Ⅵ. ORGANIZATION AND ACTIVITIES OF RNC

RIKEN Accel. Prog. Rep. 56 (2023)

Research Facility Development Division
Instrumentation Development Group
Rare RI-ring Team
1. Abstract

The aim of Rare-RI Ring (R3) is to measure the masses of short-lived unstable nuclei far from the beta-stability line. In particular,a high-precision mass measurement for nuclei located around the r-process path (rare-RI) is required in nucleosynthesis point of view.
Through the commissioning experiments by 2017, we conﬁrmed the high ability of R3 as a storage ring capable of handling one event,and demonstrated that it is possible to perform the time-of-ﬂight Isochronous Mass Spectrometry (IMS) in shorter than 1 millisecond.
In 2018, we performed mass measurement experiments for the ﬁrst time. In 2020, the kicker system was modiﬁed to ﬂatten themagnetic ﬁeld distribution, and the performance study using unstable nuclei was successfully conducted. In 2021, we performed massmeasurement experiments using the upgraded kicker system. The results of the experiment dispelled concerns about the results of theprevious experiment that the injection angle might greatly aﬀected the accuracy of the mass determination. On the other hand, a failure
occurred in the kicker power supply that interfered with the progress of experiments. The cause of the failure was investigated andresolved in this ﬁscal year. We have plan to conduct a beam test to conﬁrm the stable operation of the kicker system before resuming
the mass measurement experiments.

2. Major Research Subjects(1) Further improvement of mass measurement eﬃciency and precision(2) Precision mass measurement for rarely produced isotopes related to r-process

3. Summary of Research ActivityIn the commissioning experiments up to 2017, we conﬁrmed the unique performances of R3 and demonstrated the time-of-ﬂight
isochronous mass measurement method. We have realized in forming the precise isochronous ﬁeld of less than 5 ppm with wide
momentum range of ∆p/p = ±0.5%. Another performance required for R3 is to eﬃciently seize hold of an opportunity of the massmeasurement for rare-RIs produced unpredictably. It was realized by constructing the Isotope-Selectable Self-trigger Injection (ISSI)
scheme which pre-identiﬁed rare-RI itself triggers the injection kicker magnets. Key device was a fast response kicker system that has
been successfully developed. Full activation of the kicker magnetic ﬁeld can be completed within the ﬂight time of the rare-RI froman originating point (F3 focal point in BigRIPS) of the trigger signal to the kicker position in R3.
Since R3 circulates, in principle, only one event, we fabricated high-sensitive beam diagnostic devices in the ring. One of them isa cavity type of Schottky pick-up installed in a straight section of R3. The Schottky pick-up successfully monitored a single 78 Kr36+
ion circulation with the measurement time of less than 10 milliseconds in the ﬁrst commissioning experiment. We also conﬁrmed that
it is useful for ﬁne tuning of the isochronous ﬁeld.
We performed mass measurement in the third commissioning experiment by using unstable nuclei which masses are well-known.
The masses of 79 As, 77 Ga, 76 Zn, and 75 Cu relative to 78 Ge were deduced with the accuracy of several ppm. In addition, we have
improved the extraction eﬃciency to 2% by considering the matching condition between the emittance of injection events and theacceptance of R3 in the fourth commissioning experiment. This extraction eﬃciency was suﬃcient to conduct the accepted two
proposals: mass measurements of Ni isotopes and south-west region of 132 Sn.
In November 2018, we conducted the ﬁrst experiment using R3 to measure the masses for 74, 76 Ni in 4 days. After that, we alsomeasured the masses for 122 Rh, 123, 124 Pd, and 125 Ag in 4.5 days. These nuclei were successfully extracted from R3 with the eﬃciencyof 1–2%. However, unexpected deviation from the evaluated values of literature remained in the masses obtained by detailed analysis.
This was thought to be due to the following two reasons. One is that due to the kicker ﬁeld distribution is not ﬂattened, the injection
angle is diﬀerent between the reference and target nucleus, and therefore the relative value of TOF is incorrect. The other is that theabsolute value of beta or magnetic rigidity determined for each extracted event is incorrect.
In 2020, we modiﬁed the kicker system to ﬂatten the magnetic ﬁeld distribution as well as to dispel the concerns of the results ofﬁrst experiment. As a result of performance study using unstable nuclei, we succeeded in forming the kicker ﬁeld with 100 ns ﬂat-top
for injection and long ﬂat-top of 350 ns or more for extraction. The experimental eﬃciency had been improved by a factor of two or
more than previous condition because all nuclides can be extracted at once thanks to the long ﬂat-top.
Using this upgraded kicker system, we measured the mass for 74 Ni again in April 2021. Although the kicker ﬁeld distribution wasﬂattened, the results of masses were the same as those of the ﬁrst experiment. In other words, concerns about the eﬀects of diﬀerences
in injection angles have been dispelled. As a subsequent analysis, it was clariﬁed that the reason is the second concern described above.
Recently, the mass of 123 Pd was determined precisely and its eﬀect on heavy element synthesis was investigated with collaborators.
We concluded that the composition around 123 Pd observed in the solar system can only be reproduced using the new mass values, andpublished a paper in early 2022. The ﬁnal mass values of other measured nuclei, such as Ni-isotopes, will be determined soon.
The insulation breakdown of a ceramic capacitor used as the charging circuit of the kicker system, which occurred frequently
during the experiment conducted in November 2021, signiﬁcantly aﬀected the progress of the experiment. We investigated the causeof the problem, but found no external factors, and considered the problem to be the capacitor itself. It was diﬃcult to determine theexact cause due to so severe damage, but it was found that the central part of the three-layer structure was particularly damaged inside
the molded resin. Therefore, we decided to use a single-layer capacitor to avoid overheating of the central part. A ﬁve-days continuous
test was conducted in November 2022 to compare the old and new capacitors. As a result, the old capacitor failed again while the new- 306 -Ⅵ. ORGANIZATION AND ACTIVITIES OF RNC

