Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 53

Angle-tunable wedge degrader for an energy-degrading RI beamline†

The Optimized Energy-Degrading Optics (OEDO)
beamline1) was established at RIBF to produce 10–50MeV/nucleon beams by slowing down secondary beams
from the BigRIPS separator. This beamline can extend the energy range of RI beams that can be produced in RIBF to optimize nuclear reactions such as
nucleon transfer and Coulomb excitation.
The angle-tunable degrader, one of the key elementsin the OEDO beamline, is a monoenergetic degraderthat reduces the beam energy and spread of a beam.
It consists of two Al sheets with a quadratic cross section, the quadratic coeﬃcients of which have opposite
signs. While its central thickness is ﬁxed at 3 mm,
the wedge angle varies from 0 to 40 mrad according to
the relative positions of the two sheets. The eﬀective
area is ±30 mm (H) × ±50 mm (V), which is wide
enough for the typical beam width, ±20 mm. The average thickness deviations for sheets 1 and 2 are 33
and 58 µm, respectively. The total central thickness
can be increased by introducing an additional Al ﬂatdegrader immediately behind our system.
Figure 1 shows the whole system. For parallel andseparate movement of the sheets, the system includes
guides for the sheets, two parallel rails, and two linear
stepper motors. Each motor drives each Al sheet along
the corresponding rail. Because the motors operate
independently by remote control, we can optimize thewedge angle using real-time data.
Normalized CountJ. W. Hwang,∗1 S. Michimasa,∗1 S. Ota,∗1 M. Dozono,∗1 N. Imai,∗1 K. Yoshida,∗2 Y. Yanagisawa,∗2K. Kusaka,∗2 M. Ohtake,∗2 D. S. Ahn,∗2 O. Beliuskina,∗1 N. Fukuda,∗2 C. Iwamoto,∗1 S. Kawase,∗3K. Kawata,∗1 N. Kitamura,∗1 S. Masuoka,∗1 H. Otsu,∗2 H. Sakurai,∗2 P. Schrock,∗1 T. Sumikama,∗2H. Suzuki,∗2 M. Takaki,∗1 H. Takeda,∗2 R. Tsunoda,∗1 K. Wimmer,∗4 K. Yako,∗1 and S. Shimoura∗1∗3∗4

FWHM =
5.4 MeV/u40

12.6 MeV/u

20−20−100102030

An experiment for evaluating the system performance was carried out at the OEDO beamline in RIBF.
A 171-MeV/nucleon 79 Se beam was produced by thein-ﬂight ﬁssion of a 345-MeV/nucleon 238 U beam impinged on a 5-mm-thick Be target and was transported
to the OEDO beamline. The degrader system at thedispersive focus (FE9) in the OEDO beamline was optimized to obtain the 40-MeV/nucleon beam. The central thickness was set to 6 mm by placing a 3-mm-thick
ﬂat Al plate behind the system.
The performance of energy compression was evaluated by comparing the outgoing energy deviations using a monoenergetic degrader (the optimized wedgeangle) to a homogeneous degrader (0 mrad setting).
Figure 2 shows that the spread in the homogeneouscase is 12.6 MeV/nucleon in full width at half maximum (FWHM). The monoenergetic degrader reduced
the spread to 5.4 MeV/nucleon, which is similar to theestimated value of 5.2 MeV/nucleon. The performanceof the system was estimated as satisfactory and its ﬂexible wedge angle is useful to deal with many diﬀerent
nuclides and energies for various purposes.

Fig. 1. 3D design of the angle-tunable degrader system.

∗260Fig. 2. Distributions of the outgoing energy spread using
a homogeneous and monoenergetic degrader. All distributions were normalized to have the same number ofevents.



∗180Outgoing Energy Deviation (MeV/u)

†Homogeneous
Monoenergetic

0−30




100

Reference

Condensed from the article in Prog. Theor. Exp. Phys.
2019, 043D02 (2019), see also references therein
Center for Nuclear Study, the University of Tokyo
RIKEN Nishina Center
Department of Advanced Energy Engineering Science,Kyushu University
Department of Physics, the University of Tokyo1) S. Michimasa et al., Prog. Theor. Exp. Phys. 2019,
043D01 (2019).

- 16 -