Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 48 (2015)

Fission Barrier Studies of Neutron-Rich Nuclei via the (p,2p)
Reaction: Status of NP1306-SAMURAI-14 Experiment
D. Mücher,∗1,∗2 S. Reichert,∗1,∗2 M. Sako,∗2 M. Sasano,∗2 A. Andreyev,∗3 T. Aumann,∗4 H. Baba,∗2M. Böhmer,∗1 M. Dozono,∗5 N. Fukuda,∗2 R. Gernhäuser,∗1 W. F. Henning,∗6 K. Hirose,∗3N. Inabe,∗2 D. Kameda,∗2 N. Kobayashi,∗2 T. Kobayashi,∗7 Y. Kondo,∗8 T. Kubo,∗2 Y. Kubota,∗2,∗5 R. Lang,∗1L. Maier,∗1 Y. Matsuda,∗9 S. Mitsuoka,∗4 T. Motobayashi,∗2 T. Nakamura,∗8 N. Nakatsuka,∗9 S. Nishimura,∗2I. Nishinaka,∗3 K. Nishio,∗3 R. Orlandi,∗3 H. Otsu,∗2 V. Panin,∗2 S. Sakaguchi,∗2 H. Sato,∗2 Y. Shimizu,∗2L. Stuhl,∗2 T. Sumikama,∗2 H. Suzuki,∗2 H. Takeda,∗2 Y. Togano,∗2 T. Uesaka,∗2 J. Yasuda,∗2 K. Yoneda,∗2
and J. Zenihiro∗2
The aim of NP1306-SAMURAI-14 is to experimentally determine fission barrier heights in neutron-richnuclei ”north east” of doubly-magic 208 Pb using RIB’sat RI Beam Factory (RIBF) at RIKEN. The results
will drastically improve our experimental knowledge
on fission barriers away from stability which is poorat present. Neutron-rich nuclei around Z ≈ 82 andN > 126 are good subjects to be studied because theisospin dependence of fission barriers in those nuclei arequite controversial: The Rotating Liquid Drop Model
predicts an increase of barrier heights with neutron
number, while modern, more microscopic fission models make completely different and, depending on themodel, quite varying behaviors (see e.g.1) and references therein). The behavior of fission barriers towards neutron-rich nuclei has a direct impact on ther-process2) , which is believed to produce about half ofthe stable heavy elements in the universe.
The basic idea is to use the quasifree (p,2p) reaction
in inverse kinematics to excite the nuclei of interest.
Heavy neutron-rich nuclei provided at the RI Beam
Factory bombard a hydrogen target and measurements
of proton four-momenta tells us an excitation energyof the neutron-rich isotopes. In case that fission takes
place, the two fission fragments are detected and analyzed with the large-acceptance SAMURAI spectrometer3) . This allows to extract a threshold energy forthe fission process.
For this experiment, heavy secondary beams must
be produced from a primary 238 U beam and identified
using BigRIPS. This is a difficult task as the secondary
fragments after the production target have very similar
A/Q ratios compared to the primary beam, which has
to be stopped in the beam dump after the D1 magnet. A first test was performed from the BigRIPS
team in autumn 2014 to produce neutron-rich Polonium (Z=84) nuclei (N. Inabe et al. in the present∗1∗2∗3∗4∗5∗6∗7∗8∗9Physics Department E12, Technical University Munich
RIKEN Nishina Center
ASRC JAEAIKP, Technical University Darmstadt
Center for Nuclear Studies, University of Tokyo
Physics Devision, Argonne National Laboratory
Department of Physics, Tohoku University
Department of Physics, Tokyo Institute of Technology
Department of Physics, Kyoto University

volume of this APR).
In April 2014, we performed a one day test at theSAMURAI setup using a stable 238 U beam. The experimental setup was basically identical to the one used forexperiment NP1306-SAMURAI-17 (M. Sasano in thepresent volume of this APR). To keep the beam intensity below 5 · 104 pps in the SAMURAI area, the 238 Ubeam was produced as a secondary beam, using a 5 mm
Be production target. The best setting was found bychoosing the Q=87+ charge state after the target, followed by a Q=90+ charge state after a thin aluminum
degrader. In this way, 40 kHz beam rate after BigRIPS
was achieved using 0.75 pnA primary beam intensity.
The beam purity was better then 98 %. The beam hita 1 cm liquid hydrogen target. A multiplicity triggerof the WINDS array (see reports of the present volume of the APR) was used to trigger on possible (p,2p)
events close to the target. These data are currently analyzed by S. Reichert. Indeed, based on time-of-flightand QDC information, coincident protons follow theexpected kinematics of quasi-free scattering. However,in order to achieve the desired missing mass resolution,a new high-resolution proton-detection system is currently under development in collaboration of the TU
Munich and RIKEN. The status of the development ofthis setup can be found in the report of S. Reichert etal. in the present volume of this APR.
The test experiment also gives first results about
the response and performance of various detectors of
the SAMURAI setup on high-Z beams. The mass andcharge resolution for the identification of fission fragments is currently analyzed by M. Sako (see report in
the present volume of this APR). Furthermore the data
allow us to study the tracking efficiency of coincident
fission fragments. A first analysis about the identification of fission fragments using the FDC1 detector wasdone by S. Reichert (see report in the present volumeof this APR).
References
1) P. Moeller et al.: Phys. Rev. C 79, 064304 (2009).
2) G. Martinez-Pinedo et al.: Prog. Part. Nucl. Phys. 59(2007) 199-205.
3) T. Kobayashi et al.: Nucl. Inst. Meth. B 317, 294304(2013).

- 47 -完全版2014_本文.indd 47

15/10/16 17:41