Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Search for the double Gamow-Teller giant resonance using double
charge exchange reaction
A. Sakaue,∗1 T. Uesaka,∗2 and K. Yako∗1 for the RIBF-141R1 Collaboration∗1∗2

Center for Nuclear Study, University of Tokyo
RIKEN Nishina Center-6-preliminary
20

48

Ca(12C, 12Be(0+2))
250 MeV/nucleon
0 < θ < 0.3 deg

15dσ/dΩdE [a.u.]

The double Gamow-Teller (DGT) transition is the nuclear process where the spin and isospin are changedtwice by a (στ )2 operator, where σ and τ are the spin
and isospin operators, respectively. It is theoretically
predicted that there is a giant resonance which occupies
most of the whole DGT transition strength in a highexcitation energy region.1) This resonance, called theDGT giant resonance (DGTGR), remains undiscovered
experimentally. The observable of the DGTGR such as
the strength or the width will provide information about
the two-phonon excitations with spin-dependent correlations. In addition, it is suggested that such observables
will give a constraint to the nuclear matrix element ofneutrinoless double β decay, which has a large ambiguity in the current theoretical calculation.2)
We observe DGTGR using the double charge exchange
reaction (12 C, 12 Be(0+2 )) at RIBF. We employ an experimental method developed in the pionic atoms spectroscopy3) using a part of BigRIPS, from the F0 focal
plane to the F5 focal plane, as a spectrometer. A reaction target at F0 is bombarded with the primary beamof 12 C and the (12 C, 12 Be(0+2 )) reaction is induced. Themomentum of the ejected particle is obtained by measuring the position at F5 and thus the excitation energyof the residual nucleus of F0 is deduced. To select eventswith spin-ﬂip mode, 12 Be is transported to the stopperof 9 Be at F8 and delayed γ rays deriving from 12 Be(0+2))
decaysintothegroundare detected by DALI2; 12 Be(0+2state by emitting an electron-positron pair with the lifetime of 331 ± 12 ns.4)
We successfully detected the delayed-γ rays from1248
Be(0+Ca target at F0 in the experiment2 ) for theconducted at RIBF in May 2021 using this method forthe ﬁrst time.5) The signal-to-background ratio is estimated to approximately 5 : 1 for the events with photonsof 500 ± 100 keV detected in DALI2. It is improved fromthe one of the pilot experiment performed at RCNP, approximately 1 : 1.6)
The preliminary spectrum of the position at F5 forthe measurement of the 48 Ca target is shown in Fig. 1.
Here the event selection is such that the timing of thephoton is between 20 ns and 700 ns after from the timing of prompt γ-rays, and the energy is in the regionof 510 ± 100 keV. Each color corresponds to the scattering angle of 12 Be at F0 target in the laboratory system, which is reconstructed from the position and theangle measured at F5, within 0.0–0.3◦ (blue line, bluehatched), 0.3–0.6◦ (red line, green hatched), and 0.6–0.9◦ (black line, black hatched). In this spectrum, thekinematical position aberration with the scattering an-0.3 < θ < 0.6 deg
0.6 < θ < 0.9 deg

1050−8048Ti Eex [MeV]−60−40−20F5x [mm]4002020Fig. 1. Preliminary result of position measured at F5. Lineand hatching colors correspond to selected regions of scattering angles: 0–0.3◦ (blue line, blue hatched), 0.3–0.6◦(red line, green hatched), and 0.6–0.9◦ (black line, blackedhatched).

gle is corrected for. The preliminarily calibrated energyof 48 Ti is shown below the ﬁgure. The energy resolution
and the angular resolution was evaluated as 1.6 MeV
and 0.17◦ , respectively. The main background is an accidental coincidence of 12 Be and γ rays from room backgrounds. The amount of the background is estimated toabout 5% with the event selection by the gate of DALI2
described above. This background is subtracted by assuming that its energy spectrum is same as events thatare not gated by DALI2. In the region of the position
from −10 mm to 0 mm, which corresponds to aboutfrom 23 to 30 MeV in the excitation energy where theDGTGR is expected to be observed2) , there seem to beenhancements at −2 mm and −8 mm especially in theforward angle, though statistics are limited. The structure around −20 mm, on the other hand, is likely due
to the events of the 12 C(12 C, 12 Be)12 Og.s. reaction fromthe graphene sheet attached to the 48 Ca target.
In order to interpret the structure, we are now working
on the comparison of the angular distribution with thecalculated ones.
References
1) N. Auerbach et al., Annu. Phys. 192, 77 (1989).
2) N. Shimizu et al., Phys. Rev. Lett. 120, 142502 (2018).
3) T. Nishi et al., Nucl. Instrum. Method Phys. Res. B 317,290 (2013).
4) S. Shimoura et al., Phys. Lett. B 654, 87 (2007).
5) A. Sakaue et al., RIKEN Accel. Prog. Rep. 55, 9 (2022).
6) M. Takaki et al., CNS Annu. Rep. 2016, CNS-REP-96, 3(2018).