Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 49 (2016)

In-beam γ-ray spectroscopy of 55 Sc
D. Steppenbeck,∗1 S. Takeuchi,∗2 N. Aoi,∗1,∗3 P. Doornenbal,∗1 M. Matsushita,∗4 H. Wang,∗1 H. Baba,∗1S. Go,∗5 J. Lee,∗1,∗6 K. Matsui,∗7 S. Michimasa,∗4 T. Motobayashi,∗1 D. Nishimura,∗1,∗8 H. Sakurai,∗1,∗7Y. Shiga,∗1,∗9 P.-A. Söderström,∗1 T. Sumikama,∗1 R. Taniuchi,∗1,∗7 J. J. Valiente-Dobón,∗10 and K. Yoneda∗1The study of the evolution of shell structure has
played an important role in the fields of experimental and theoretical nuclear physics over recent decades.
On the experimental front, significant advances havebeen made owing to progress in the production rates
of exotic nuclei at radioactive isotope beam facilities
worldwide. A few noteworthy examples of shell evolution include the onset of the neutron magic number
N = 16 in exotic oxygen1,2) , and the weakening of
the traditional magic number N = 28 approaching theneutron drip line3) . Moreover, in the pf shell, the onsetof a new subshell closure at N = 32 has been reported
along the Ca4,5) , Ti6,7) , and Cr8,9) isotopic chains owing to the migration of the νf5/2 orbital as protons areremoved from the πf7/2 state10) . More recently, therobustness of the N = 32 subshell closure below theZ = 20 core, namely, in K11) and Ar12) isotopes, hasbeen highlighted. An additional subshell closure waspredicted13) to occur at N = 34 in Ti and Ca isotopes;
however, the experimental data provided no evidence
for this shell gap in 56 Ti7,14) . Later work on the spectroscopy of 54 Ca did, however, indicate the presenceof a sizable subshell closure at N = 34 from the energy of the first 2+ level15) . The present work on 55 Sc
aims at the study of the significance of the N = 34
gap along the Z = 21 isotopic chain, and the evolutionof this neutron magic number as protons are added tothe πf7/2 orbital.
The experiment was performed using 70 Zn30+ ionswith the BigRIPS separator to provide a fast radioactive beam—optimized for the transmission of 55 Sc—
that was focused on a 10-mm-thick 9 Be reaction target at the eighth focal plane of the spectrometer. Thebeam constituents were identified on an event-by-event
basis using particle magnetic rigidities (Bρ), times offlight (T ), and energy losses in an ionization chamber (∆E)16) . The target at F8 was surrounded by
the DALI2 γ-ray detector array to measure transitions
from nuclear excited states populated by the reactions;
ions emerging from the target position were identified
by the ZeroDegree spectrometer using the same general (Bρ–T –∆E) techniques as for BigRIPS.

103

Be(55Sc, 55Sc+γ )XCounts / 25 keV

9102

RlBFpreliminary
1005001000

1500

20002500

3000

Transition energy (keV)

Fig. 1. (colour) Doppler-corrected γ-ray energy spectrum
deduced from the inelastic scattering of 55 Sc on a 9 Be
target. The black dashed line is a double exponential
fit to the background and the blue dashed lines are theresults of GEANT4 simulations; the solid red line is thetotal (sum) fit. Results are preliminary.

The γ-ray energy spectrum deduced from theBe(55 Sc, 55 Sc+γ)X inelastic scattering reaction ispresented in Fig. 1. This work was previously reported,amongst other results, in Ref.17) . No results on excitedstates in 55 Sc were available prior to the present experiment. From the spectrum in Fig. 1, we report newtransitions in 55 Sc at energies of 0.71(1) and 1.54(2)MeV. Additional results on the low-lying structure of55
Sc will be reported elsewhere.
9References
1) R. Kanungo et al., Phys. Rev. Lett. 102, 152501 (2009).
2) C. R. Hoffman et al., Phys. Lett. B 672, 17 (2009).
3) B. Bastin et al., Phys. Rev. Lett. 99, 022503 (2007).
4) A. Gade et al., Phys. Rev. C 74, 021302(R) (2006).
5) F. Wienholtz et al., Nature 498, 346 (2013).
6) R. V. F. Janssens et al., Phys. Lett. B 546, 55 (2002).
7) D.-C. Dinca et al., Phys. Rev. C 71, 041302(R) (2005).
8) J. I. Prisciandaro et al., Phys. Lett. B 510, 17 (2001).
9) A. Bürger et al., Phys. Lett. B 622, 29 (2005).
10) T. Otsuka et al., Phys. Rev. Lett. 95, 232502 (2005).
11) M. Rosenbusch et al., Phys. Rev. Lett. 114, 202501(2015).
12) D. Steppenbeck et al., Phys. Rev. Lett. 114, 252501(2015).
13) T. Otsuka et al., Phys. Rev. Lett. 87, 082502 (2001).
14) S. N. Liddick et al., Phys. Rev. Lett. 92, 072502 (2004).
15) D. Steppenbeck et al., Nature 502, 207 (2013).
16) T. Kubo et al., Nucl. Instrum. Meth. B 204, 97 (2003).
17) D. Steppenbeck et al., JPS Conf. Proc. 6, 020019 (2015).

RIKEN Nishina Center
Dept of Physics, Tokyo Institute of Technology∗3 RCNP, Osaka University∗4 Center for Nuclear Study, University of Tokyo∗5 Dept of Science and Engineering, University of Tennessee∗6 Dept of Physics, University of Hong Kong
∗7 Dept of Physics, University of Tokyo∗8 Dept of Physics, Tokyo University of Science∗9 Dept of Physics, Rikkyo University∗10 Laboratori Nazionali di Legnaro∗1∗2- 41 -