RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-1. Nuclear Physics

Yield development of KEK isotope separation system
Y.X. Watanabe,∗1 Y. Hirayama,∗1 M. Mukai,∗2,∗3 Y. Kakiguchi,∗1 P. Schury,∗1 M. Oyaizu,∗1 H. Miyatake,∗1
M. Wada,∗1,∗2 M. Ahmed,∗1,∗3 S. Kimura,∗2,∗3 J.Y. Moon,∗4 J.H. Park,∗4 H. Ishiyama,∗4 S.C. Jeong,∗4 andA. Taniguchi∗5∗1∗2∗3∗4∗5Extraction Efficiency

We have been developing KEK isotope separationsystem (KISS)1) for lifetime measurements of neutronrich nuclei around N = 126, relevant to r-process nucleosynthesis.2) The multinucleon transfer (MNT) reaction between the 136 Xe beam and 198 Pt target wasused at KISS, and it is one of the promising candidates to eﬃciently produce those neutron-rich nuclei.3)
The target was located in a gas cell, and the reaction
products were stopped and neutralized in the argon
gas ﬁlled in the gas cell. They were transported to theexit of the gas cell by laminar gas ﬂow, where they wereirradiated by the lasers to be ionized for a speciﬁc element by laser resonance ionization technique. Those
extracted ions were mass-separated and transported tothe measurement area for β-decay spectroscopy.
So far, we have successfully extracted radioactive
isotopes (RIs) of 199 Pt and 196,197,198 Ir.4) The accessible isotopes were limited to the vicinity of the198
Pt target because of the low extraction eﬃciency(∼1×10−4 ) and limited beam intensity (∼20 pnA) toprevent heat damage at the target. The low extraction
eﬃciency might be due to the re-neutralization of thelaser-ionized isotopes caused by the radiation from thedense plasma in the argon gas, which is induced by
the primary beam.5) We have developed a doughnutshaped gas cell and a rotating target to accept a more
intense beam without injecting it into the gas cell.
In parallel, we have started to investigate other possible reaction pairs to achieve more eﬃcient production of the desired isotopes. GRAZING calculations6)
indicate larger cross sections using the 238 U beam ascompared with the 136 Xe beam. The ﬁrst attempt toextract RIs produced in the MNT reactions between
the 238 U beam and the 198 Pt target at KISS is reported
here.
The well-studied ﬁxed-target gas cell was used in thismeasurement for comparison with 136 Xe beam data. A198
Pt target with a thickness of 11 mg/cm2 in the gas
cell was bombarded by the 238 U beam accelerated up
to 10.75 MeV/nucleon by RRC. The beam energy on
the target was tuned by energy degraders to approximately 8.5 MeV/nucleon, which is the optimal value
from the GRAZING calculations. Three plastic scintillator telescopes were used to detect β-rays by mea-10-4

10-50123Beam intensity (pnA)

45Fig. 1. Beam intensity dependence of extraction eﬃciency
measured for 199 Pt.

suring the extracted RIs, which were implanted into
aluminized Mylar tape at the detection area to identify them by measuring their lifetimes.
Only one species of isotopes, 199 Pt, was identiﬁedin this measurement because of the low extraction efﬁciency and limited beam intensity. To investigate theintensity limitation, the beam intensity dependence ofthe extraction eﬃciency was measured. Figure 1 shows
the result. The extraction eﬃciency was deﬁned as theratio the between number of extracted ions in the measurements and the number of ions ejected from the target in the calculations. The extraction eﬃciency was
1×10−4 at a beam intensity of approximately 1 pnA,
and it decreased as the beam intensity increased. The
absolute extraction eﬃciency and its beam-intensity
dependence are similar to those in the case with the136
Xe beam. Because the beam intensity was limited
up to 5 pnA by the heat damage of the energy degrader
foils and the target foil, the extraction eﬃciency was
not available for higher beam intensity. In the next
experiment, we will measure the extraction rates athigher intensities using rotating degraders and target
with the doughnut-shaped gas cell for 199 Pt and otherRIs.
References
1) Y. Hirayama et al., Nucl. Instr. Meth. B 353, 4 (2015).
2) S. C. Jeong et al., KEK Report 2010-2 (2010).
3) Y. X. Watanabe et al., Phys. Rev. Lett. 115, 172503(2015).
4) Y. Hirayama et al., RIKEN Accel. Prog. Rep. 49, 148(2016).
5) M. Huyse et al., Nucl. Instrum. Methods Phys. Res.
Sect. B 187, 535 (2002).
6) A. Winther, Nucl. Phys. A 572, 191 (1994); Nucl.
Phys. A 594, 203 (1995); Program GRAZING,
http://www.to.infn.in/ nanni/grazing/

Wako Nuclear Science Center (WNSC), Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator
Research Organization (KEK)
RIKEN Nishina Center
Department of Physics, University of Tsukuba
Institute for Basic Science (IBS)
Research Reactor Institute, Kyoto University- 70 -