II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Trace-back method for dispersion matching conditions of primarybeams at RIBF
A. Sakaue,∗1 D. S. Ahn,∗1 H. Baba,∗1 N. Fukuda,∗1 N. Fukunishi,∗1 N. Inabe,∗1 K. Itahashi,∗1 K. Kusaka,∗1T. Nishi,∗1 Y. Shimizu,∗1 T. Sumikama,∗1 H. Suzuki,∗1 H. Takeda,∗1 T. Uesaka,∗1 Y. Yanagisawa,∗1K. Yoshida,∗1 S. Y. Matsumoto,∗2,∗3 R. Sekiya,∗2,∗3 Y. K. Tanaka,∗3 K. Yako,∗4 and H. Geissel∗5∗1∗2∗3∗4∗5

RIKEN Nishina Center
Department of Physics, Kyoto University
RIKEN Cluster for Pioneering ResearchCNS, University of Tokyo
GSI Helmholtzzentrum für Schwerionenforschung GmbH

800

10700

xF0 [mm]

600
5500
400

0300
200

−5−0.2

100
−0.10
δ [%]

0.1

100300250

xF0 [mm]

We are preparing for an experiment to search for
double Gamow-Teller giant resonance (RIBF-141R1,
DGTGR) at RIBF. We use a part of BigRIPS, F0-F5,
as a spectrometer;1) this is a common setup for spectroscopic experiments at the pionic Atom Factory (RIBF135R1, piAF).
In these experiments, the energy spread of the primary beam has the largest contribution to the energy
resolution. To remedy this situation, we are developing
dispersion-matching optics, where the energy spread iscanceled out under the condition that the dispersion at
the target position F0 (x|δ)F0 is 34.1 mm/% for DGTGR
and 44.6 mm/% for piAF.
In order to satisfy these conditions with suﬃcient precision, we need to develop diagnostic methods of thephase distributions at F0 based on tracking informationand the tuning method by using the optical elementsin the upper stream. Owing to severe radiation conditions, detectors in the upstream sections require radiation hardening, and the information obtained in the section is limited. Here, we employ the trace-back method
to obtain the phase distributions from distributions measured by tracking detectors at the F3, F5, and F7 planes.
In this method, a set of the horizontal position x, angle
a, and relative momentum deviation from the reference
particle δ (= δp/p) at F3 is converted to one at a certain plane by multiplying with a transfer matrix. Thex and a at F3 are instantly obtained by measuring thetrajectory of the beam using the tracking detectors. The
δ is deduced from the horizontal positions x at F5 andF7, which are dispersive and achromatic focal planes,respectively.
We attempted to apply the method by analyzing
data taken in June 2018 for study of the superconducting ring cyclotron-BigRIPS (SRC-BigRIPS) opticalsystem. A primary beam of 18 O8+ with an energy of230 MeV/nucleon was transported to F7. The beam trajectory was measured by parallel-plate avalanche counters (PPACs) at F3, F5, and F7 for diﬀerent settings ofoptics between SRC and F0.
We obtained reconstructed phase distributions at F0by using the trace-back method. Figure 1 shows thedistributions at F0 for two diﬀerent settings of optics,
which are (x|δ)F0 = 34.1 mm/% for the upper panel
and 44.6 mm/% for the lower panel. Each slope corresponds to the dispersion at F0, and the deduced values

5200
150010050−5−0.2−0.10δ [%]0.10Fig. 1. Correlation between the deduced x and δ at F0 for
two diﬀerent settings: (x|δ)F0 = 34.1 mm/% (upperpanel) and (x|δ)F0 = 44.6 mm/% (lower panel). Fitting
results obtained using linear functions are shown with redlines. Each slope corresponds to the deduced dispersion.

are 30.8 mm/% and 37.3 mm/%, respectively. The ﬁtregion corresponds to the momentum spread for the welltuned beam (σ ∼ 0.03%).2) The precision of determination of the dispersion at F0 is approximately 3 mm/%,which is suﬃcient as the experimental resolution. The
absolute values of dispersion in this measurement wereslightly diﬀerent from the ideal values. By referring to
the reconstructed distribution, we can tune the optics
so as to cancel the diﬀerence. We are now reﬁning thetuning method to provide an online feedback based on
the diagnosis.
In summary, we have developed the trace-backmethod for tuning the dispersion at F0 so as to fulﬁll
the matching condition. We are now working on an optimization of the tuning method for the beam time.
References

1) T. Nishi et al., Nucl. Instrum. Methods Phys. Res. B 317,290 (2013).
2) T. Nihsi, Doctoral Thesis, University of Tokyo (2015).
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