Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 53

New ion-optical operating modes of BigRIPS and ZeroDegree
spectrometer for production and separation of high-quality rareisotope beams and high-resolution spectrometer experiments†
H. Takeda,∗1 Y. K. Tanaka,∗2 H. Suzuki,∗1 D. S. Ahn,∗1 B. Franczak,∗2 N. Fukuda,∗1 H. Geissel,∗2E. Haettner,∗2 N. Inabe,∗1 K. Itahashi,∗1 T. Komatsubara,∗1 T. Kubo,∗1,∗3 K. Kusaka,∗1 S. Y. Matsumoto,∗4D. Murai,∗1 T. Nishi,∗1 M. Ohtake,∗1 C. Scheidenberger,∗2 Y. Shimizu,∗1 T. Sumikama,∗1 H. Ueno,∗1T. Uesaka,∗1 Y. Yanagisawa,∗1 and K. Yoshida∗1BigRIPS1) is a powerful two-stage in-ﬂight separator
that has been used for frontier experiments with exotic
nuclei for more than a decade. The ion-optical system is very versatile owing to the multi-stage structureof BigRIPS combined with the ZeroDegree spectrometer2) or the Superconducting Ring Cyclotron (SRC).
Various optical modes can be ﬂexibly realized according to the purpose of experiments. Two categories ofdevelopments are presented here.
One is a new operating mode of BigRIPS aiming ata higher ion-optical resolving power. BigRIPS itself
has a two-stage structure consisting of F0-F2 and F3F7 with a matching section F2-F3 in between. In eachstage, the isotopic separation of projectile fragments
at an overall achromatic focus is performed in the Bρ∆E-Bρ operation with an achromatic energy degrader
positioned in a dispersive focal plane. In principle, thetwo stages act independently and their isotopic separation powers can be added or subtracted dependingon the sign of the optical magniﬁcation in the matching section F2-F3. In the standard operating mode ofBigRIPS, the two spatial separations with energy degraders are subtractive. The ion-optical solution foradding the Bρ-∆E-Bρ separation power (the additivemode) is obtained by adding an x-focus at F2.5. The
spatial-separation performance of the standard and additive modes of BigRIPS is demonstrated in the simulation with diﬀerent combinations of achromatic F1and F5-degraders. With the resulting increased spatial
separation power, the isotopic background can be substantially reduced. On the other hand, the standardmode has a slightly better overall transmission and issuperior in providing an isotope cocktail focused on asmall-area detector or a secondary target. One canfavorably use either the standard or the new additivemode depending on the experimental requirements andpriorities.
Higher ion-optical resolving powers of the ﬁrst andsecond BigRIPS stages are also investigated with thegoals of further reducing the background and yielding

access to new isotopes of heavier elements. First, theresolving power of the second stage was doubled (thehigh-resolution Ddouble mode3) ). Improved particle
identiﬁcation was conﬁrmed by the ﬁrst test experiments. The simulation of a test case indicated that theincreased resolving power of both the ﬁrst and second
stages in the additive mode can reduce the backgroundto a large extent. The actual use of the proposed andprepared additive, standard, and higher-resolution ionoptical modes primarily depends on the detailed goalsof future BigRIPS experiments.
The other development is a dispersion-matched system with BigRIPS for high-resolution spectrometer experiments. The BigRIPS and ZeroDegree spectrometer are presently two independent, coupled achromaticsystems. A new dispersion-matched mode of BigRIPSand ZeroDegree will enable novel experiments. In thisexperimental scenario, the F8 focal plane is dispersive and accommodates the secondary target. The secondary target can be viewed, e.g., by the γ-ray detector
array, and thus, high-resolution momentum measurements after a removal reaction can be performed atF11 in coincidence with γ-spectroscopy.
For high-resolution spectroscopy experiments withhigh-intensity light projectiles, SRC and BigRIPS canbe operated as a dispersion-matched system. In thismode, the dispersive beamline from SRC to F0 and theBigRIPS stages up to F5 form an overall achromaticsystem. The momentum spread of high-intensity primary beams from the SRC is canceled in the position
spectra at F5, for instance, in the experiments studying pionic states. The ﬁrst experimental tests of theproposed ion-optical mode have been successfully performed.
The described diﬀerent ion-optical developments will
be a base for novel BigRIPS experiments with exotic
nuclei and mesic atoms, and thus, contribute to a better understanding of the strong interaction and properties of hadronic matter.
References†∗1
∗2
∗3
∗4

Condensed from the article in Nucl. Instrum. Methods Phys.
Res. B 463, 515 (2020)
RIKEN Nishina Center
GSI Helmholzzentrum für Schwerionenforschung GmbH
Facility for Rare Isotope Beams and National Superconducting Cyclotron Laboratory, Michigan State University
Department of Physics, Kyoto University- 101 -1) T. Kubo, Nucl. Instrum. Methods Phys. Res. B 204, 97(2003).
2) T. Kubo et al., Prog. Theor. Exp. Phys. 2012, 03C003(2012).
3) H. Suzuki et al., RIKEN Accel. Prog. Rep. 46, 155(2013).