RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-1. Nuclear Physics

0DVVDQGǃ-decay measurement of neutron-rich nuclei around N=56
A. Estrade,䠆1 D. S. Ahn,䠆2 M. Amthor,䠆3 O. Aktas,䠆4 H. Baba,䠆2 S. Bae,䠆5 P. Coleman-Smith,䠆6 T. Davinson,䠆7P. Doornenbal,䠆2 N. Fukuda,䠆2 C. Griffin,䠆7 S. Gaire,䠆1 Z. Ge,䠆2 J. Ha,䠆5 L. Harkness-Brennan,䠆8 T. Isobe,䠆2 N. Inabe,䠆2D. Kahl,䠆7 G. G. Kiss,䠆2 T. Kubo,䠆2 S. Kubono,䠆2M. Labiche,䠆6 G. Lorusso,䠆9 G. Lotay,䠆10 Y. Litvinov,䠆11 C. Lizarazo,䠆12B. Moon,䠆13 F. Montes,䠆14 K. Matsui,䠆2 S. Momiyama,䠆15 S. Naimi,䠆2 S. Neupane,䠆1 S. Nishimura,䠆2 C. Nita,䠆16V Phong,䠆2 F. Recchia,䠆17 S Riccetto,䠆18 H. Sakurai,䠆15 Y. Saito,䠆15 P. Schrock,䠆10 Y. Shimizu,䠆2 P-A Söderström,䠆2H. Suzuki,䠆2 T. Sumikama,䠆2 H. Takeda,䠆2 V. Werner,䠆12 K. Wimmer,䠆15 W. Witt,䠆12 P. J. Woods䠆7 and K. Yoshida䠆2The origin of the elements around Zr (Z=40) is one of themost intriguing questions regarding nucleosynthesis of theheavy elements during the rapid neutron-capture process(r-process). In recent years, the observation of elemental
abundances in a growing sample of extremely metal-poorstar has shown a remarkably robust pattern of abundancesin the heavier elements (beyond Te, at Z=52), in very good
agreement with the r-process abundance pattern derived
from the Solar System 1). This provides evidence that ther-process is a mechanism contributing to the chemical
evolution of the Galaxy from its early stages. On the other
hand, the scenario for the elements in the Sr to Ag region ismore complex2). The abundances do not show such a clear
correlation with the solar r-process pattern. For example,VRPH VWDUV SUHVHQW DQ RYHUDEXQGDQFH RI $§ LVRWRSHV
Additional nucleosynthesis processes, such as a weakr-process acting in neutrino-driven winds of core-collapse
supernovae, have been proposed as an explanation2).
Astrophysical model calculations consistently show thatthe uncertainty in the available nuclear physics input
contributes in a significant way to the uncertainty of theirresults. For some models, the region around 92Se studied inour experiment, has been identified as having a highimpact3). The region is critical to disentangle thecontribution of the strong r-process from othernucleosynthesis processes. Our new data will help toaddress the question of what, if any, the contribution of thestrong r-process to elements lighter than Ag (Z=47) is, or
which alternative processes could be responsible for thesynthesis of these elements.
The objective of our experiment, performed in June of2016, LVWRPHDVXUHIRUWKHILUVWWLPHWKHPDVVDQGȕ-decay
half-lives of a number of neutron-ULFKLVRWRSHVLQWKH=§region. The experiment combines a ȕ-decay measurement
using the Advanced Implantation Detector Array (AIDA)
with a simultaneous mass measurement using thetime-of-flight (TOF) technique4). A neutron-rich beam in the92
Se region was produced by fragmentation of a 345 MeV/u
primary beam of 238U on a Be target. In addition to theseparation of the secondary beam ions, BigRIPS was usedfor the TOF mass measurement. Its optics were tuned toHigh Resolution Mode, using a TOF flight path from the F3
to the F11 focal plane positions. The beam ions were thenimplanted in the double-sided silicon strip detectors ofAIDA, which was placed at the focal plane of theZero-Degree Spectrometer and surrounded by the EURICA
gamma detector array.
Figure 1 shows a particle-identification plot for theexperiment, with a line indicating the limit of known
nuclear masses. We expect to measure new decay half-livesof 10 isotopes in the region of Ge to Kr region, and themass for up to 10 isotopes. The data analysis for the massand half-live measurement is in progress, as well as that for
decay spectroscopy with EURICA.

䠆1

Central Michigan University
RIKEN Nishina Center䠆3Bucknell University
䠆4Middle East Technical University
䠆5Seoul National University
䠆6
STFC Daresbury Laboratory
䠆7
University of Edinburgh
䠆8University of Liverpool
䠆8
National Physical Laboratory
䠆10
University of Surrey
䠆11GSI䠆12
TU Darmstadt
䠆13
Korea University
䠆14Michigan State University
䠆15
University of Tokyo䠆16
IFIN Horia Hulubei Bucharest
䠆17
University and INFN Padova,
䠆18
University and INFN Perugia
䠆2Fig. 1. Particle identification plot for secondary beam ionsin the experiment, showing the limit of previously measured
masses (with three isotopes labelled as reference).
References
1) I. U. Roederer et al.: Astrophys. J. Lett. 747, L8 (2012).
2) C. Hansen et al: Astrophys. J. 797, 123 (2014).
3) S. Brett et al: Eur. Phys. J. A 48, 184 (2012).
4) M. Matos et al: Nucl. Instr. Meth. A 696, 171 (2012).

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