Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 53

First success of RI-beam separation and particle identification fornuclei with atomic number Z > 82 at RIKEN RI beam factory†
T. Sumikama,∗1 T. Kubo,∗1,∗2 N. Fukuda,∗1 D. S. Ahn,∗1 H. Takeda,∗1 Y. Shimizu,∗1 H. Suzuki,∗1 N. Inabe,∗1D. Murai,∗1,∗3 H. Sato,∗1 K. Kusaka,∗1 Y. Yanagisawa,∗1 M. Ohtake,∗1 and K. Yoshida∗1:D1LiBe50

Full
BeLiHeLi HHeFullHeLi

8280
2.43

∗1∗2
∗3

2.44

2.45

2.46

2.47

2.482.49

2.5

2.51

2.52

Fig. 2. Particle-identification plot of the RI beams, which
was obtained by applying software gates of −5 < x <5 mm on the F3 and F7 positions.

HeH FullHe HLi HeBe Li82

Fig. 1. RI-beam separation scheme at the first stage ofthe BigRIPS separator in the case of a 2-mm-thick aluminum degrader. The purple, green, red, orange, andpink solid lines show the selected RI beams for thecharge-state change ∆QF1 = 0, +1, +2, +3, and +4
at F1, respectively. The blue dashed lines show nuclei
having the same A/Q as 208 Rn with Q = 84.
†120884+
86Rn122

A/ Q

126

50

108684

238UFullH88eee
ipe-lik i-lik e-lik -lik ll str: L : H 1: H Fu
11:
DD D1
D82

102

90

Be208RnBe92

ZThe separation of heavy RI beams is diﬃcult at
RIKEN RIBF, because the charge state Q of ions frequently changes in certain materials including a wedge
degrader used for the RI-beam separation. In an earlierstudy,1) a large amount of in-ﬂight ﬁssion fragments
prevented the good separation of heavy RI beams.
In the present study, the charge-state change in thewedge degrader was taken into consideration in termsof the RI-beam puriﬁcation. We found that a 208 Rn
beam can be well separated from the ﬁssion fragments
when He-like and fully-stripped ions are selected at
the ﬁrst and second dipole magnets, respectively. Figure 1 shows the selected isotopes when an aluminum
degrader with a thickness of 2 mm is used at F1 of theBigRIPS separator.2) This is an extension of a ﬁgurefor the full-strip ions for both dipole magnets givenin Ref. 3). The ﬁssion fragments with atomic numbers between 25 and 65 are required to have Li-like or
Be-like ions at the ﬁrst dipole. The probabilities of Lilike and Be-like ions for 160
85 Tb was estimated using theGlobal code4) to be 4.7 × 10−4 and 1.8 × 10−6 , respectively. Because of these low probabilities, the ﬁssion
fragments are expected to be well eliminated.
An experiment to produce a 208 Rn beam was performed at RIKEN RIBF. The 3-mm-thick beryllium
was used as the production target. The thickness ofthe aluminum wedge degraders at F1 and F5 were 2and 1 mm, respectively. The He-like and fully-strippedions were chosen at the ﬁrst and second halves of theﬁrst stage of BigRIPS, respectively. The He-like ionswere chosen at the second stage of BigRIPS. To compare the separated RI beams with those in Fig. 1, awide slit setting of ±20 mm was applied at the achromatic foci F2 and F7. The ﬁssion fragments were nottransported to the second stage of BigRIPS even for
the wide slit. Software gates were applied to the positions measured at F3 and F7 to simulate the RI beams
with a narrow slit setting. Figure 2 shows a particleidentiﬁcation plot with gates of −5 < x < 5 mm at F3and F7. The purity of 208 Rn reached 25%. Therefore,RIKEN RIBF is now ready for user experiments in thisregion of the nuclear chart.
References

1) N. Inabe et al., RIKEN Accel. Prog. Rep. 48, 73 (2015).
2) T. Kubo, Nucl. Instrum. Methods Phys. Res. B 204, 97(2003).
3) J. P. Dufour et al., Nucl. Instrum. Methods Phys. Res.
A 248, 267 (1986).
4) C. Scheidenberger et al., Nucl. Instrum. Methods Phys.
Res. B 142, 441 (1998).

Condensed from the article in Nucl. Instrum. Methods Phys.
Res. B 463, 237 (2020)
RIKEN Nishina Center
FRIB/NSCL, MSU
Department of Physics, Rikkyo University- 65 -