Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 50 (2017)

Mass measurements of neutron-rich isotopes in the vicinity of 54 Ca
using the TOF–Bρ technique
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,∗6
D. 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,∗2
R. Yokoyama,∗1 and K. Yoshida∗2

∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8m/q(exp) - m/q(ref) (keV/q)

The mass of an atomic nucleus is a fundamental
quantity because it reﬂects the sum of all interactions
within the nucleus. Mass measurements can directlyprobe changes in the shell structure in nuclei far fromstability, which is called “shell evolution.” The presence of subshell gaps at N = 32 and 34 around calcium
isotopes has attracted much attention over recent years(for example, see Refs. 1–3). To investigate the shellevolution at N = 32 and 34, mass measurements ofneutron-rich nuclei in the vicinity of 54 Ca were performed at the RIKEN RI Beam Factory.
The masses were measured using the TOF–Bρ technique. The time-of-ﬂight (TOF) of an ion was measured by a pair of diamond detectors with extremely
high time resolutions. The dispersion-matched operation of the High-Resolution Beam Line and theSHARAQ spectrometer allowed high-precision measurements of the beam momenta. Details of the experimental setup are described in Ref. 4.
The reference nuclei with known masses used formass calibration were 52–54 Ca, 49,51–53 K, 46–48 Ar,
43–46
Cl, 41,42 S, 38–42 P, and 36–40 Si. These nuclides
have mass precisions in the literature of < 320 keV
and do not have known long-lived (T1/2 > 100 ns)
isomeric states. Masses were deduced by ﬁtting witha fourth-order calibration function in TOF, the positions and angles at the focal planes F3 and S2, and thehorizontal position at the dispersive focus S0, which isrelated to Bρ. Figure 1 shows the diﬀerences between
the deduced m/q values and the values reported in theliterature for the reference nuclei. Using the standard
deviation of the data points, the systematic error in themass determination was estimated to be 6.1 keV/q.
The deduced A/Q spectrum is presented in Fig. 2.
In the present experiment, the achieved mass resolution was typically ∼ 1.0 × 10−4 (σ). We measured
21 nuclides with unknown masses: 62–64 V, 58–62 Ti,

40

Preliminary30

40204452

Ca10Ar

39P46SS 53
Ca49

K46

Ar51

ArK53

K41

PPPSi

52

4038

4239K

47

-10Cl

42410 43Cl

Si48

ClP

38

54

SiCa

36

Si

37

Si

-2045

Cl-30
-40
2.5

2.55

2.6

2.65

2.7

2.75

2.8

2.85

2.9
A/Q

Fig. 1. Diﬀerences between the deduced and reference m/q
values as a function of A/Q. The blue band indicatesthe region within the systematic error. Error bars represent the uncertainties in the reference values.

Sc, 55–57 Ca, 54 K, 50–52 Ar, 48,49 Cl, and 46 S. The
mass precisions for 55,56 Ca were evaluated to be lessthan 300 keV. Discussion regarding the deduced massvalues is in progress.
Counts (a.u.)

58–60

45

Preliminary

Cl

104

4448

Ar

42

Cl

S

47

Ar

4651

Cl

K

50

K

43

S
49

Ar
41
P
Sc

40

P
Sc

103

46

53

39

41

S

49

K

Ar

Ca

P

59

Ti

54

60

Ti

Ca

36

Si

102

52

Ca

38

P

P

Ar

45

S

Sc

53

55

Ca

58

4250
58

Si

Si

Sc

Cl

38

3755

47

K
VV

Cl

S

52

63

62
43

4457

56

Ti

6139SiTi

48

KCl43

P

40

Si5961

V56

Sc46

Ca64

102.5

VS51

Ti Ar

6249

54

48

K

2.55

2.6

2.65

2.7

2.75

2.8

ClK

2.85

2.9
A/Q

Fig. 2. A/Q spectrum deduced from the present experiment. Nuclides whose masses have not been measured
previously are indicated with red letters.

Center for Nuclear Study, University of Tokyo
RIKEN Nishina Center
JINA-CEE and Department of Physics, University of Notre
Dame
Department of Physics, Kyoto University
Research Center for Nuclear Physics, Osaka University
Department of Physics, Rikkyo University
Department of Physics, Tokyo University of Science
National Superconducting Cyclotron Laboratory, Michigan
State University

References
1) 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).

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