Ⅵ. ORGANIZATION AND ACTIVITIES OF RNC

RIKEN Accel. Prog. Rep. 56 (2023)

Research Facility Development Division
Instrumentation Development GroupSLOWRI Team
1. Abstract

The SLOWRI team develops, operates and improves SLOW RI beam production devices and related systems, and uses thesedevices and systems to conduct experimental programs at RIBF. As the heart of SLOWRI, we have developed RF carpet type cryogenic He gas catchers (RFGC). Two RFGCs are currently in operation to deliver slow RI beams converted from fast RIs producedwith fragmentation or inﬂight ﬁssion on RIPS and BigRIPS. These RFGCs are directly combined with muti-reﬂection time-of-ﬂight
mass spectrographs (MRTOF-MS) for precise mass measurements tailored to user’s experimental programs. Especially, the RFGC
combined with a MRTOF-MS installed behind ZeroDegree spectrometer of BigRIPS is being operated symbiotically reusing RIs fromother experiment conducted upstream without extra costs. To date, masses on more than 80 RIs provided with BigRIPS have beensuccessfully measured. A lot of user’s experimental programs for precise mass measurements will be conducted in SLOWRI projectsoon. An Ar gas catcher at F2 of BigRIPS was also installed to rescue RI that would otherwise be discarded without being used forexperiments. The RI is caught with the Ar gas catcher, laser resonant ionized and delivered as a slow RI beam in parasitic operating
mode (PALIS). The oﬀ- and online-commissioning of PALIS is underway.

2. Major Research Subjects(1) Development, operation and upgrade of SLOW RI beam production devices and related systems(2) Development and operation of multi-reﬂection time-of-ﬂight mass spectrographs and to conduct precision mass measurementsof short-lived nuclei(3) Development of a parasitic slow RI beam production method using resonance laser ionization

3. Summary of Research Activity(1) Development of RF carpet type cryogenic He gas catchers (RFGCs)
The fast (>100 MeV/nucleon) RI beams can be stopped in helium gas and extracted as slow (<100 eV) RI beams from a RFcarpet type cryogenic He gas cell (RFGC). At RIBF, two RFGCs are in operation at the SLOWRI project: one is a 30-cm-long RFGC
consisting of DC ring electrodes and a RF carpet. It is located behind GARIS-II at E6 of RIBF and is used in combination with aMRTOF-MS for precise mass measurements of superheavy nuclides. Also, a 9 MBq 252 Cf ﬁssion source has recently been installed
just in front of the He gas catcher. Even oﬀ-line, mass measurements on ﬁssion fragments have continued and several ﬁrst massmeasurements were performed.
Second one is a 50-cm-long RFGC installed behind ZeroDgree spectrometer of BigRIPS. This RFGC contains a three stage RFcarpet structure: a gutter RF carpet (1st carpet) for the collection thermal ions in the cell into a small slit, a narrow (about 10 mm)
traveling-wave RF-carpet (2nd carpet) for collection of ions from the gutter carpet and for transporting the ions towards the exit, and asmall RF carpet for extraction from the gas cell. The oﬀ-line test has been completed in FY2019. The on-line commissioning has beensuccessfully performed symbiotically using RIs provided with BigRIPS during HiCARI campaign in FY2020. During the on-line
commissioning, precise mass measurements were also performed. In FY2021, the ﬁrst experiment approved in NP-PAC of RNC hasbeen performed, which has aimed the mass measurement in the vicinity of the double magic nucleus of 78 Ni. As the result, masses of74
Ni and 75 Ni have been measured with high precision less than 20 keV. In FY 2022, a 248 Cm source has been installed just behindthe mylar window of the gas cell. The measured extraction eﬃciencies on the ﬁssion products emitted from the source after stoppingin the He gas have been currently reached several to about 30% of the sum of singly-, doubly- and triply-charged ions, depending onelements. Several improvements in order to improve the eﬃciencies are underway using the ﬁssion fragments from the source. Massmeasurements on experimental programs already approved in NP-PAC of RIBF will be performed soon. A ﬁnal version of the RFGC,
1 m long and with increased helium gas pressure, is also in preparation and will be installed in SD4 at BigRIPS.
(2) Conduct of precise mass measurements using MRTOF-MSs combined with RFGCs
The MRTOF, called as SHE-MASS, is combined with the RFGC behind GARIS-II of E6. Mass measurements of superheavy
elements of Db isotopes have been conducted. As a result, the mass on 257 Db superheavy nucleus was determined for the ﬁrst time.
The mass measurements on superheavy nuclide, proton rich nuclide and ﬁssion products emitted from intense Cf source have been
performed.
The MRTOF-MS, called as ZD-MRTOF, is located behind ZDS of BigRIPS in combination with the 50 cm-long RFGC. Currently,
the mass resolving power has been reached at 1 million. Since the location of the ZD-MRTOF is just in front of the beam dump ofZDS, mass measurements have been conducted symbiotically re-using RIs from other experiment conducted upstream without extra
costs. To date, the masses on more than 80 RIs provided from BigRIPS have been measured. Among them, three isotope masses havebeen measured for the ﬁrst time and mass uncertainties of eleven isotope have been signiﬁcantly improved from the previous ones.
For example, the mass uncertainties have been reduced down to the order of 10 keV for 56, 58 Ti and 56–59 V and nonexistence of the N= 34 empirical two neutron shell-gaps on Ti and V isotopes are revealed experimentally with the new precision achieved.
We have a plan to increase this versatile and portable instrumentation at RIBF: an MRTOF-MS is being installed behind GARISIII, and a plan is underway to install an MRTOF-MS with the RFGC behind SD4 at BigRIPS, which will lead to more opportunities
to study unexplored nuclear species.

