Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 48 (2015)

Missing-mass spectroscopy of tetra-neutron system via exothermic
double-charge exchange reaction 4 He(8 He,8 Be)4n
K. Kisamori,∗1,∗2 S. Shimoura,∗2 H. Miya,∗1,∗2 M. Assie,∗3 H. Baba,∗1 T. Baba,∗4 D. Beaumel,∗1,∗3M. Dozono,∗1 T. Fujii,∗1,∗2 N. Fukuda,∗1 S. Go,∗1,∗2 F. Hammache,∗3 E. Ideguchi,∗5 N. Inabe,∗1 M. Itoh,∗6D. Kameda,∗1 S. Kawase,∗2 T. Kawabata,∗4 M. Kobayashi,∗2 Y. Kondo,∗1,∗7 T. Kubo,∗1 Y. Kubota,∗1,∗2M. Kurata-Nishimura,∗1 C. S. Lee,∗1,∗2 Y. Maeda,∗8 H. Matsubara,∗1 S. Michimasa,∗2 K. Miki,∗5 T. Nishi,∗1,∗9S, Noji,∗10 S. Ota,∗2 S. Sakaguchi,∗1,∗11 H. Sakai,∗1 Y. Sasamoto,∗2 M. Sasano,∗1 H. Sato,∗1 Y. Shimizu,∗1A. Stolz,∗10 H. Suzuki,∗1 M. Takaki,∗2 H. Takeda,∗1 S. Takeuchi,∗1 A. Tamii,∗5 L. Tang,∗2 H. Tokieda,∗2M. Tsumura,∗4 T. Uesaka,∗1 K. Yako,∗2 Y. Yanagisawa,∗1 and R. Yokoyama∗2
neutron system, as shown in Fig. 1. The spectrum
contains 28 events including background events. The
background events were estimated from the the number of events that was not identified as multi-particle
in one bunch in the MWDC at F6. There were only
2.2 ± 1.0 events in the whole spectrum region, which isalmost negligible compared to the selected 28 events.
The excitation energy (E4n ) of the spectrum was calibrated using the peak position of the 8 Li(1+ ) state andthe scale of magnetic rigidity of the SHARAQ spectrometer. 8 Li was produced by the inverse kinematicsof the (p,n) reaction of 8 He. The uncertainty of theenergy determination accuracy was 1.25 MeV at theone sigma level. The resolution of the spectrum was
1.16 MeV at the one sigma level.
The events were concentrated in the low-excitationenergy region of 0 < E4n < 2 MeV and a continuum
were observed at a region of E4n > 2 MeV. The meanof the four events at 0 < E4n < 2 MeV was 0.83 ±0.25 MeV. The possibility of the resonance state at thelow-energy region and the shape of the continuum will
be discussed in comparison with results of a theoretical calculation assuming the correlation of di-neutronclusters.
+>+).?/@/2/4+5
events / 4 MeV

Since the report on candidates of a bound tetraneutron system1) , multi-neutron systems in nuclei haveattracted considerable attention on both the experimental and theoretical fronts. On the other hand,later theoretical studies using ab-initio calculations2)
have suggested that the tetra-neutron cannot exist as
a bound system but possibly as a resonance system.
We performed missing-mass spectroscopy of the 4nsystem via the exothermic double-charge exchange reaction 4 He(8 He,8 Be)4n3) . The primary goal of the experiment was to determine the energy level and its
width of the 4n system. In order to produce the 4nsystem with a small momentum transfer of less than
20 MeV/c, a secondary beam of 8 He with a large internal energy was used.
The experiment was performed at the RIKEN RI
Beam Factory (RIBF) using the SHARAQ spectrometer and a liquid He target system4) . We measured themomentum of the 8 He beam at BigRIPS-F6 with theHigh-Resolution Beamline and the momenta of two alpha particles, which were the decay products of the8Be ejectile, with the SHARAQ spectrometer. The incident beam energy of 8 He was 186 MeV/u.
Events resulting from the 4 He(8 He,8 Be)4n reaction
were selected. Two α particles from the 8 Be ejectile
were detected simultaneously at the final focal planeof the SHARAQ spectrometer. Furthermore, a method
to reconstruct trajectories for more than two particles
under a high-intensity beam (∼2 MHz) was developed5). In order to a obtain good signal-to-noise ratio, it
was important to identify multi-particles in one bunch,which have the possibility to create the background
events. These events were rejected using the MultiWire Drift Chamber (MWDC) at F6.
We obtained the missing-mass spectrum of a tetra-:2
14
12:8
10:9!"#$%&'()*
background

Preliminary!"#$%&%'(")

wave
wave packet
packetjust
just after
after reaction
reaction

8A

resonance
resonance

6=continuum
continuum

42
28
09-20
789

∗1-10
7:9

09

10:9

20
89

30;9402950<960=9excitationenergy of 4n (MeV)+,#-.".-')/+)+&%0/'1/2)/34+56

Fig. 1. Missing-mass spectrum of tetra-neutron system.

RIKEN Nishina Center
Center for Nuclear Study, the University of Tokyo∗3 IPN, Orsay∗4 Department of Physics, Kyoto University∗5 Research Center Nuclear Physics, Osaka University∗6 Cyclotron and Radioisotope Center, Tohoku University∗7 Department of Physics, Tokyo Institute of Technology∗8 Faculty of Engineering, University of Miyazaki∗9 Department of Physics, the University of Tokyo∗10 National Superconducting Cyclotron Laboratory, Michigan
State University∗11 Faculity of Science, Kyushu University∗2References
1) F. Marques et al.: Phys. Rev. C 65, 044006 (2002)
2) S. Pieper: Phys. Rev. Lett. 90, 252501 (2003)
3) K. Kisamori et. al.: Accelerator Progress Rep. 45 (2012)4) M. Kurata-Nishimura et. al.: Accelerator Progress Rep.
46 (2013)
5) K. Kisamori et. al.: Accelerator Progress Rep. 46 (2013)-7-

完全版2014_本文.indd 7

15/10/16 17:39