Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 51 (2018)

Identification of New Neutron-rich Isotopes in the Rare-EarthRegion Produced by 345 MeV/nucleon 238 U†
N. Fukuda,∗1 T. Kubo,∗1,∗11 D. Kameda,∗1 N. Inabe,∗1 H. Suzuki,∗1 Y. Shimizu,∗1 H. Takeda,∗1 K. Kusaka,∗1Y. Yanagisawa,∗1 M. Ohtake,∗1 K. Tanaka,∗1 K. Yoshida,∗1 H. Sato,∗1 H. Baba,∗1 M. Kurokawa,∗1 T. Ohnishi,∗1N. Iwasa,∗2 A. Chiba,∗2 T. Yamada,∗2 E. Ideguchi,∗3 S. Go,∗3 R. Yokoyama,∗3 T. Fujii,∗3 H. Nishibata,∗4K. Ieki,∗5 D. Murai,∗5 S. Momota,∗6 D. Nishimura,∗7 Y. Sato,∗8 J. Hwang,∗8 S. Kim,∗8 O. B. Tarasov,∗9D. J. Morrissey,∗9 and G. Simpson∗1068

Searches for new isotopes at the RIKEN RI Beam
Factory1) have been conducted since its commissioning in 2007, expanding the region of accessible nuclei in the nuclear chart.2–6) To explore the uncharted region of far-from-stability nuclei, we performed a search for new isotopes in the neutronrich rare-earth region using a 345 MeV/nucleon
238
U beam with the BigRIPS in-ﬂight separator.7) We observed a total of 29 new neutron153
rich isotopes:
Ba, 154,155,156 La, 156,157,158 Ce,
156,157,158,159,160,161
Pr, 162,163 Nd, 164,165 Pm, 166,167 Sm,169
171
173,174
Eu,Gd,Tb, 175,176 Dy, 177,178 Ho, and
179,180
Er.
Neutron-rich nuclei in the rare-earth region wereproduced by the in-ﬂight ﬁssion of a 345 MeV/nucleon
238
U beam with a beryllium target. The intensity of
the 238 U beam was approximately 0.2–0.3 particle nA.
The experiment was carried out with two diﬀerent settings of the BigRIPS separator, each of which wasaimed at the discovery of new neutron-rich isotopes
in the region of atomic number Z of around 59 and64, which are referred to as the Pr and Gd settings,respectively. The target thicknesses were 4.0 mm and4.9 mm for the Pr and Gd settings, respectively. The
particle identiﬁcation (PID) was performed event-byevent based on the ∆E-TOF-Bρ method, deducing Z
and the mass-to-charge ratio A/Q of the fragments.8)
Figures 1(a) and 1(b) show the Z vs A/Q PID plotsobtained with the Pr and Gd settings, respectively.
New isotopes are clearly identiﬁed in the plots, thanks
to the excellent relative A/Q resolutions of 0.034% and0.036% for the Pr and Gd settings, respectively.

(a)

66
64

104103

Z

62102

60
58

1056
54

2.65

A/ Q

2.7

70

2.75

(b)

1103

68
66
102

Z

64
62
60

10585654

2.65

A/ Q

2.7

2.75

1Fig. 1. Particle identification plots of Z vs A/Q for thefragments produced in the 238 U + Be reaction at345 MeV/nucleon: (a) data obtained with the Pr setting during a 54.3-hour measurement period and (b)data obtained with the Er setting during a 45.8-hourmeasurement period. The red lines indicate the knownlimit. The new isotopes observed in this work are indicated by the red solid circles.

†Condensed from the article in J. Phys. Soc. Jpn. 87, 014202(2018)∗1 RIKEN Nishina Center∗2 Department of Physics, Tohoku University∗3 Center for Nuclear Study, University of Tokyo∗4 Department of Physics, Osaka University∗5 Department of Physics, Rikkyo University∗6 School of Environmental Science and Engineering, Kochi
University of Technology∗7 Department of Physics, Faculty of Liberal Arts and Sciences,
Tokyo City University∗8 Department of Physics and Astronomy, Seoul National University∗9 National Superconducting Cyclotron Laboratory, Michigan
State University∗10 Laboratoire de Physique Subatomique et de Cosmologie deGrenoble∗11 Present address: FRIB/NSCL, Michigan State University

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