II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Re-measurement of the 4 He(8 He, 8 Be) reaction

In our previous study, the candidate resonance ofthe 4n system (tetra-neutron) was determined using the4He(8 He, 8 Be)4n reaction with a 186 MeV/nucleon 8 He
beam.1) A new measurement with better statistics andbetter accuracy was performed to conﬁrm the existenceof the tetra-neutron system.2)
The intensity of the 8 He beam was 3.5 × 106 particles per second at the liquid helium target, and approximately twice compared to that of the previous experiment. Low pressure multi-wire drift chambers (LPMWDCs) were installed at the focal planes F3, F6, andF-H10(S0), to measure the trajectory and momentum ofthe beam. The time reference to determine the drift timein LP-MWDCs was obtained from a plastic scintillator
at F3.
We present the analysis of the LP-MWDCs developed
to eliminate accidental coincidence events induced by thehigh intensity beam. The 8 He beam from the SRC hada bunch structure with a periodic cycle of 73 ns. There
are two cases of accidental coincidence as illustrated inFig. 1(a) and (b). The ﬁlled circles represent the particle, which triggered the data acquisition. The other par(a)beam(b)

Triggered bunch

Next bunchtime

TRF = 73 ns
Fig. 1. Time structure of the beam bunch. The solid circles show particles triggering data acquisition, and theopen circles are accidental particles. (a) Beam contains
both the ‘Triggered bunch’ and ‘Next bunch.’ (b) Twoparticles are in the triggered bunch.
∗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
Faculry of Engineering, Univ. of Miyazaki
Laboratorie de Physique Corpusculaire, IN2P3-CNRS, ENSICAEN et Université de Caen
Department of Physics, the University of TokyoCounts (a.u.)

S. Masuoka,∗1,∗2 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,∗2 Y. Maeda,∗6F. M. Marqués,∗7 M. Matsushita,∗1 T. Nakamura,∗5 N. Orr,∗7 H. Sakai,∗2 H. Sato,∗2 P. Schrock,∗1 L. Stuhl,∗1,∗2T. Sumikama,∗2 H. Suzuki,∗2 H. Takeda,∗2 K. Taniue,∗6 T. Uesaka,∗2 K. Wimmer,∗8 K. Yako,∗1 Y. Yamaguchi,∗1Y. Yanagisawa,∗2 K. Yoshida,∗2 and J. Zenihiro∗23500
3000
2500
2000
1500
1000
500
3024681012×10

Sum of drift charge (a.u.)

Fig. 2. Energy distribution of LP-MWDC.

ticle in the next bunch (open circle) hits in (a), whereas
two particles together hit in the same bunch in (b). In
the analysis, we carefully treated these events, which
created multiple hits within the maximum drift time of
120 ns in the LP-MWDCs. The events of Fig. 1(a) weresuccessfully identiﬁed by selecting the drift-time regionof the LP-MWDC and the total traveling time from F3
to S2 focus corresponding to the beam energy.
In the case of Fig. 1(b), we simply eliminated such
events because the triggered beam particle and accidental particle in the same bunch cannot be distinguished.
To identify such cases, we estimated the total energy
spectrum obtained by using the Time-Over-Thresholddata of its signal. Figure 2 shows the total energy distributions measured at LP-MWDC; two peaks are visible.
The peak at higher energies results from multiple-hit
events, and 75% of the multiple hit events were rejectedby selecting energies below 6,000. This value is consistent with the probability of an occurrence of pile up
events.
After the treatment, the tracking eﬃciency of thebeam was 95%. In the present experiment, one of theLP-MWDCs installed at F-H10 was damaged under theintense irradiation of the 8 He beam and operated witha low eﬃciency of 48%. The overall tracking eﬃciencyof the 8 He beam is therefore 80%, which still ensuresbetter statistics than the previous experiment.
Data analysis is in progress toward the ﬁnal result.
References

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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