I. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Experimental studies of the two-step scheme with an intense radioactive132
Sn beam for next-generation production of very neutron-rich nuclei†

Production yields (cps)The usefulness of a two-step scheme with a 132 Sn
beam proposed1) for the eﬃcient production of midheavy very-neutron-rich RIs was investigated, as an alternate method for the in-ﬂight ﬁssion of a 238 U beam
(one-step scheme). The two-step scheme is a combination of an isotope-separation online (ISOL) system witha thick U target and a high-intensity proton beam as
the ﬁrst step, and the projectile fragmentation of reaccelerated RI beams (e.g., 132 Sn) as the second step.
We measured production cross sections beyond 125 Pd,
up to which the cross sections had already been measured at GSI together with the neighboring RIs,2) toevaluate the yields of RIs using the two-step scheme witha 132 Sn beam. The 278-MeV/nucleon 132 Sn beam was
supplied from BigRIPS, and the very neutron-rich RIs
around the neutron number N = 82 were produced in
the ZeroDegree spectrometer with a 5.97-mm Be target.
Yields obtained by the two-step and one-step schemes
were estimated based on the measured cross sections,and we examined whether and to what extent the twostep scheme at future 1-MW beam facilities can reachfurther into the neutron-rich region.
Figure 1 shows the production yields of Pd isotopes
obtained using the two-step scheme with a 132 Sn beam108

Pd isotopes

106
104
102

two-step

010 two-step(COFRA)
one-step-2
10116 118 120 122 124 126 128 130 132

Mass numberFig. 1. Yield comparison between the two-step scheme witha 132 Sn beam (orange circles) and the one-step in-flightfission of a 238 U beam (blue squares) for Pd isotopes.
†∗1
∗2
∗3
∗4
∗5Condensed from the article in Phys. Rev. C 102, 064615(2020)RIKEN Nishina Center
Center for Nuclear Study, University of Tokyo
National Superconducting Cyclotron Laboratory, Michigan
State University
Division of Physics, Argonne National Laboratory
Physik Department, Technische Universität MünchenZ

H. Suzuki,∗1 K. Yoshida,∗1 N. Fukuda,∗1 H. Takeda,∗1 Y. Shimizu,∗1 D. S. Ahn,∗1 T. Sumikama,∗1 N. Inabe,∗1T. Komatsubara,∗1 H. Sato,∗1 Z. Korkulu,∗1 K. Kusaka,∗1 Y. Yanagisawa,∗1 M. Ohtake,∗1 H. Ueno,∗1 T. Kubo,∗1S. Michimasa,∗2 N. Kitamura,∗2 K. Kawata,∗2 N. Imai,∗2 O. B. Tarasov,∗3,∗1 D. P. Bazin,∗3,∗1 J. Nolen,∗4,∗1 andW. F. Henning∗4,∗5,∗15048
46
44
4240
65

70

75

80

N

102
10
110−110−210−310−410−5

Fig. 2. Ratios of the Y2step with a 132 Sn beam to the Y1step .
Orange and blue regions indicate that the two- and onestep schemes are more useful than the other, respectively.
Dark-blue square dots represent the supernova r-process
path.

and the one-step scheme with 1-MW beam powers. The
two-step yields (Y2step ) decrease more slowly with neutron numbers than the one-step yields (Y1step ), andY2step becomes larger than Y1step beyond 124 Pd. The
ratio of Y2step /Y1step around the N = 82 region is shownin Fig. 2. This ratio systematically increases, as moving
away from the stability line. Thus, the two-step scheme
is more favorable than the one-step scheme to yield moreneutron-rich region beyond our results, especially for theregion of the supernova r-process path. The calculated
Y2step with a cross-section formula COFRA4) indicatedby black lines reproduces the experimental results well.
ISOL systems can provide various RIs over the nuclear chart; thus, a wide region of very neutron-rich RI
beams can be produced by the two-step scheme. By using the ISOLDE yield database3) and the cross-section
formula COFRA, the Y2step in the whole region was estimated. The Y2step was expected to be larger than theY1step around the neutron-rich N = 50, 60, 82, and 90,including the supernova r-process path. The two-step
scheme is considered a powerful tool to open a new window into the unknown region of mid-mass to heavy, very
neutron-rich nuclei.
References

1) K. Helariutta et al., Eur. Phys. J. A 17, 181 (2003).
2) D. Pérez-Loureiro et al., Phys. Lett. B 703, 552 (2011).
3) ISOLDE Yield Database web page,
http://isoyields-classic.web.cern.ch .
4) COFRA web page, http://www.usc.es/genp/cofra .

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