Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 49 (2016)

RI-beam production using BigRIPS separator in regions heavier thanthose belonging to lead isotope
T. Sumikama,∗1 N. Inabe,∗1 N. Fukuda,∗1 H. Takeda,∗1 Y. Shimizu,∗1 H. Suzuki,∗1 D. S. Ahn,∗1 D. Murai,∗2H. Sato,∗1 K. Kusaka,∗1 Y. Yanagisawa,∗1 M. Ohtake,∗1 K. Yoshida,∗1 and T. Kubo∗195

The regions of the nuclear chart that are heavier
in terms of atomic weight, i.e. with an atomic number Z ∼ 80 and more, are the key to understand thenucleosynthesis of elements up to uranium. In our previous study,1) an RI beam with Z > 80 was produced
using the projectile-fragmentation reaction of a 238 U
beam at RIBF. A 0.3-mm thick degrader was used in
the BigRIPS separator2) for the RI beam separation,
but a large amount of ﬁssion fragments unexpectedly
decreased a purity. In the present study, an RI beamheavier than the lead isotope was produced using athicker degrader, with a thickness of 2 mm, to eliminate the ﬁssion fragments.
The RI beam around 208
86 Rn was produced via theprojectile-fragmentation reaction of a 345-MeV/u 238 U
beam. The production target was a 3-mm thick
Be. To separate the primary 238 U beam, the He-likeions (charge-state Q = Z − 2) were selected at theﬁrst dipole D1. The 2-mm-thick wedge degrader wasplaced at the ﬁrst momentum dispersive focal plane
F1, and the fully-stripped ions were selected at thesecond dipole D2 after F1. Under these conditions,the charge-state combination for the ﬁssion fragmentswith Z < 60 was restricted to Li-like and fully stripped
ions at D1 and D2, respectively, or Z − Q ≥ 4 at D1.
Due to a low probability of these combinations, the ﬁssion fragments were separated very well. The secondwedge degrader with a thickness of 1 mm was usedin the middle of the second stage of BigRIPS for further puriﬁcation. The He-like ions were selected at
all the dipole magnets used in the second stage. The
particle-identiﬁcation (PID) plot measured at the second stage of BigRIPS is shown in Fig. 1 (a). Thecontaminants in the form of the ﬁssion fragments werenegligibly eliminated. The PID was conﬁrmed by the γrays emitted from a known isomeric state in 208 Rn, asshown in Fig. 1 (b). The high purity of 208 Rn allowed
us to complete the γ-ray measurement within 30 minutes. Figure 2 shows the mass to charge ratio for theRn isotopes. Each Rn isotope was well separated fromthe others. A small component with a diﬀerent charge
state may be included within the width of the distribution because the A/Q of the nucleus with A = 208
and Q = 84, for example, is very close to that with
A = 203 and Q = 82 or A = 213 and Q = 86. To distinguish the charge states, the total kinetic energy wasmeasured by using six stacked Si detectors, each witha thickness of 1 mm. This analysis is still in progress.
∗1
∗2(a)

90
102

Z

85
208
84+86Rn122

80
30Counts

75

70
2.36

2.38

2.4

2.42

2.44

2.46

636 keV
778 keV100

2.4810325 keV(b)

200

200

400

600

800 1000

Energy (keV)

2.5

2.52

2.54

1

2.56

A/ Q

Fig. 1. (a) Particle identification plot for the RI beam measured at the second stage of BigRIPS. The atomic number Z versus the mass to charge ratio is shown. (b) The
γ-ray energy spectrum of the known isomeric state in
208
Rn, indicated by the circle in (a). Three γ-ray peaks
can be clearly observed.

207

20884+

84+

209

Rn

206

Rn

10

205

84+

Rn

84+

210

Rn

384+

Rn

84+

Counts

Rn

102

204

21184+

Rn

84+

Rn

10

206 83+ 211 85+/ 87 Fr85 At

1
2.4

2.42

2.44

2.46

2.48

2.5

2.52

2.54

A/ Q

Fig. 2. Mass to charge ratio of the radon isotope (Z = 86)
obtained from Fig. 1 (a) with the gate of 85.8 < Z <86.2. The radon isotopes with Q = 84 were observed
between the masses 204 and 211, as can be seen for the83+ 211/ 87 Fr85+ shows aisotopes labeled using lines. 206
85 At
typical example of the contaminants for isotopes withZ = 85 and 87.

References
1) N. Inabe et al.: RIKEN Accel. Prog. Rep., Vol. 48(2015), p. 73.
2) T. Kubo et al.: Prog. Theor. Exp. Phys., 03C003 (2012).

RIKEN Nishina Center
Department of Physics, Rikkyo University- 70 -