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

Atomic-number identiﬁcation of heavy RI beams using the energy
loss in a Xe-based gas
T. Sumikama,∗1 N. Fukuda,∗1 and M. Yoshimoto∗1

P10 gas (1.00 atm)
Z-Q = 1 -> 2
Z-Q = 2 -> 1Xe (90%) + CH4 (10%) gas (0.39 atm)
60
Z-Q = 1 -> 2Z-Q = 2 -> 1L (cm)80

Effective length of ionization chamber

20150

200

250

300

350

Energy (MeV/u)

Fig. 1. Partial mean free path length of the change in thecharge state in the ionization chamber as a function ofthe energy of the 208 Rn beam.
∗1RIKEN Nishina CenterCounts

4000
3000

Rn-208 (Z=86)
At-206 (Z=85)(a)
P10 gasAr (90%) + CH4 (10%)
1 atm

300 MeV/u

2000
1000
1100

6000

1150

1200

1250

1300

1350

Rn-208 (Z=86)(b)
Xe (90%) + CH4 (10%) gas
0.39 atm

At-206 (Z=85)

300 MeV/u

4000
2000
0

1100

1150

1200

1250

Energy deposit (MeV)

1300

1350

Fig. 2. Monte Carlo simulation of the energy deposit in theionization chamber. Half of the energy-loss straggling in
ATIMA was applied for the energy deposit.5)

An experimental study of PID using a Xe-based gas
was already conducted, and a good Z resolution wasobtained.6,7)
References

40

0of a Xe-based mixed gas (Xe 90% + CH4 10%). The gaspressure was determined so that the energy deposit at
300 MeV/nucleon is same as that of the P10 gas. Evenat 300 MeV/nucleon, L is shorter than that of P10 at
185 MeV/nucleon, indicating a better Z resolution.
The energy loss in the ionization chamber was simulated by using the energy-loss code ATIMA4) andGLOBAL. The ﬂuctuation in Q was taken into account by the Monte Carlo method. The Z resolution ofthe 180-MeV/nucleon 210
85 At beam was simulated to be0.46%, which is consistent with the experimental valueof 0.45%. Figure 2 shows the energy deposit of the 300MeV/nucleon 208 Rn and 206 Ac beams into the eﬀective
region of the ionization chamber. The Z resolution is
improved from 0.60% for the P10 gas to 0.39% for theXe-based gas. This result is consistent with the discussion of Fig. 1.

CountsRadioactive isotope (RI) beams produced at RIBF
are tagged event-by-event with the atomic number Z
and mass-to-charge ratio A/Q determined using beamline detectors. For heavy RI beams, particle identiﬁcation (PID) becomes diﬃcult owing to the change in Q
inside the beam-line detectors because Z is determinedfrom the energy loss depending on Q2 . Blobs in the PID
plot were clearly visible for the 208 Rn case1) but not forthe 220 Th case.2) The relative Z resolutions were 0.45%
and 0.69% (1σ) for the 185-MeV/nucleon 208 Rn beamand 315-MeV/nucleon 220 Th beam, respectively. Theworse resolution was considered to be due to the energyloss straggling caused by charge-state ﬂuctuation in thegas of the ionization chamber. The diﬀerence between
these two cases indicates the impact of the energy dependence of the charge-state ﬂuctuation.
In this paper, the diﬀerence in the Z resolutions isdiscussed in regards to the energy dependence of thecross section of the change in Q in the gas using theGLOBAL code.3) Figure 1 shows the energy dependence of the partial mean free path length L, givena change in Z − Q from 1 to 2 or from 2 to 1. Theenergy dependence of L is mainly for the electronpickup reaction. In the 208 Rn case, the mean valueof the equilibrium charge-state distribution ⟨Q⟩ is 84.5
at 185 MeV/nucleon. Since ⟨Q⟩ is a decimal, thecharge state must change multiple times in the ionization chamber to make the eﬀective Q in a single
event closer to 84.5. L (Z−Q = 1→2) is roughly 1/2 ofthe eﬀective length of the ionization chamber, as indicated by the dotted line in Fig. 1. Thus, even if Z − Qchanges from 2 to 1, it could be back. In contrast, L(Z−Q = 1→2) is longer than the eﬀective length at
315 MeV/nucleon. Z − Q might not change to 2 once itbecomes 1. This is mainly the nature of Ar gas, which
accounts for 90% of the P10 gas. Figure 1 also shows L

1) T. Sumikama et al., Nucl. Instrum. Methods Phys. Res.
B 463, 237 (2020).
2) N. Fukuda et al., RIKEN Accel. Prog. Rep. 54, 81(2021).
3) C. Scheidenberger et al., Nucl. Instrum. Methods Phys.
Res. B 142, 441 (1998).
4) https://web-docs.gsi.de/~weick/atima/ .
5) Y. Sato et al., RIKEN Accel. Prog. Rep. 46, 159 (2013).
6) M. Yoshimoto et al., 19th Int. Conf. on Electromagnetic
Isotope Separators and Related Topics (EMIS XIX),
Daejeon, South Korea, Oct. 3–7, 2022.
7) M. Yoshimoto et al., in this report.

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