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RIKEN Accel. Prog. Rep. 56 (2023)(3) Development of a parasitic slow RI beam production method using resonance laser ionization (PALIS)
More than 99% of RIs produced in projectile ﬁssion or fragmentation at BigRIPS of RIBF are simply dumped into the ﬁrst dipole
magnet and the slits. The SLOWRI project proposed a new method called PALIS, which uses a compact Ar gas catcher and resonant
laser ionization to rescue such precious RI. The thermalized RIs in a cell ﬁlled with Ar gas can be quickly neutralized and transported
to the exit of the cell by gas ﬂow. Irradiation of resonance lasers at the exit ionizes neutral RI atoms selectively. PALIS is located at
F2 of BigRIPS and is undergoing oﬀ- and on-line commissioning.
At F2, due to high radiation from a beam dump, it was found to be not easy to handle ions using electric ion guides. Therefore,a 70-cm-long gas pipe from the Ar gas cell was newly installed to transport RIs to relatively low radiation area thanks for the Ar gas
ﬂow. In FY2021, we have conﬁrmed the transport of ions of interest downstream of the ion guide behind the gas pipe using α-emitting
Ac isotopes provided with BigRIPS. Also, we have found a lot of contaminant ions from the gas cell, which are originated fromimpurities in the gas. To reduce the inﬂuence of such contaminant, a quadrupole mass ﬁlter has been installed downstream of the ion
guide. In FY2023, an on-line test for resonant laser ionization is planned.

Members
Team Leader
Hironobu ISHIYAMA
Senior Research Scientist
Takao KOJIMA
Research Scientist
Aiko TAKAMINETechnical Scientist
Tetsu SONODA
Research Consultant
Hideki IIMURA
Visiting Scientists
Minoru TANIGAKI (Kyoto Univ.)

Hideki TOMITA (Nagoya Univ.)

