II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

The ONOKORO project—Toward comprehensive understanding onclustering in heavy nuclei
T. Uesaka,∗1 J. Zenihiro,∗2,∗1 K. Ogata,∗3 J. Tanaka,∗1 R. Tsuji,∗2,∗1 Y. Hijikata,∗2,∗1 K. Higuchi,∗1,∗4
S. Takeshige,∗1,∗5 K. Yahiro,∗2 S. Kurosawa,∗6 K. Yoshida,∗7 Y. Chazono,∗3 M. Takano,∗8 S. Typel,∗9 andH. Baba∗1 for the ONOKORO Collaboration
A new research project, “ONOKORO project,” hasstarted to investigate clustering in heavy nuclei using
cluster knockout reactions. Do nuclear systems, in general, have an inherent orientation that breaks uniformity? In this research, we try to answer this question
by studying formation mechanisms of deuteron, triton,
3He, and α clusters in stable and unstable nuclei in awide mass range.
Since early days of nuclear physics, α cluster in nuclei have been discussed, ﬁrst to explain α decays ofheavy nuclei, while clustering is now mainly discussed
for light nuclei with masses lighter than 30. On theother hand, there are signiﬁcant progress in theories
of cluster formation in inﬁnite nuclear matter.1,2) Important predictions of the theories are that clusters develop signiﬁcantly in the low-density region below 1/10of the saturation density, and the cluster abundanceschange signiﬁcantly with the neutron-proton asymmetry. Typel predicted also such a phenomenon would
also occur in the low-density surface of heavy nuclei.3)
Our previous experiment for the 112–124 Sn(p, pα) reaction has conﬁrmed Typel’s prediction and has demonstrated the existence of α clusters in those nuclei.4)
The ONOKORO project will extend the scope toall the light clusters of d, t, 3 He, α in stable and unstable nuclei from calcium (Z = 20) to heavy nuclei
beyond lead (Z = 82). The (p, pX) cluster knockout
reactions, where X denotes d, t, 3 He or α, are used to
extract information of clusters in nuclei. Characteristic points of the ONOKORO project are:• d, t, 3 He, α clusters are studied on the same footing• Both stable and unstable nuclei are covered• Normal and inverse kinematics experiments arecombinedSimultaneous studies of d, t, 3 He, α are essentially
important because the diﬀerent clusters would manifest diﬀerent aspects of clustering depending on its
spin and isospin. As is discussed in Refs. 2) and 3),
dependence on neutron excess is essential in studies ofclustering in heavy ﬁnite nuclei and in nuclear matter,
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8
∗9and thus the access to unstable nuclei is the core of theproject.
In order to cover both stable and unstable nuclei, wewill perform experiments at intermediate-energy accelerator facilities in Japan, the RIKEN RIBF, the QST
HIMAC, and the Osaka University RCNP cyclotron
facility. At RCNP, knockout reaction experiments under normal kinematics for stable nuclear targets are
conducted using proton beams and double-arm magnetic spectrometers, Grand Raiden and LAS, to obtainhigh energy-resolution data. State-by-state separationof the ﬁnal states enabled by the high resolution is aunique capability in experiments at RCNP. At RIBF
and HIMAC, inverse-kinematics experiments will becarried out using unstable and stable heavy-ion beams,
respectively. The inverse kinematics experiment has
the advantage of covering a broad kinematic regionand simultaneous measurements for all clusters of d, t,3He, α.
In this project, we will construct a detector array,TOGAXSI,5) designed speciﬁcally for inverse kinematics knockout reaction experiments. The array consistsof 100-μm pitch silicon strip detectors and GAGG:Cescintillators. GAGG:Ce scintillators have a good time
response of 1 ns rise time and <100 ns decay time.
Another good feature of GAGG:Ce is no hygroscopicnature.
The cluster knockout reaction theory based on theimpulse approximation will be reﬁned to derive thecluster abundance from the experimental cross sections accurately. In particular, a theory that treats
the breakup eﬀect, which is essential in the knockout reaction of weakly-bound deuterons, will be developed using the continuum-discretized coupled-channel
method.6)
By comparing the cluster abundance obtained fromthe experimental data and the reaction theory with thenuclear structure and nuclear matter theories, we willverify the hypothesis of cluster formation in nuclear
matter and clarify the mechanism of cluster formation.
References

RIKEN Nishina Center
Department of Physics, Kyoto University
Research Center for Nuclear Physics, Osaka University
Department of Physics, Toho University
Department of Physics, Rikkyo University
Institute for Materials Research, Tohoku University
Advanced Science Research Center, Japan Atomic Energy
AgencyDepartment of Physics, Waseda University
Institut für Kernphysik, Technisch Universität Darmstadt

1) S. Typel et al., Phys. Rev. C 81, 015803 (2010).
2) Z. -W. Zhang, L. -W. Chen, Phys. Rev. C 95, 064330(2017).
3) S. Typel, Phys. Rev. C 89, 064321 (2014).
4) J. Tanaka et al., Science 371, 260 (2021).
5) T. Uesaka et al., RIBF Construction proposal NP2112SAMURAI72.
6) Y. Chazono et al., in this report.

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