Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

New type of spectroscopy via heavy-ion double charge exchange(12 C, 12 Be(0+2 )) reaction
M. Takaki,∗1 T. Uesaka,∗2 S. Shimoura,∗1 N. Aoi,∗3 M. Dozono,∗2 S. Gotanda,∗4 T. Hashimoto,∗3 Y. Kanaya,∗4T. Kawabata,∗5 K. Kisamori,∗1,∗2 M. Kobayashi,∗1 Y. Kubota,∗1,∗2 Y. Maeda,∗4 M. Matsushita,∗1S. Michimasa,∗1 K. Miki,∗3 E. Milman,∗2 S. Ota,∗1 M. Sasano,∗2 A. Tamii,∗3 M. Tsumura,∗5 and J. Zenihiro∗2
One of the most interesting features in atomic nuclei is the variety of spin and isospin responses. TheGamow–Teller (GT) transition is the simplest spin–
isospin response within one-phonon excitations, andit has been well studied. In contrast, data on multiphonon excitations have been scarce. The double GT
giant resonance (DGTGR)1) is the most basic twophonon excitation mode. However, DGTGRs have notbeen observed so far. The discovery of the DGTGR isan essential step in extending the research of the spin–
isospin responses to multi-phonon space. Another interest for studying DGTGR relates to its relevance inneutrino physics; the DGT transition is induced by
the same transition operator as the ββ-decay is , i.e.,στ στ . However, the ββ-decay has quite small DGTstrength. A major part of the DGT strength is concentrated among highly excited states in DGTGR. A
promising spectroscopic method to search for DGTGRs is through heavy-ion double charge exchange(HIDCX) reactions, which can induce two-phonon excitations with spin and isospin transfer by two units.
In 2011, we conducted a HIDCX 12 C(18 O, 18 Ne)12 Be
reaction experiment and found a large cross section
12of 1.5 µb/sr for the second 0+ (0+Be at2 ) state in◦2)
120 . This is probably because all the initial C(0+g.s. ),+12),
andﬁnalBe(0)statesareintermediate 12 B(1+g.s.
2
dominated by a 0h̄ω conﬁguration3–5) . This led us toa new idea to use the (12 C, 12 Be(0+2 )) reaction as atool to investigate DGTGRs. In this probe, the excitation energy of target nuclei are measured using amissing–mass technique. Several ﬁnal states in 12 Be
can degrade the signal-to-noise ratio of an observed
spectrum in the method. The key of this probe is toavoid the contamination by tagging the two 511–keV
γ-rays emitted back-to-back from the e+ e− decay withthe mean lifetime of 331 ns6) . In order to demonstrate
the feasibility of the delayed γ-ray tagging method,18
we performed the HIDCX 18 O(12 C, 12 Be(0+2 )) Ne reaction measurement using the Grand Raiden (GR)spectrometer at RCNP, Osaka University. The primary 12 C beam at 100A MeV bombarded a 20-mg/cm2
H2 18 O ice target. The momenta of outgoing particles were analyzed using GR. The two 511–keV γ-rays
from 12 Be(0+2 ) were detected using a NaI(Tl) array sur∗1∗2∗3∗4∗5rounding a plastic-scintillator stopper at the GR focalplane. Figure 1 shows the GR horizontal position spectra. The position corresponds to the excitation energyof 18 Ne. The peak of the spectrum without the γ-ray
tagging, which originates from the 18 Ne ground state,
is rather broad and has a tail. The broadening is probably due to contributions from diﬀerent ﬁnal states in
12Be, and the tail originates from accidental coincidence events of 9 Li and 6 He. On the other hand, ina red spectrum with γ-ray tagging, the peak indicatedby a red arrow is narrower, and the background has
mostly vanished. The obtained energy resolution was∼3 MeV mainly because of an energy–loss diﬀerence inthe target, and thus the diﬀerence between the peak
positions of the two spectra is within the resolution.
The result of the test experiment shows the feasibilityof the gamma-ray tagging method. Our next step is toapply this method to nuclei exhibiting ββ-decay, suchas 48 Ca.
Counts (A.U.)

22020018018121218Ne(gnd)18O( C, Be) Ne w/o γ -ray tag

18+ 1812 12O( C, Be(0 )) Ne w/ γ -ray tag × 102160
140120
100
80
60

1818

O(12C,12Be(0+2 ;2.24 MeV)) Ne(gnd)preliminary

4020
0-400 -300 -200 -100

0100200 300 400
GR FP X (mm)Fig. 1. Horizontal position spectra of the GR focal planewith or without γ-ray tagging. For the red spectrum,detection eﬃciency of 10% is considered for the NaI(Tl)array.

References
1) D.C. Zheng, L. Zamick, N. Auerbach: Ann. of Phys.
197, 343 (1990).
2) M. Takaki et al.: RIKEN Accel. Prog. Rep. 46, 29(2013).
3) F.C. Barker: J. Phys. G 2, L45 (1976).
4) H. Fortune and R. Sherr: Phys. Rev. C 74 (2006)
024301.
5) R. Meharchant et al.: Phys. Rev. Lett. 108 (2012)
122501.
6) S. Shimoura et al.: Phys. Lett. B 560 (2003) 31; ibid654, (2007) 87.

Center for Nuclear Study, The University of Tokyo
RIKEN Nishina Center
Research Center for Nuclear Physics, Osaka University
Department of Applied Physics, University of Miyazaki
Department of Physics, Kyoto University- 18 -