Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Decay properties of 68,69,70Mn†
G. Benzoni,∗1 A.I. Morales,∗1,∗2 H. Watanabe,∗3,∗4 L. Coraggio,∗5 A. Bracco,∗1,∗2 P. Doornenbal,∗3G. Lorusso,∗3 S. Nishimura,∗3 H. Sakurai,∗3 P.A. Söderström,∗3 T. Sumikama,∗6 Z.Y. Xu,∗7for the RIBF80 and the EURICA collaborations.
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In recent years the evolution of magic numbers moving away from the valley of stability has been a hottopic both from experimental and theoretical points ofview. In this context, one region in the chart of nuclides that is attracting particular attention lies around78
Ni: this is a key region to study the path toward theN=50 shell closure and its implications on the astrophysical r-process. In particular, the study of the evolution of excited states for the Cr, Fe, Zn, Ge isotopes
provides a stringent test for shell model calculations
leading to N=50.
The experiment described here was performed at
RIKEN in May 2013, as part of the EURICA campaign
at the Radioactive-Isotope Beam Factory (RIBF) facility . The nuclear species were produced by meansof in-flight fission of a 238 U beam at a bombarding energy of 345 MeV/nucleon. The experiment collected
data for an equivalent time of 3 days with an average primary beam intensity of 10 pnA. The resulting
fragments were transported and separated in the BigRIPS separator and Zero-Degree spectrometer down
to the final focal plane. They were implanted in the5 silicon detectors of the WAS3ABi array1) which wassurrounded by the EURICA spectrometer2) coupled to18 small volume LaBr3 (Ce) scintillator detectors, forfast-timing measurements3) . The yields for the mothernuclei, after implantation, were: 6700 68 Mn ions, 4300
69Mn ions and 400 70 Mn ions.
The experimental data confirm the β-decay spectrum of 68 Mn reported previously in Ref.4) even if we
see different relative population of the transitions at
1250 keV and 1514 keV. In addition, γ-ray energies forthe decay of 69 Fe and 70 Fe are extracted for the firsttime. For the odd isotope, the limited statistics do notallow the extraction of γ − γ coincidences. Its decay
spectrum shows a substantial contribution from theβ-delayed neutron channel, which preferentially seems
to populate specific low-J states. By systematics andsupported by recent shell-model calculations, we assign the γ rays seen in the spectrum of 70 Fe to the+depopulation of the 2+
1 and 41 levels.
The proposed level schemes following the β decay of†
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FeFig. 1. Partial level schemes following the β decays 68 Mn→ 68 Fe (left) and 70 Mn → 70 Fe (right).

even-mass isotopes are shown in Fig. 1. The comparison of the decay patterns of 68 Mn → 68 Fe and 70 Mn→ 70 Fe shows a sudden change: in the case of 70 Mn→ 70 Fe, the feeding seems to preferably go to the proposed 4+ state, while the 2+ state is mainly fed by internal decay coming from the higher-lying states. The
sudden change of the spin population in the daughter
nuclei indicates different spin of the ground state of
the mother nuclei.
The experimental results have been compared to ashell-model calculation performed with the CD-Bonn
NN potential in the Vlow−k approach5) extended to68,70
Fe isotopes. It is found that the experimental
R4/2 ratio is properly reproduced by the calculations
only with the inclusion of the 1d5/2 neutron orbital inthe valence space. This is interpreted, as for Cr isotopes, in terms of the interplay between the quadrupole
correlations of the ν1d5/2 and ν0g9/2 orbitals and themonopole component of the π0f7/2 - ν0f5/2 interaction,
thus driving the deformation in the neutron-rich Cr-Feregion. Since the maximum of quadrupole deformation
has not been reached yet, investigating heavier Fe isotopes is of foremost interest to assess the robustness of
the N=50 shell closure below 78 Ni.

Condensed from the article in PLB, Vol.751, 107 (2015)
INFN sezione di Milano, MilanoDipartimento di Fisica, Universitá degli Studi di Milano,Milano
RIKEN Nishina Center, Riken
IRCNPC, School of Physics and Nuclear Engineering, Beihang University, Beijing
INFN sezione di Napoli, Napoli
Department of Physics, Tohoku University, Sendai
Department of Physics, University of Tokyo, Tokyo

References
1) S. Nishimura, Prog. Theor. Exp. Phys. 2012 (2012)
03C006.
2) P.A. Söderström, Nucl. Instr. Meth. B317 (2013) 649.
3) Z. Patel et al., RIKEN Accel. Prog. Rep. 47 (2014) 13.
4) S.N. Liddick et al., Phys. Rev. C87 (2013) 014325.
5) L. Coraggio et al., Phys. Rev. C89 (2014) 024319.

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