RIKEN Accel. Prog. Rep. 56 (2023)

capacitor was ﬁne. We will replace all the capacitors and conduct a performance test of the kicker system in 2023.

Members
Team Leaders
Tetsuya OHNISHI

Masanori WAKASUGITechnical Scientist
Yoshitaka YAMAGUCHI

List of Publications & Presentations
Publications[Original Papers]
H. F. Li, S. Naimi, T. M. Sprouse, M. R. Mumpower, Y. Abe, Y. Yamaguchi, D. Nagae, F. Suzaki, M .Wakasugi, H. Arakawa, W. B. Dou,D. Hamakawa, S. Hosoi, Y. Inada, D. Kajiki, T. Kobayashi, M .Sakaue, Y. Yokoda, T. Yamaguchi, R. Kagesawa, D. Kamioka,T. Moriguchi, M. Mukai, A. Ozawa, S. Ota, N. Kitamura, S. Masuoka, S. Michimasa, H. Baba, N. Fukuda, Y. Shimizu, H. Suzuki,H. Takeda, D. S. Ahn, M. Wang, C. Y. Fu, Q. Wang, S. Suzuki, Z. Ge, Yu. A. Litvinov, G. Lorusso, P. M. Walker, Zs. Podolyak,and T. Uesaka, “First application of mass measurements with the Rare-RI Ring Reveals the solar r-process abundance at A = 122 andA = 123,” Phys. Rev. Lett. 128, 152701 (2022).
ナイミ・サラ, リ・ホンフー, 山口由高, 上坂友洋, 「元素起源の謎解明に向けた超高速質量測定法の実現」, Isotope News 785, 32(2023).

Presentations[Domestic Conferences/Workshops]

山口貴之 (招待講演), 「多価イオンビームによる２光子稀崩壊の観測」, 新学術領域研究「宇宙観測検出器と量子ビームの出会い.
新たな応用への架け橋」, 領域研究会, オンライン, 2022 年 5 月 20–21 日.
篠崎稔 (口頭発表), 「稀少 RI リングのための GAGG:Ce シンチレータを用いた重イオンビーム飛行時間兼全エネルギー検出器の開発」, 日本物理学会 2022 年秋季大会, 岡山市 (岡山理科大学), 2022 年 9 月 6–8 日.
神田真矩 (口頭発表), 「稀少 RI リングのためのプラスチックシンチレータと WLS ファイバーを用いた位置検出器の開発」, 日本
物理学会 2022 年秋季大会, 岡山市 (岡山理科大学), 2022 年 9 月 6–8 日.
大久保研吾 (口頭発表), 「デルタ線を利用した重イオンビーム飛行時間検出器の開発」, 日本物理学会 2023 年秋季大会, オンライン, 2023 年 3 月 15–19 日.
佐々木健太 (口頭発表), 「稀少 RI リングのためのシンチレーションファイバーを用いた位置検出器の開発」, 日本物理学会 2023
年秋季大会, オンライン, 2023 年 3 月 15–19 日.

Press Release「元素起源の謎の解明に向けた世界最速質量測定が始動—稀少 RI リングを用いた短寿命放射性同位体の質量測定に成功—」 , 理
化学研究所, 筑波大学, 埼玉大学, 東京大学, 2022 年 4 月 28 日.

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