II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Spectroscopy of pionic atoms in tin isotopes by (d, 3 He) reactions∗1Department of Physics, Kyoto University
RIKEN Nishina Center∗3 JINA and Department of Physics, University of Notre Dame
∗4 Department of Physics, Tokyo Institute of Technology∗5 GSI Helmholtzzentrum für Schwerionenforschung GmbH
∗6 Center for Nuclear Study, University of Tokyo∗7 Department of Physics, Toho University∗8 Japan Atomic Energy Agency∗9 Department of Physics, Saitama University∗10 Research Center for Nuclear Physics, Osaka University∗2- 10 -      Fig. 1. Position distribution observed by p(d, 3 He)π 0 reactions. Blue, red and green lines represent data with CH2target at −3, 0, and +6 mm from beam center, respectively.

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We performed systematic spectroscopy measurements of pionic tin isotopes by (d, 3 He) reactions. We
measured the masses of the reaction products, i.e., pionic atoms, by missing mass spectroscopy. The incident
deuteron beam energy was chosen as 500 MeV to enhance the formation cross-sections of the pionic atoms.
We installed A = 112, 115, 117, 119, 122, and 124 tin
isotopes at F0.
3He emitted by an energy of ∼360 MeV wasmomentum-analyzed by using the BigRIPS. We installed two low-pressure multi-wire drift chambers1)(MWDCs) at F5, and measured 3 He tracks. The
MWDCs were operated with 30 kPa isobutane and had
suﬃcient eﬃciency. We installed plastic scintillators atF5 and F7 for time-of-ﬂight measurements. Combining
the time-of-ﬂight information with the measured energy
loss at each scintillator, 3 He particles were identiﬁed.
One of the largest sources of resolution is the momentum spread of the primary beam, which was determinedas δp/p = 0.03% (σ). We constructed dispersion matching optics by analyzing the beam momentum at F0 tohave a dispersion of 44.6 mm/%. We developed a novel
method of tuning the optical condition, called “traceback method.”2) In this method, the particle tracks at
F3 and F5 are measured and the matrix elements in F0F3-F5 are evaluated. Finally the trajectories are traced
back to F0, and the phase distributions are deduced.
Figure 1 shows the position spectra of p(d, 3 He)π 0
reactions at F5 using a CH2 target. The spectra weremeasured by placing the target at horizontal positions−3, 0, and +6 mm from the beam center. The beam
spot size was enlarged to approximately 15 mm (σ) for
the very large dispersion of 44.6 mm/% at the target.
Without the dispersion matching conditions, the displacement of 3.0 mm at the target position causes a
5.5-mm shift at the F5 focal plane and severe deterioration of the energy resolution. In the spectra, we observe similar position distributions for the three target
positions, which suggesting a nearly perfect realization

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S. Y. Matsumoto,∗1 H. Asano,∗2 H. Baba,∗2 G. P. A. Berg,∗3 M. Dozono,∗1,∗2 H. Fujioka,∗2,∗4 N. Fukuda,∗2N. Fukunishi,∗2 H. Geissel,∗5 S. Hanai,∗6 T. Harada,∗2,∗7 T. Hashimoto,∗8 S. Hayakawa,∗5 K. Higuchi,∗4Y. Hijikata,∗1,∗2 K. Horikawa,∗4 N. Inabe,∗2 K. Itahashi,∗2 K. Kusaka,∗2 Y. Ma,∗2 R. Matsumura,∗9,∗2S. Michimasa,∗6 N. Nakatsuka,∗4,∗2 T. Nishi,∗2 S. Ota,∗10 A. Sakaue,∗6 F. Sakuma,∗2 M. Sasano,∗2 R. Sekiya,∗1,∗2Y. Shimizu,∗2 T. Sumikama,∗2 D. Suzuki,∗2 H. Suzuki,∗2 H. Takeda,∗2 Y. K. Tanaka,∗2 J. Tanaka,∗2 R. Tsuji,∗1,∗2T. Uesaka,∗2 K. Yako,∗6 Y. Yanagisawa,∗2 H. Weick,∗5 K. Yoshida,∗2 M. Yoshimoto,∗2 and J. Zenihiro∗1,∗2      Fig. 2. Position spectrum measured using 124 Sn target.

of the dispersion matching conditions.
Figure 2 shows a typical position spectrum observedin the production setting. We employed an 124 Sn target
with a width of 6 mm and accumulated the data for
approximately 50 min. Pionic atoms in 1s and 2p states
are noticeable as peak structures.
We will continue the data analysis in terms of energy
calibration, resolution improvement, and other factors.
References
1) S. Y. Matsumoto et al., RIKEN Accel. Prog. Rep. 50,205 (2017).
2) A. Sakaue et al., RIKEN Accel. Prog. Rep. 54, 86 (2021).

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