List of Publications & Presentations
Publications[Original Papers]
S. Iimura, M. Rosenbusch, A. Takamine, Y. Tsunoda, M. Wada, S. Chen, D. S. Hou, W. Xian, H. Ishiyama, S. Yan, P. Schury, H. Crawford,
P. Doornenbal, Y. Hirayama, Y. Ito, S. Kimura, T. Koiwai, T. M. Kojima, H. Koura, J. Lee, J. Liu, S. Michimasa, H. Miyatake,J. Y. Moon, S. Nishimura, S. Naimi, T. Niwase, A. Odahara, T. Otsuka, S. Paschalis, M. Petri, N. Shimizu, T. Sonoda, D. Suzuki,Y. X. Watanabe, K. Wimmer, and H. Wollnik, “A new study of the N = 32 and N = 34 shell gap for Ti and V by the ﬁrst high-precision
MRTOF mass measurements at BigRIPS-SLOWRI,” Phys. Rev. Lett. 130, 012501 (2023).
M. Rosenbusch, M. Wada, S. Chen, A. Takamine, S. Iimura, D. Hou, W. Xian, S. Yan, P. Schury, Y. Hirayama, Y. Ito, H. Ishiyama,S. Kimura, T. Kojima, J. Lee, J. Liu, S. Michimasa, H. Miyatake, M. Mukai, J. Y. Moon, S. Nishimura, S. Naiumi, T. Niwase,T. Sonoda, Y. X. Watanabe, and H. Wolnik, “The new MRTOF mass spectrograph following the ZeroDegree spectrometer at RIKEN’s
RIBF facility,” Nucl. Instrum. Methods Phys. Res. B 1047, 167824 (2023).
Y. Hirayama, M. Mukai, Y. X. Watanabe, P. Schury, H. Nakada, J. Y. Moon, T. Hashimoto, S. Iimura, S. C. Jeong, M. Rosenbusch,
M. Oyaizu, T. Niwase, M. Tajima, A. Taniguchi, M. Wada, and H. Miyatake, “In-gas-cell laser resonance ionization spectroscopy of200, 201Pt,” Phys. Rev. C 106, 034326 (2022).

Presentations[International Conferences/Workshops]
S. Iimura (invited), “A new study of N = 34 subshell structure probed by high-precision mass measurements,” JSPS/NSFC/NRF A3Foresight Program, “Nuclear Physics in the 21st Century,” Osaka, Japan, February 13–15, 2023.
A. Takamine, M. Rosenbusch, M. Wada, S. Iimura, D. Hou, W. Xian, S. Chen, J. M. Yap, H. Ishiyama, P. Schury, S. Nishimura, T. Niwase,
S. Kimura, Y. Hirayama, Y. Ito, T. M. Kojima, J. Lee, J. Liu, S. Michimasa, H. Miyatake, J. Y. Moon, M. Mukai, S. Naimi, T. Sonoda,
H. Ueno, P. Vi, Y. X. Watanabe, S. Yan, T. T. Yeung, and H. Wollnik, “Development of the new helium gas catcher and nuclear massmeasurements with the new MRTOF-MS system behind the ZeroDegree spectrometer at RIKEN BigRIPS,” The 19th International
Conference on Electromagnetic Isotope Separators and Related Topics (EMIS 2022), Daejong, Korea, October 3–7, 2022.
M. Rosenbusch, M. Wada, S. Chen, A. Takamine, S. Iimura, D. Hou, W. Xian, S. Yan, P. Schury, Y. Hirayama, Y. Ito, H. Ishiyama,
S. Kimura, J. Lee, J. Liu, S. Michimasa, H. Miyatake, J. Y. Moon, S. Nishimura, S. Naimi, T. Niwase, Y. X. Watanabe, P. Vi, H. Wollnik, and J. M. Yap, “High-precision MRTOF mass measurements of radioactive isotopes at RIKEN’s RIBF facility: Recent projects
for mirror potentials, wideband mass accuracy, and ion selection,” The 19th International Conference on Electromagnetic Isotope
Separators and Related Topics (EMIS 2022), Daejong, Korea, October 3–7, 2022.

