Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 52 (2019)

Re-measurement of the 4 He(8 He, 8 Be) reaction∗1∗2∗3∗4∗5∗6∗7∗8100
9080(a)

7060
5040
3020
1000Counts (a.u.)

In our previous study, the candidate resonanceof 4n system (tetra-neutron) was found using the4He(8 He, 8 Be)4n reaction with a 186 MeV/nucleon 8 Hebeam.1) A new measurement with better statistics andbetter accuracy was performed to evidence the existence of the tetra-neutron system.2)
The intensity of the 8 He beam (3.5×106 particles per
second at the liquid helium target) was approximately
twice compared to that of the previous experiment.
A pair of cathode readout drift chambers (CRDCs)
was used at the ﬁnal focal plane of the SHARAQ spectrometer. To improve the resolution, the positions andangles of the CRDCs were precisely calibrated in thesoftware analysis by using the reference data of a 4 He
beam. We achieved position resolutions of 0.40 and
0.62 mm FWHM and angular resolutions of 1.12 and1.87 mrad FWHM in the horizontal and vertical directions, respectively. Ion optical analysis was then performed to evaluate the ion transport matrix elementsof both the beam line and the SHARAQ spectrometer. The scattering angle of the reaction was deduced
from the angle measured by the CRDCs after taking
the ion transport matrix into account. Higher orderaberrations were thus corrected for. We obtained aresolution of 4.7 and 2.8 mrad FWHM in the horizontal and vertical directions, respectively.
In the previous experiment, the systematic uncertainty was 1.25 MeV mostly due to the fact that noreference data were measured in the momentum regionof the tetra-neutron resonance. In this experiment,
we measured the 1 H(3 H, 3 He)n reaction by using a 3 H
beam at 310 MeV/nucleon (8.3 Tm) and a CH2 target.
The advantage of this reaction is that the rigidity ofboth the beam and the ejectile are the same as thatof the 4 He(8 He, 8 Be)4n reaction at 186 MeV/nucleon.
Therefore, the reaction of reference and the reactionof physics can be measured in the same momentum
window without changing the magnet settings.
Figure 1 (a) shows the kinematical curve of the1H(3 H, 3 He) reaction. The scattering angle is plotted

Scattering angle (mrad)

S. Masuoka,∗1 S. Shimoura,∗1 M. Takaki,∗1 S. Ota,∗1 S. Michimasa,∗1 M. Dozono,∗1 C. Iwamoto,∗1K. Kawata,∗1 N. Kitamura,∗1 M. Kobayashi,∗1 R. Nakajima,∗1 H. Tokieda,∗1 R. Yokoyama,∗1 D. S. Ahn,∗2H. Baba,∗2 N. Fukuda,∗2 T. Harada,∗3 E. Ideguchi,∗4 N. Imai,∗1 N. Inabe,∗2 Y. Kondo,∗5 T. Kubo,∗2Y. Maeda,∗6 F. M. Marqués,∗7 M. Matsushita,∗1 T. Nakamura,∗5 N. Orr,∗7 H. Sakai,∗2 H. Sato,∗2 P. Schrock,∗1L. Stuhl,∗1,∗2 T. Sumikama,∗2 H. Suzuki,∗2 H. Takeda,∗2 K. Taniue,∗6 T. Uesaka,∗2 K. Wimmer,∗8 K. Yako,∗1Y. Yamaguchi,∗1 Y. Yanagisawa,∗2 K. Yoshida,∗2 and J. Zenihiro∗2350
300

0.2

0.4

0.6

0.8

11.2

1.4
1.6
1.8
2
Missing momentum (%)

0.4

0.6

0.8

11.2

1.4
1.6
1.8
2
Missing momentum (%)

(b)

250
200
150
100
50
000.2

Fig. 1. (a) Plot of the scattering angle against the missingmomentum. The kinematical curve of 1 H(3 H, 3 He)n reaction is visible. (b) Missing momentum spectrum aftercorrecting for the scattering angle in (a).

against the missing momentum, which is given by themomentum diﬀerence between the 3 H beam and the3He ejectile. Figure 1 (b) shows the spectrum of themissing mass momenta after correcting for the scattering angle dependence. Given the known Q-value of the1H(3 H, 3 He) reaction, the missing mass momentum of
the peak seen in the ﬁgure was taken as a reference.
The uncertainty of the excitation energy of 4n is evaluated to be 26 keV RMS due to the uncertainty of thepeak position in Fig. 1 (b).
Further analysis is ongoing.
References

Center 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, University of Miyazaki
Laboratorie de Physique Corpusculaire, IN2P3-CNRS, ENSICAEN et Université de Caen
Department of Physics, the University of Tokyo

1) K. Kisamori et al., Phys. Rev. Lett. 116, 052501 (2016).
2) S. Masuoka et al., RIKEN Accel. Prog. Rep. 50, 198(2017).

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