RIKEN Accel. Prog. Rep. 50 (2017)

Research Activities 1991–2016

Direct reaction experiments with fast RI beams for studying
properties of exotic nuclei and astrophysical nuclear processes
T. Motobayashi ∗1
One of the ﬁrst experiments using fast radioactive
isotope (RI) beams, which RIKEN started to provide in the early 1990s, was on the Coulomb dissociation of 14 O. The experiment was performed with 14 O
beams and a 208 Pb target by a group mainly composedof researchers from Rikkyo University, ISN Grenoble,RIKEN, and University of Tokyo.1) The 13 N(p, γ)14 O
reaction in the hot CNO cycle was indirectly studied. The Grenoble team brought an array of CsI(Tl)scintillators, which was used together with a positionsensitive silicon detector array and associated electronics developed by the Rikkyo group for detecting outgoing particles in coincidence.
This study has unique properties: it is the ﬁrst experiment for astrophysical reactions involving short-lived
nuclei, the ﬁrst Coulomb dissociation measurement for
explosive nuclear burning, an international collaboration that combines experimental devices developed byrespective groups, and a good example of direct reaction studies using fast (87.5 MeV/nucleon in this case)
RI beams in inverse kinematics, where the nuclei in thebeam are of interest and the target serves as the reaction probe.
The radiative width of the ﬁrst 1− state in 14 O,
the key parameter for evaluating astrophysical reaction
rates, was successfully extracted, and it agreed witha direct measurement at the laboratory of Louvainla-Neuve with slow 13 N beams performed in a similar
period. These pioneering experiments resolved a longstanding question on the 13 N(p, γ)14 O astrophysical reaction rate.
Another direct reaction experiment in inverse kinematics was on the neutron-rich N = 20 nucleus 32 Mg
performed at RIKEN in 1994. The reduced transition
probability B(E2 :0+ −2+1 ) was successfully determinedby Coulomb excitation.2) Again, the inverse kinematics, where fast radioactive 32 Mg beams were incident on
a 208 Pb target, was employed. The population of theﬁrst 2+ state in 32 Mg was identiﬁed by measuring its
deexcitation γ rays, in contrast to the above-mentioned
Coulomb dissociation, in which
where the particle decay fromthe unbound 1− state in 14 O was measured by detecting
the proton and 13 N in coincidence.
The extracted large B(E2) value of 454 ± 78 e2 fm4
supports the idea of disappearance of the N = 20 shellclosure in 32 Mg. This B(E2) measurement was performed for the ﬁrst time long after the study determining the low 2+ energy of 895 keV in the late 1970s.
For this type of experiments, a γ detector array,
DALI, was built. It consists of ≈60 NaI(Tl) scintil∗1lators and has a large eﬃciency with sensitivity to theγ-ray emission angle, which is indispensable for correcting a large Doppler shift of photons from fast-movingnuclei. To adopt higher-energy beams of RIBF, a newarray called DALI2 with ≈170 crystals was developed
to cope with larger Doppler shifts. DALI and DALI2
have been and are still used in various experiments.3)
The successes of these studies demonstrated the usefulness of direct reaction with fast RI beams, and triggered extensive studies of the structure of exotic nuclei
and astrophysical reactions in explosive burning. Theywere performed at RIKEN and other facilities such as
NSCL-MSU, GANIL, and GSI.
Coulomb dissociation was also applied to the7Be(p, γ)8 B reaction, the major process of producing
high-energy neutrinos in the sun. Several experiments
provided new independent results free from systematic
problems caused by the radioactive 7 Be target used in(p, γ) measurements in normal kinematics. These studies contributed to make the 7 Be(p, γ)8 B reaction rates
more reliable and, therefore, to place the discussions ofthe solar neutrino problem on ﬁrmer ground.4)
In addition to the Coulomb excitation, various direct reactions have been studied. The inelastic scattering of light particles such as proton, deuteron, orα in this energy regime is dominated by the nuclear
interaction and have diﬀerent sensitivities to the ﬁnalstates. For example, for ﬁnding the 1− state in 12 Be,
the Coulomb excitation and proton inelastic scattering
are compared, where the former is more sensitive tothe  = 1 excitation.5) We also found that the inelastic
proton scattering is one of the most eﬃcient probes toobserve unknown low-lying levels. Nucleon removal reactions are commonly used, since they populate morestates compared with the inelastic scattering.
Many new properties in exotic nuclei were discoveredby these direct reactions in inverse kinematics. Recent
examples at RIKEN are ﬁndings of a “soft” nature of32
Mg by the proton inelastic scattering populating its
ﬁrst 4+ state 6) and a large N = 34 shell gap in 54 Ca
expected form the location of the 2+ state populated
by nucleon removal reactions.7)
References
1) T. Motobayashi et al., Phys. Lett. B 264, 259 (1991).
2) T. Motobayashi et al., Phys. Lett. B 346, 9 (1995).
3) S. Takeuchi et al., Nucl. Instrum. Methods Phys. Res.
A 763, 596 (2014).
4) T. Motobayashi, Nucl. Phys. A 693, 258 (2001), andtherein.
5) N. Iwasaki et al., Phys. Lett. B 491, 8 (2000).
6) S. Takeuchi et al., Phys. Rev. C 79, 054319 (2009).
7) D. Steppenbeck et al., Nature 502, 207 (2013).

Chief Scientist of Heavy Ion Nuclear Physics Laboratory(2002–2010)

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