Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 48 (2015)

Direct mass measurements of neutron-rich Ca isotopes beyond
N = 34
M. Kobayashi,∗1 S. Michimasa,∗1 Y. Kiyokawa,∗1 H. Baba,∗2 G.P.A. Berg,∗3 M. Dozono,∗1 N. Fukuda,∗2T. Furuno,∗4 E. Ideguchi,∗5 N. Inabe,∗2 T. Kawabata,∗4 S. Kawase,∗1 K. Kisamori,∗1,∗2 K. Kobayashi,∗6
T. Kubo,∗2 Y. Kubota,∗1,∗2 C.S. Lee,∗1,∗2 M. Matsushita,∗1 H. Miya,∗1 A. Mizukami,∗7 H. Nagakura,∗6D. Nishimura,∗7 H. Oikawa,∗7 S. Ota,∗1 H. Sakai,∗2 S. Shimoura,∗1 A. Stolz,∗8 H. Suzuki,∗2 M. Takaki,∗1H. Takeda,∗2 S. Takeuchi,∗2 H. Tokieda,∗1 T. Uesaka,∗2 K. Yako,∗1 Y. Yamaguchi,∗6 Y. Yanagisawa,∗2R. Yokoyama,∗1 and K. Yoshida∗2
The shell evolution in nuclei far from stability is oneof the main subjects of nuclear physics. Nuclear mass
is one of the most fundamental quantity providing information on the shell structure. The neutron numbersof 32 and 34 have been suggested to be candidates of
new magic numbers in the Ca isotopes1) . Recently
the masses of 53 Ca and 54 Ca were measured, and theshell closure at N = 32 was established2) . The present
work aims at studying the nuclear shell evolution at
N = 32, 34 by direct mass measurements of neutronrich nuclei in the vicinity of 54 Ca.
The experiment was performed at the RIKEN RI
Beam Factory (RIBF). The masses were measured directly by the TOF-Bρ method. Neutron-rich isotopes
were produced by fragmentation of a 70 Zn primarybeam at 345 MeV/nucleon in a 9 Be target. The fragments were separated by BigRIPS, and transported inthe High Resolution Beam Line to the SHARAQ spectrometer. The beam line and SHARAQ were operated
in the dispersion matching mode allowing a momentum resolution of 1/14700.

Fig. 1. Schematic view of the beamline and the detectors
used in the experiment.
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8Center for Nuclear Study, University of Tokyo
RIKEN Nishina Center
Department of Physics, University of Notre Dame
Department of Physics, Kyoto University
RCNP, Osaka University
Department of Physics, Rikkyo University
Department of Physics, Tokyo University of Science
NSCL, Michigan State University

A schematic view of the beamline with the detectors used in the experiment is shown in Fig. 1. The
TOF was measured with newly developed diamonddetectors3) installed at BigRIPS-F3 and the ﬁnal focal plane of SHARAQ (S2). The ﬂight path length
between F3 and S2 is 105 m along the central ray.
We installed two low pressure multi-wire drift chambers (LP-MWDCs)4) at both F3 and S2 to correct theﬂight pass lengths using the tracking information on
an event-by-event basis. The Bρ value was measured
by a parallel plate avalanche counter (PPAC) locatedat the target position of SHARAQ (S0). At S2, wemounted two silicon strip detectors for identiﬁcationof the atomic numbers of the fragments. To identify
the isomers, which leads to a systematic shift towards
higher masses, we placed a plastic stopper downstreamof S2 and a γ-detector array consisting of 2 Ge cloverand 16 NaI(Tl) detectors. Details of this system canbe found in Ref.5) .
Figure 2 shows the preliminary particle identiﬁcation of the secondary beams. The total yield of 55 Ca
was on the order of several thousands. Many species ofreference nuclei over a broad range of A and Z were observed, which were used in the mass calibration. Further analysis is in progress.

Fig. 2. Particle identiﬁcation of the secondary beams.

References
1) T. Otsuka et al.: Phys. Rev. Lett. 87 082502 (2001).
2) F. Wienholtz et al.: Nature 498 349 (2013).
3) S. Michimasa et al.: Nucl. Instr. Meth. B 317 305 (2013).
4) H. Miya et al.: Nucl. Instr. Meth. B 317 317 (2013).
5) Y. Kiyokawa et al.: in this report.

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