RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-1. Nuclear Physics

Measurement of 4 He(8 He,8 Be) reaction for verifying tetra-neutron
resonance
S. Masuoka,∗1 S. Shimoura,∗1 M. Takaki,∗1 D. S. Ahn,∗2 H. Baba,∗2 M. Dozono,∗1 N. Fukuda,∗2 T. Harada,∗3E. Ideguchi,∗4 N. Imai,∗1 N. Inabe,∗2 C. Iwamoto,∗1 K. Kawata,∗1 N. Kitamura,∗1 M. Kobayashi,∗1 Y. Kondo,∗5T. Kubo,∗2 Y. Maeda,∗6 F. M. Marqués,∗7 M. Matsushita,∗1 S. Michimasa,∗1 R. Nakajima,∗1 T. Nakamura,∗5N. Orr,∗7 S. Ota,∗1 H. Sakai,∗2 H. Sato,∗2 P. Schrock,∗1 L. Stuhl,∗1,∗2 T. Sumikama,∗2 H. Suzuki,∗2 H. Takeda,∗2K. Taniue,∗6 H. Tokieda,∗1 T. Uesaka,∗2 K. Wimmer,∗8 K. Yako,∗1 Y. Yamaguchi,∗1 Y. Yanagisawa,∗2
R. Yokoyama,∗1 K. Yoshida,∗2 and J. Zenihiro∗2
rate of the secondary beam at F3 was increased fromabout 2.0 MHz to 3.5 MHz. Six low-pressure multiwire drift chambers (LP-MWDCs) were installed for
tracking the beam. “F6” was set as a dispersive focalplane, so that the momentum of the beam could bemeasured by the focus position. At “S0,” a liquid He
target system (CRYPTA) was installed. At the ﬁnal
focal plane, “S2,” 2 α particles from the decay of outgoing 8 Be were detected using 2 cathode readout drift
chambers (CRDCs).
S2 Y (mm)The existence of nuclei composed only of neutrons
has been discussed for over half a century, but it has
not been conﬁrmed yet. In 2002, a candidate boundstate of the tetra-neutron, which consists of four neutrons, was reported.1) An ab-initio calculation suggested that there might be a tetra-neutron (4n) resonance, but a bound 4n was not reproduced.2) Anexperimental search for the 4n resonance state conducted using the exothermic double charge exchange(DCX) 4 He(8 He,8 Be)4n reaction was performed at theSHARAQ spectrometer in RIBF3) . As a result, fourcandidate events were found with a 4.9σ signiﬁcancelevel, and the energy of the 4n resonance was determined as E4n = 0.83 ± 0.65(stat.) ± 1.25(syst.) MeV.
To conﬁrm the existence of 4n resonance, we performed
a new measurement with higher statistics and withsmaller energy uncertainty.

reaction

60p(t,3He)

4020He(8He,8Be)

40-20-40-60-150-100-50050100150
S2 X corrected (mm)Fig. 2. A preliminary hit pattern of the 1 H(t,3 He) reaction
at S2 for the energy calibration with a reaction kinematics curve. The X and Y axes represent the missingmomentum and the vertical scattering angle, respectively.

In the present experiment, the method of missingmomentum calibration was changed to reduce the systematic error of the 4n energy. As a reference for theenergy, the 1 H(t,3 He) reaction was measured with atriton beam that has the same magnetic rigidity as
the 8 He beam (8.3 Tm). The energy can be calibrated without changing the magnetic settings. Figure 2 shows an S2 image of outgoing 3 He particles fromthe 1 H(t,3 He) reaction. The red line indicates the ﬁtted kinematics curve of the reaction. The thresholdenergy of the 4n state can be determined from thecurve. Further analysis is now in progress.

Fig. 1. Detector setup of BigRIPS/SHARAQ beam line.

Figure 1 shows a schematic view of the setup forthis experiment. A primary 18 O beam was accelerated
to about 230 MeV/nucleon by AVF+RRC+SRC. The
intensity of the primary beam was about 700 particle nA. The energy of the secondary 8 He beam wasabout 186 MeV/nucleon. The beam intensity was increased from that in the previous experiment. The
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8Center for Nuclear Study, the University of Tokyo
RIKEN Nishina Center
Department of Physics, Toho University
Research Center for Nuclear Physics, Osaka University
Department of Physics, Tokyo Institute of Technology
Faculty of Engineering, Univ. of MiyazakiLaboratorie de Physique Corpusculaire, IN2P3-CNRS, ENSICAEN et Université de Caen
Department of Physics, the University of Tokyo

References
1) F.M. Marqués et. al., Phys. Rev. C 65, 044006 (2002).
2) S.C. Pieper, Phys. Rev. Lett. 90, 252501 (2003).
3) K. Kisamori et al., Phys. Rev. Lett. 116, 052501 (2016).

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