RIKEN Accel. Prog. Rep. 47 (2014)

HIGHLIGHTS OF THE YEAR

Production rates of new neutron-rich rare-earth nuclei via in-flight fission of a345 MeV/nucleon 238U beam
D. Kameda,*1 T. Kubo, *1 N. Fukuda,*1 H. Takeda,*1 H. Suzuki,*1 K. Yoshida,*1 K. Kusaka,*1 K. Tanaka,*1 Y. Yanagisawa,*1M. Ohtake,*1 N. Inabe,*1 H. Sato,*1 Y. Shimizu,*1 H. Baba,*1 M. Kurokawa,*1 D. Nishimura,*1,*2 T. Ohnishi,*1 N. Iwasa,*1,*3A. Chiba,*1,*3 T. Yamada,*1,*3 E. Ideguchi,*4 S. Go,*1,*5 R. Yokoyama,*5 T. Fujii,*5 H. Nishibata,*1,*6 K. Ieki,*1,*7 D. Murai,*1,*7S. Momota,*1,*8 Y. Satou,*9 J. W. Hwang,*1,*9 S. Kim,*1,*9 O. B. Tarasov,*1,*10 D. J. Morrissey,*1,*10B. M. Sherrill,*1,*10 and G. Simpson*11The reaction mechanism of in-flight fission is not clear
yet due to the complexity of the abrasion fission process, inwhich many kinds of fissile nuclei can contribute to theproduction of fission fragments. The measurement of theproduction rates of various fission fragments is useful not
only to plan various experiments but also to understandsuch a reaction mechanism. In 2011, using a 345
MeV/nucleon 238U beam with a Be target, we searched for
new isotopes and isomers whose atomic numbers roughly
range from 56 to 68.1) Here, we report on the production
rates of new isotopes, in addition to the improvements inparticle identification (PID) compared to our previous
report.2)
Fission fragments were separated and analyzed using theBigRIPS separator. We adopted two settings that targeted
the regions of nuclei around 159Pr and 168Gd. The
mass-to-charge ratio (A/Q) was deduced from the time offlight (TOF) and magnetic rigidity measurements obtainedusing BigRIPS. The atomic number (Z) was deduced fromthe TOF and energy losses, which were measured using a
stack of Si detectors in the focal plane. The PID plot in theregion of 159Pr is presented in Fig. 1. Since our previous
report,2) we have modified the gate conditions to remove
background events and improved the resolutions of A/Q andZ by carrying out detailed analysis. As a result, in the twosettings, we identified a total of 26 new isotopes: 153Ba,154,155,156
La, 156,157158Ce, 156,157,158,159,160Pr, 162,163Nd, 164,165Pm,
166,167
Sm, 169Eu, 171Gd, 173,174Tb, 175,176Dy, 177Ho, and 179Er.
The production rates of fully stripped fragments arepresented in Fig. 2 along with the LISE++ calculations.3)
Here, for the LISE++ abrasion fission (AF) model, weadopted the same parameters as those used in the previousexperiment.4) This AF model reproduced the production
rates in the region of Z = 20 to 49 fairly well.4,5) In thepresent region of Z = 56 to 65, however, the calculations areorders of magnitudes smaller than the measured rates. Thisdifference might be due to the AF model parameters, whichwere optimized to reproduce limited cross-section data for Z= 20 to 46.3) Further systematic studies are now in progress.

Fig. 1. Z versus A/Q PID plot in the region of 159Pr. The redlines indicate the known limits of neutron-rich isotopes.

Fig. 2. Measured production rates of (a) even-Z and (b)odd-Z isotopes in the region of 159Pr. The red arrows
indicate the production rates of new isotopes. The LISE++
calculations are denoted by the dashed or solid curves
without circles (see text).

RIKEN Nishina Center
Department of Physics, Tokyo University of Science＊3
Department of Physics, Tohoku University＊4
Research Center for Nuclear Physics, Osaka University＊5
Center for Nuclear Study, University of Tokyo＊6
Department of Physics, Osaka University＊7
Department of Physics, Rikkyo University＊8
School of Environmental Science and Engineering, Kochi Universityof Technology＊9Department of Physics and Astronomy, Seoul National University＊10
National Superconducting Cyclotron Laboratory, Michigan StateUniversity＊11LPSC-IN2P3＊1＊2

References

1) D. Kameda et al.: RIKEN Accel. Prog. Rep. 45, 117 (2012).
2) D. Kameda et al.: RIKEN Accel. Prog. Rep. 46, 20 (2013).
3) O. B. Tarasov and D. Bazin: Nucl. Instr. Meth. B 266, 4657
(2008) and references therein.
4) T. Ohnishi et al.: J. Phys. Soc. Jpn. 79, 073201 (2010).
5) H. Suzuki et al.: Nucl. Instr. Meth. B 317, 756 (2013).

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