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RIKEN Accel. Prog. Rep. 56 (2023)

M. Rosenbusch, “Exploring exotic nuclei by high-precision MRTOF mass measurements: The new ion catcher and mass spectrographat RIKEN’s RIBF facility,” The 8th International Conference on Trapped Charged Particles and Fundamental Physics (TCP 2022),
Glashütten, Germany, September 25–30, 2022.
H. Ishiyama, “Toward mass measurement of neutron-rich nuclei in the vicinity of N = 126,” Workshop for Uniqueness of the 208 Pb beam
at RIBF, Online, September 27, 2022.
M. Rosenbusch (invited), “New nuclear masses, recent and present developments, and future opportunities of the MRTOF-MS at theZeroDegree spectrometer,” RIBF Users Meeting 2022, Online, September 20–22, 2022.
S. Iimura (invited), “A new study of the N = 32 and N = 34 shell gap for Ti and V by the ﬁrst high-precision MRTOF mass measurements
at BigRIPS-SLOWRI,” RIBF Users Meeting 2022, Online, September 20–22, 2022.
A. Takamine (invited), “Nuclear mass measurements with the new MRTOF-MS system at the ZeroDegree spectrometer of BigRIPS,”
28th International Nuclear Physics Conference (INPC 2022), Cape Town, South Africa, September 11–16, 2022.
H. Ishiyama (invited), “Present status of SLOWRI,” SSRI-PNS collaboration meeting 2022, Online, September 1–2, 2022.

[Domestic Conferences/Workshops]
M. Rosenbusch, S. Iimura, A. Takamine, Y. Tsunoda, M. Wada, S. Chen, D. S. Hou, W. Xian, H. Ishiyama, S. Yan, P. Schury, H. Crawford,P. Doornenbal, Y. Hirayama, Y. Ito1, S. Kimura, T. Koiwai, T. M. Kojima, H. Koura, J. Lee, J. Liu, S. Michimasa, H. Miyatake,J. Y. Moon, S. Naimi, S. Nishimura, T. Niwase, A. Odahara, T. Otsuka, S. Paschalis, M. Petri, N. Shimizu, T. Sonoda, D. Suzuki,
Y. X. Watanabe, K. Wimmer, and H. Wollnik, “The SLOWRI/MRTOF-MS project at BigRIPS/ZeroDegree,” 日本物理学会 2022 年度
春季年会, オンライン, 2023 年 3 月 23–25 日.
飯村俊, M. Rosenbusch, 高峰愛子, 和田道治, S. Chen, D. Hou, J. Liu, W. Xian, S. Yan, P. Schury, 木村創大, 庭瀬暁隆, 伊藤由太, 園田哲, 小島隆夫, 渡辺裕, 平山賀一, 宮武宇也, S. Naimi, 道正新一郎, 西村俊二, 小田原厚子, 石山博恒, 角田佑介, 清水則孝, 大塚孝治,「BigRIPS SLOWRI における ZD-MRTOF 装置を用いた中性子過剰 Sc, Ti, V 核の系統的核構造研究」, 日本物理学会 2022 年度秋季大会, 岡山市 (岡山理科大学), 2022 年 9 月 6–8 日.
庭瀬暁隆, 渡辺裕, 平山賀一, 向井もも, P. Schury, A. N. Andreyev, 飯村俊, 石山博恒, 鄭淳讃, 宮武宇也, M. Rosenbusch, 谷口秋洋,
和田道治, 「精密質量測定による新同位体 241 U の発見」
, 日本物理学会 2022 年度秋季大会, 岡山市 (岡山理科大学), 2022 年 9 月
6–8 日.

[Seminar]

M. Rosenbusch (invited), “A ﬁrst review of the SLOWRI-MRTOF mass spectrograph following the ZeroDegree spectrometer at BigRIPS,”RIBF seminar, Wako, Japan, February 14, 2023.

Press Release

マルコローゼンブッシュ, 飯村俊, 他, 「チタン・バナジウム中性子過剰同位体で新魔法数の消失を観測—精密質量測定による原子核構造のより深い理解に期待—」, KEK, RIKEN, 大阪大学, 2023 年 1 月 6 日.

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