RIKEN Accel. Prog. Rep. 47 (2014)

HIGHLIGHTS OF THE YEAR

Evidence for a new nuclear ‘magic number’ in 54 Ca†
D. Steppenbeck,∗1 S. Takeuchi,∗2 N. Aoi,∗3 P. Doornenbal,∗2 M. Matsushita,∗1 H. Wang,∗2 H. Baba,∗2N. Fukuda,∗2 S. Go,∗1 M. Honma,∗4 J. Lee,∗2 K. Matsui,∗5 S. Michimasa,∗1 T. Motobayashi,∗2 D. Nishimura,∗6T. Otsuka,∗1∗5 H. Sakurai,∗2∗5 Y. Shiga,∗7 P.-A. Söderström,∗2 T. Sumikama,∗8 H. Suzuki,∗2 R. Taniuchi,∗5Y. Utsuno,∗9 J. J. Valiente-Dobón,∗10 and K. Yoneda∗2
Over recent years, the evolution of nuclear shellstructure in exotic, neutron-rich nuclei has attracted
much attention on both the experimental and theoretical fronts. In the neutron-rich f p shell, the onset of
the N = 32 subshell closure is well established fromthe structural characteristics of 52 Ca1,2) , 54 Ti3,4) and56
Cr5,6) . This subshell gap is reproduced successfully
by numerous theoretical predictions. In the frameworkof tensor-force-driven shell evolution7) , the onset of theN = 32 subshell closure results as a direct consequenceof a sizable νp3/2 –νp1/2 gap, which presents itself as
the νf5/2 orbital shifts up in energy owing to a weakening of the attractive πf7/2 –νf5/2 interaction as protons
are removed from the πf7/2 orbital. Another important manifestation of some theories is the prediction ofa large subshell gap at N = 34, which develops if theνf5/2 orbital lies sufficiently high in energy above theνp1/2 orbital. It has already been shown that no significant N = 34 subshell gap exists in 56 Ti4,8) or 58 Cr6,9)
and, therefore, the size of the energy gap in 54 Ca is
an important structural characteristic that requires experimental input. Moreover, the single-particle statesof 53 Ca should also reflect the nature of the N = 34
subshell closure in isotopes far from stability.
The structures of 54 Ca and 53 Ca were investigatedusing in-beam γ-ray spectroscopy at the RIBF to address this issue. A primary beam of 70 Zn30+ ions at 345
MeV/u was used to create a radioactive beam containing 55 Sc and 56 Ti, which was focused on a 10-mm-thickFig. 2. (colour) Doppler-corrected γ-ray energy spectra for(a) 54 Ca and (c) 53 Ca. Insets (b) and (d) indicate γ
rays in coincidence with the 2043- and 1753-keV lines.

Be reaction target located inside the DALI2 γ-ray detector array at F8. Reaction products were identified
with the ZeroDegree spectrometer (see Fig. 1).
The energy spectra for 54 Ca and 53 Ca deduced in thepresent work are presented in Fig. 2. The most intense
peak in the 54 Ca spectrum, the line at 2043(19) keV,+ground-state transition.
is assigned as the 2+
1 → 0
Several other weaker lines are also reported. The relatively high energy of the 2+
1 state reflects the doubly
magic nature of 54 Ca and provides direct experimental
evidence for the onset of a sizable subshell closure in
N = 34 isotones far from stability. Shell-model calculations adopting a modified GXPF1B Hamiltonian
indicate that the strength of the N = 34 subshell gap
in 54 Ca (the νp1/2 –νf5/2 SPO energy gap) is in factcomparable to the N = 32 subshell gap in 52 Ca (theνp3/2 –νp1/2 SPO energy gap) (see original Letter fordetails). In the 53 Ca spectrum, the 1753(15)-keV transition is reported for the first time, while the line at2227(19) keV is consistent in energy with a transitionpreviously measured in a decay study10) .

Fig. 1. (colour) Particle identification plots measured by(a) the BigRIPS separator and (b) the ZeroDegree spectrometer. The black circle indicates 54 Ca events.

References
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†Condensed from the article in Nature 502, 207 (2013).
∗1 Center for Nuclear Study, University of Tokyo.
∗2 RIKEN Nishina Center.
∗3 RCNP, University of Osaka.
∗4 Center for Mathematical Sciences, Aizu University.
∗5 Department of Physics, University of Tokyo.
∗6 Department of Physics, Tokyo University of Science.
∗7 Department of Physics, Rikkyo University.
∗8 Department of Physics, Tohoku University.
∗9 Japan Atomic Energy Agency.
∗10 Laboratori Nazionali di Legnaro.

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