Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 49 (2016)

Time-of-Flight 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∗2achieved for Z ∼ 20 nuclei. The total yield of 55 Ca,
which is one of the target nuclei, is several thousands.
It is expected that the mass of 55 Ca can be deducedwith the precision of ∼3 × 10−6 .

The nuclear mass is one of the fundamental quantities used in investigating nuclear structure propertiessuch as shell closures and new magic numbers. The
neutron numbers of 32 and 34 have been theoreticallysuggested to be candidates of new magic numbers in
the Ca isotopes1) . Recently, the prominent shell closure at N = 32 was established by measurements ofthe massses of 53 Ca and 54 Ca2) . Moreover, the exci54
tation energy of the 2+
Ca was reported3) ,1 state inand the result suggests the existence of an N = 34 shellclosure in 54 Ca. For establishment of the closed-shellnature of 54 Ca, mass measurements of the Ca isotopes
beyond N = 34 are essential. The present work aims
at studying the nuclear shell evolution at N = 32 and34 by direct mass measurements of neutron-rich nuclei
in the vicinity of 54 Ca.
We performed the nuclear mass measurement at theRIKEN RI Beam Factory using the SHARAQ spectrometer. The masses were measured directly by theTOF-Bρ technique. The experimental setup is presented in detail by Kobayashi et al.4) .
Here, current status of the data analysis is described.
We analyzed the data taken by diamond detectors,
which were employed for timing measurement. Timeresolution was estimated to be 12 ps (σ), which is amuch better value than that in the previous measurement using 12 N beams at 320 AMeV5) . This improvement of the time resolution can be interpreted in termsof the energy deposit in the diamond detector, whichwas ∼60–100 MeV (∼10 MeV) in the present (previous) experiment.
The mass to charge ratio (A/Q) was reconstructedby the Bρ value from the beam tracking with up tofourth-order aberrations, and calibrated by using thereference nuclei such as 53–54 Ca, 49,51–52 K, 46–47 Ar,
43–46
Cl, 41–42 S, 39–42 P, and 38 Si. Figure 1 shows an
A/Q spectrum for one of the reference isotopes, 54 Ca.
The relative mass resolution of ∼1/8000 (σ) has been
∗1∗2
∗3
∗4
∗5
∗6
∗7
∗8Fig. 1. A/Q spectrum for a reference nucleus, 54 Ca.

At present, energy loss in a PPAC, which was usedfor the Bρ measurement, has not been considered inthe mass calibration. It will be included in the ﬁtting to reduce the residuals of the calibration. Further
analysis is in progress.
References1) T. Otsuka et al.: Phys. Rev. Lett. 87, 082502 (2001).
2) F. Wienholtz et al.: Nature 498, 349 (2013).
3) D. Steppenbeck et al.: Nature 502, 207 (2013).
4) M. Kobayashi et al.: RIKEN Accel. Prog. Rep. 48, 59(2015).
5) S. Michimasa et al.: Nucl. Instr. Meth. B 317, 305(2013).

Center for Nuclear Study, University of Tokyo
RIKEN Nishina Center
JINA and the 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- 48 -