intensity of 600 particle nA was accelerated to 345 MeV/nucleon and impinged on a 11mm-thick 9 Be primary target to produce a secondary
beam for 56, 58 Ti. The beam fragments were separated and identiﬁed by the BigRIPS using the standard∆E−TOF−Bρ method. The secondary beam impinged on a 1-mm-thick Au target to induce a Coulomb
excitation reaction. A 3-mm-thick Be target was also
used to determine the nuclear contribution to the excitation. The reaction residuals were identiﬁed by theZeroDegree spectrometer. De-excitation γ rays weredetected by the HiCARI array7) at the F8 focal plane.
The preliminary particle identiﬁcation plots are shownin Fig. 1. Tails will be removed by further analysis.
Currently, the data are under analysis. The preliminary Doppler-corrected energy spectra show the clear
transition peaks at known energies of the ﬁrst 2+ statein both 56, 58 Ti. Moreover, candidates of peaks arefound in coincidence with the one-neutron knockout
channel from 56, 58 Ti, which will reveal the conﬁguration of the ground-state wave function of these nuclei.

∗1
∗2
∗3

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19Fig. 1. Particle identification plots for both the incoming
beam and outgoing residuals. The top panel shows the56
Ti setting and the bottom the 58 Ti setting.

References

1) H. L. Crawford et al., Phys. Rev. Lett. 110, 242701(2013).
2) S. M. Lenzi, F. Nowacki, A. Poves, K. Sieja, Phys. Rev.
C 82, 054301 (2010).
3) A. Poves, J. Sánchez-Solano, E. Caurier, F. Nowacki,Nucl. Phys. A 694, 157 (2001).
4) M. Honma, T. Otsuka, B. A. Brown, T. Mizusaki, Eur.
Phys. J. A 25, s01, 499 (2005).
5) Y. Tsunoda et al., Phys. Rev. C 89, 031301(R) (2014).
6) L. Coraggio, A. Covello, A. Gargano, N. Itaco, Phys.
Rev. C 89, 024319 (2014).
7) K. Wimmer et al., in this report.

Department of Physics, University of Tokyo
RIKEN Nishina Center
IEM-CSIC-5-1