Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 53

Isomeric-to-ground state ratio of 128 Sn measured by Rare RI Ring
H. F. Li,∗1,∗2,∗3 S. Naimi,∗1 D. Nagae,∗1 Y. Abe,∗1 F. Suzaki,∗1 Y. Yamaguchi,∗1 M. Wakasugi,∗1 S. Omika,∗4K. Inomata,∗4 H. Arakawa,∗4 S. Hosoi,∗4 K. Nishimuro,∗4 Y. Inada,∗4 T. Kobayashi,∗4 D. Kajiki,∗4
D. Hamakawa,∗4 W. B. Dou,∗4 T. Yamaguchi,∗4 M. Mukai,∗5 T. Moriguchi,∗5 R. Kagesawa,∗5 D. Kamioka,∗5A. Ozawa,∗5 S. Suzuki,∗2 Z. Ge,∗2 C. Y. Fu,∗2 Q. Wang,∗2 M. Wang,∗2 S. Ota,∗6 S. Michimasa,∗6N. Kitamura,∗6 S. Masuoka,∗6 D. S. Ahn,∗1 H. Suzuki,∗1 N. Fukuda,∗1 H. Takeda,∗1 Y. Shimizu,∗1Y. A. Litvinov,∗7 G. Lorusso,∗8 and T. Uesaka∗1

wherec T1 and T0 are the time-of-flight (TOF) of the nucleus of interest and reference nucleus, respectively, andBρ1 is the magnet rigidity of the nucleus of interest.1)
The unknown mass m1 is determined relative to the massof the isochronous reference nucleus m0 . Bρ tagging isperformed at the momentum-dispersive focal plane F5 ofBigRIPS by measuring the horizontal position with twoparallel-plate avalanche counters (PPACs). The TOF ofthe nuclei in R3 was measured using the E-MCP detector2) at S0 of SHARAQ and a plastic scintillator placed
698220
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18698200

Amp 1

17.21  2.55

16

Mean 1693707.81  0.34

14Sigma1

2.34  0.30

Amp 2

20.79  2.28

Mean 2693720.06  0.33

Sigma23.55  0.30

T1 (ns)2 / ndfcount698180
12693740
10(ns)

260

260

69821020698190(b)

8693730

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69372046937102693700
0693680 693700
1210693720
8693740 693760 693780
64
20corr

T1F5x(ns)(mm)

6938002Fig. 1. (a), (b) Correlations between T1 and Tcorr, respec1
tively, and the F5 position for 128 Sn.
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗82012818m

Sn161412128

Sn

1086

Entries260

2 / ndf

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Amp 1

17.21  2.55

Mean 1693707.81  0.34

Sigma12.34  0.30

Amp 220.79  2.27

Mean 2693720.06  0.33

Sigma2

3.55  0.30

420693680

693700693720693740corr

T1693780

693800Fig. 2. Tcorr
spectrum of 128 Sn; two Gaussian functions were1
used to fit the histogram.

behind R3.
In the autumn of 2018, the first mass measurement
campaign was conducted at R3. To measure the mass of125
Ag and 124 Pd, 3 neighbor isotones, 126 Cd, 127 In, and128
Sn, were injected in R3 as reference nuclei to determine the mean Bρ value of R3.3) The first isomeric stateof 128 Sn, the excitation energy and half-life of which areabout 2091.5 keV and 6.5 s,5) respectively, was produced
and observed during this experiment. The TOF in R3
for each particle was normalized to the same turn numbers4) to determine T1 . The correlation between the T1of 128 Sn and the F5 position is shown in Fig. 1(a). Af’s dependence
ter event-wise correction with Bρ, Tcorr
1on the F5 position is shown in Fig. 1(b). The isomericstate and ground state of 128 Sn can be well resolved inthe spectrum of Tcorr, as shown in Fig. 2. The left peak1is the ground state of 128 Sn, and the right one is theisomeric state. A function composed of the sum of two
Gaussian functions was used to fit this spectrum.
The mass resolving power of R3 can be given by(2)

which is derived from Eq. (1). The full width at half
maximum of the achieved mass resolving power is about
125,000 from the ground-state peak. The isomeric-toground state ratio for 128 Sn is 1.8(4), which is determined by the integral values’ ratio of the fitting functions.
ReferencesRIKEN Nishina Center
Institute of Modern Physics, Chinese Academy of Sciences
University of Chinese Academy of Sciences
Department of Physics, Saitama University
Institute of Physics, University of Tsukuba
Center for Nuclear Study, University of Tokyo
GSI Helmholtzzentrum für Schwerionenforschung
NPL, University of Surrey

693760(ns)

R = m/∆m = T1corr /∆T1corr ,6T1corr

Entries

22count

The excitation energy of the isomeric state and theisomeric-to-ground state ratios are very important tounderstand the nuclear structure and reactions. Direct
mass measurement can be used for measuring the excitation energy of the long-lived isomeric state and determining the isomeric-to-ground state ratio simultaneously. Rare RI Ring (R3) is an isochronous mass spectrometer in RIBF. The principle of the mass measurement at R3 is described by the following equation:
�( )2
�
�
T0
1−corr�T1
m0 T1
m0 1 �m1==T1 � 1 + ()2 , (1)q1
q 0 T0
q0 T0
m0 /q0cBρ11) A. Ozawa et al., Prog. Theor. Exp. Phys. 2012, 03C009(2012).
2) Y. Abe et al., JPS Conf. Proc. 1, 013059 (2014).
3) S. Naimi et al., RIKEN Accel. Prog. Rep. 52, 102 (2019).
4) H. F. Li et al., RIKEN Accel. Prog. Rep. 52, 52 (2019).
5) G. Audi et al., Chin. Phys. C 41, 030001 (2017).
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