Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Precise measurement of the 4 He(8 He,8 Be) reaction
S. Masuoka,∗1 S. Shimoura,∗1 M. Takaki,∗1 M. Dozono,∗1 C. Iwamoto,∗1 K. Kawata,∗1 N. Kitamura,∗1M. Kobayashi,∗1 S. Michimasa,∗1 R. Nakajima,∗1 S. Ota,∗1 H. Tokieda,∗1 R. Yokoyama,∗1 D. S. Ahn,∗2
H. 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,∗1
Y. Yamaguchi,∗1 Y. Yanagisawa,∗2 K. Yoshida,∗2 and J. Zenihiro∗2Nuclei composed of only neutrons have been discussed for over a half century. However, their existence has not been confirmed. In 2002, a candidate
bound state of the tetra-neutron, which consists of fourneutrons, 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 RIBF.3) As a result, fourcandidate events of the resonance state were foundwith a 4.9σ significance level, and the excitation energyof 4n was determined as E4n = 0.83 ± 0.65 (stat.) ±1.25 (syst.) MeV, which is close to the threshold.
To decide that the tetra-neutron state is a bound(E4n < 0) or resonance (E4n > 0) state, it is necessary
to reduce the systematic uncertainty for the excitationenergy of 4n. Then, we performed a new measurement
to obtain more statistics than the previous experimentand reduce the uncertainty of the energy of the 4nstate.
The 1 H(3 H,3 He) reaction with the same magnetic
rigidity of the 8 He beam (8.3 Tm) was used to determine a missing-mass calibration. Thus, the energy
can be calibrated without changing the magnet settings. This is the reason why we do not have to consider the scaling errors of magnetic fields. The accuracy of the excitation energy of 4n is evaluated to beapproximately 100 keV, which originated from the uncertaintly of the energy reference.
In order to obtain more statistics than the previous experiment, the intensity of the 8 He secondary
beam of 186 MeV/nucleon was approximately twicethat in the previous experiment (3.5 × 106 cps at F3focal plane).4) At the “F3 (achromatic focus for beam
trigger),” “F6 (dispersive focus),” and “S0 (achromatic focus for the secondary target),” low-pressuremulti-wired drift chambers (LP-MWDCs) were used
for tracking the beam. Events for the physics run
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were triggered by the S2 plastic scintillator at the focal
plane of the SHARAQ spectrometer. Because of thehigh-rate beam condition, a single event may containmultiple hits at the drift chambers over several beam
bunches of 13.7 MHz (RF frequency), considering themaximum drift time for the MWDCs is comparable tothe interval of the beam bunch. Hence it is necessary
to treat information of the redundant planes and otherdetectors.
In order to estimate the number of true four-neutron
events, a strict cut satisfying the condition that the twoclusters at the cathode planes of S2 have consistent energy signals with 2 α particles oeiginating in the 8 Be
is analyzed. A preliminary analysis shows an similar
event pattern in a spectrum as a function of the momenta of 8 He and 8 Be that is similar to the previousexperiment. Considering the analyzed area of the decay cone of 8 Be, we expect approximately 2–3 times
more statistics than the previous experiment.
A more precise calibration for the missing-mass spectrum taking into account the energy loss and stragglingat the detectors and the target to reduce systematic errors will be performed concurrently.
In summary, we are still attempting to purify true
events for increasing statistics with careful rejection
conditions for the background. In parallel, missingmass calibration will be tuned up to increase the accuracy of the 4n energy. Further analysis is ongoing.
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).
4) S. Masuoka et al., RIKEN Accel. Prog. Rep. 50, 198(2017).

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, Univ. of Miyazaki
Laboratorie de Physique Corpusculaire, IN2P3-CNRS, ENSICAEN et Université de Caen
Department of Physics, the University of Tokyo- 48 -