RIKEN Accel. Prog. Rep. 50 (2017)

Research Activities 1991–2016

Introduction of storage-ring experiments at RIBF
M. Wakasugi ∗1
The RI Beam Factory (RIBF) in RIKEN Nishina
center is a synergetic-use facility where the world’s mostintense high-energy exotic nuclear beams are supplied
to users worldwide mostly for nuclear physics experiments. We have introduced storage-ring-experimentcapability at the RIBF in this decade by constructing
both an electron storage ring and a heavy-ion storage
ring. The electron storage ring facility will be used
for the world’s ﬁrst electron scattering experimentsfrom unstable nuclei, and the heavy-ion storage ringis used for precision mass measurements, especially for
extremely exotic nuclei located on the r-process path.
Both have been deﬁned as core experimental instrumentations at RIBF.
The Self-conﬁning RI ion target (SCRIT), which wasdeveloped for electron scattering oﬀ unstable nuclei, isa novel internal target forming technique in an electron storage ring. The construction of the SCRIT electron scattering facility 1) was started in 2009 and almost
completed in 2015. As shown in Fig. 1, it consists ofan electron accelerator, RTM; an electron storage ring,
SR2; an ISOL-type RI beam generator, ERIS; 2) anda high-resolution spectrometer, WiSES, for analyzing
scattered electrons. The momentum transfer distribution of elastically scattered electrons from the 132 Xe
isotope extracted from ERIS was observed with threeelectron beam energies, as shown in Fig. 1, and we successfully determined the proton distribution for the ﬁrst
time.3) The luminosity reached 2 × 1027 /(cm2 s) with
108 injected target ions at an electron beam current of200 mA − 250 mA. We evaluated the performance of
the SCRIT system and demonstrated the feasibility ofelectron scattering oﬀ unstable nuclei. The ﬁrst exper-Fig. 2. Schematic view of R3 connected to BigRIPS andtypical data of revolution times measured for exotic nuclei.

iments for unstable nuclei are now under preparation.
The heavy-ion storage ring named R3 (Rare-RIRing) 4) connected to BigRIPS, as shown in Fig. 2,
was constructed in 2012–2014. Masses of nuclei aredetermined by measuring the revolution times for lessthan 1-ms accumulation under the isochronous condition. The magnetic structure yielding a high-precision
isochronism (∼ 1 ppm) is formed solely by only bending magnets equipped by trim coils. This provides not
only large acceptances of Δp/p ∼ ±0.5 % in momentum
space and ∼ 150π mm mrad in transverse emittance,
but also a high-precision mass-determination capability
with an accuracy of the order of 1 ppm even with poor
statistics. The isotope-selectable self-triggered injection (ISSI) method, which combines particle identiﬁcation in the transport line and a newly developed ultrafast response kicker, was adopted in the R3 system,
and it enables eﬃcient mass measurement for rarely
produced exotic nuclei. Since the injection kicker magnet is activated by a trigger signal produced by theisotope of interest at the F3 focal point, the isotope isidentiﬁed with certainty and accumulated in R3 one byone. As shown in Fig. 2, revolution times for exotic nuclei measured in the commissioning are independent of
their momentum spread as expected, and the mass values relative to that of the reference isotope 78 Ge wereprecisely determined. Mass measurements for 74−78 Ni
are now under preparation.

RTM

RI generator

ERIS
FRAC
LMon

SCRIT
WiSES

(1280,1550)

01

2

3

4Å@m

SR2References
1) M. Wakasugi et al., Nucl. Instrum. Methods Phys. Res.
B 317, 668 (2013).
2) T. Ohnishi et al., Nucl. Instrum. Methods Phys. Res.
B 317, 357 (2013).
3) K. Tsukada et al., Phys. Rev. Lett. 118, 262501 (2017).
4) Y. Abe et al., Phys. Scr. T 166, 014047 (2015).

Fig. 1. Schematic view of the SCRIT electron scattering
facility and the recently observed momentum transfer
distribution of elastically scattered electrons from 132 Xe.
∗1Director of Instrumentation Development Group, RIKEN
Nishina Center (2008–present)

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