Ⅰ. HIGHLIGHTS OF THE YEAR
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Development of new ionization chamber specialized for high Z
beam (II)M. Yoshimoto,∗1 N. Fukuda,∗1 R. Matsumura,∗1 D. Nishimura,∗2 H. Otsu,∗1 Y. Shimizu,∗1 T. Sumikama,∗1H. Suzuki,∗1 H. Takahashi,∗2 H. Takeda,∗1 J. Tanaka,∗1 and K. Yoshida∗1Ionization chambers (ICs) are detectors used to determine the energy deposit (∆E) of the ions with suﬃcient accuracy to distinguish the atomic number (Z) inthe BigRIPS separator and ZeroDegree spectrometer.
A new IC has been developed to improve the Z resolution of heavy ions with Z > 80 and the energy of 200–
300 MeV/nucleon,1) for which experimental proposals
are on the rise. The diﬃculty in Z determination is because of the change in the charge state,2) which is more
pronounced for higher Z beams. To overcome this effect, previous studies suggested the replacement of thecommonly used P-10 with a gas with a smaller or larger
Z.1,3)
We conducted machine studies (MS-EXP21-10, -11)
on the gas dependence of the IC at RIKEN RIBF. TheIC setup has been described in a previous study.1) We
used three gas species, P-10 gas (Ar 90% + CH4 10%),methane gas (CH4 100%) with a smaller cross sectionof the charge state changing, and xenon-based gas (Xe70% + CH4 30%) with a larger cross section of thecharge state changing. The ICs with these gases wereoperated at 620 Torr in the F7 vacuum chamber.
The ∆E in the IC was measured with 238 U90+, 91+ions at 344 MeV/nucleon. Figure 1 shows the ∆E distributions for the three gas species. The shape for theCH4 gas diﬀered signiﬁcantly between 90+ and 91+.
The lowest energy peak corresponds to 90+ passing
through the IC without charge state changing and thesecond-lowest peak corresponds to 91+.

Fig. 1. Energy deposit (∆E) distributions of the IC inmethane, P-10, and xenon-based gases irradiated with
344 MeV/nucleon 238 U90+ (blue area) and 238 U91+ (redline).

The shape diﬀerence for the P-10 gas was smaller
than that for the CH4 gas. However, the mean ﬁtted
with a normal distribution changed by 0.56% dependingon the charge states. When multiple charge states aretransmitted, the mean shift adversely aﬀects particle
identiﬁcation (PID). The energy resolution of 90+ and∗1∗2RIKEN Nishina Center
Department of Natural Sciences, Tokyo City University

91+ was 1.1%, which was worse than the 0.7% energy
resolution required to achieve the 3σ separation at Z =92 from Z = 91 isotopes. In contrast to the CH4 and P10 gases, the xenon-based gas yielded almost identicalshapes for 90+ and 91+. The diﬀerence between themeans was only 0.03%. This is because a charge-state
equilibrium was reached immediately after the injection
into the IC. The energy resolution was obtained to be
0.69%, which is suﬃcient for the 3σ separation in Z.
Hence, the xenon-based gas was demonstrated to besuitable for the IC specialized for high Z beams.
We injected the high-Z secondary beam at approximately A/Q = 2.5 into the xenon-based gas IC. The secondary beam was produced from the 345 MeV/nucleon
238
U primary beam impinging on a 4 mm-thick Be target. The magnetic rigidity of the ﬁrst dipole was 6.3 Tm
and no degraders were used in the separator. ThePID was performed using the TOF-Bρ-∆E method, asshown in Fig. 2. Further, the beam energy at F7 was
typically 264 MeV/nucleon at Z = 80 −92. Although
the blobs for He-like and H-like ions are dense, the different ion species were well separated and identiﬁed.
The averaged Z resolution of Z = 81−91 was 0.34 (1σ).
The resolution is approximately twice better than thatby the P-10 gas IC.4)

Fig. 2. PID plot of Z versus A/Q obtained with the xenonbased gas IC. The typical energy is 264 MeV/nucleon.

In conclusion, the xenon-based gas IC achieved the 3σ
separation in Z, and rendered the PID for the secondary
beams with Z > 80 at approximately 260 MeV/nucleon
practical for all experimental groups.
References
1) M. Yoshimoto et al., RIKEN Accel. Prog. Rep. 55, 70(2022).
2) H. Weick et al., Phys. Rev. Lett. 85, 2725 (2000).
3) T. Sumikama et al., in this report.
4) N. Fukuda et al., RIKEN Accel. Prog. Rep. 54, 81(2021).

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