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RIBF Users Group Thesis Awards

RIBF Users Group Thesis awards started from 2012. The awards honor the achievements of young scholars who earned his or her doctoral degree based on an RIBF experiment (using BigRIPS, GARIS, CRIB, RIPS, etc.), or a theoretical study related to physics at RIBF. The winner will be selected by the Thesis Award selection committee and be provided an opportunity to give a special presentation at the annual RIBF Users Meeting.

2024

We are pleased to announce that the RIBF Users Group Thesis Awards for 2024 will be presented to Dr. Mădălina Enciu (Technische Universität Darmstadt) for her outstanding Ph.D. thesis, entitled "Extended p3/2 neutron orbital and the N=32 shell closure in 52Ca" (Technische Universität Darmstadt, 2023). We note two major accomplishments in her thesis project. Dr. Mădălina Enciu first conducted a comprehensive investigation of the neutron shell structure of neutron-rich calcium isotopes. She successfully analyzed the 52Ca(p,pn) reaction measured at SAMURAI, integrating data from a wide range of detectors. Her analysis confirmed that 52Ca is a neutron-closed-shell nucleus. Additionally, by developing a novel method for analyzing momentum distributions, in collaboration with nuclear theorists, she indicated that the valence p3/2 neutrons in 52Ca have a large spatial extension, 0.61(23) fm larger than the f7/2 neutrons. Her thesis also includes a systematic study of the nuclei 53Ca and 54Ca where the results for the size of p and f orbitals were in agreement with her initial findings for 52Ca. The innovative method she developed has attracted considerable interest for future research activities. In the second part of her work, she made crucial contributions to the development of the silicon tracker STRASSE and its associated liquid hydrogen target, which are essential for advanced in-beam gamma and missing-mass spectroscopy. She played a key role in validating the full operation of the prototype system, particularly the readout electronics. Additionally, she advanced the development of the liquid hydrogen target, including building the necessary tools and producing target cells capable of sustaining high pressures. The target system she developed will not only benefit STRASSE experiments, but will also be applied to a variety of RIBF experiments. With best regards, Yuichi Ichikawa (RIBF-UEC chair) Hiroyoshi Sakurai (Director of Nishina Center for Accelerator-Based Science)

2023

We are pleased to announce that the RIBF Users Group Thesis Awards for 2023 will be presented to Dr. Shun Iimura (Rikkyo Univ.) and Dr. Yoshiki Chazono (RIKEN) for their outstanding contributions to the physics at RIBF. Dr. Shun Iimura receives the award for his Ph.D. thesis, entitled "Development of RF carpet type He gas cell and high-precision mass measurement of neutron-rich nuclei at around A=50−60 with MRTOF" (Osaka University, 2022). In this work, Dr. Iimura spent countless hours on the development of the helium gas stopping cell installed at the end of the ZeroDegree beamline – iteratively designing, assembling, and performing offline testing. The gas cell allows efficiently converting the relativistic beams delivered by BigRIPS to low-energy beams amenable to ion trapping experiments. With the completion of the gas cell, he devoted considerable time to the tuning of the multi-reflection time-of-flight (MRTOF) mass spectrograph – achieving mass resolving powers approaching 1,000,000. With such high resolving power, the MRTOF could achieve a mass precision of 10 keV/c2 with just 25 detected ions. Combining this with its ability to simultaneously analyze multiple species of nuclides, he was able to perform highly efficacious atomic mass measurements in parallel with in-beam gamma-decay studies being performed by separate researchers. To analyze these mass measurement data, he developed a new method of drift correction using polynomial functions to better account for fitting errors in the reference peak. The results of the high-precision atomic mass measurements showed significant deviation from previous values derived from the magnetic-rigidity measurements, and indicated a lack of N=34 shell closure for Ti and V. The high performance of MRTOF will make a harvest of the mass measurements spanning the entire nuclear chart, leading not only to the detailed studies of exotic nuclear structure but also to the understanding of nucleosynthesis in the universe. Dr. Yoshiki Chazono receives the award for his Ph.D. thesis, entitled "New reaction model for revealing deuteron inside nuclei" (RCNP, Osaka University, 2022). In this work, Dr. Chazono proposed a new reaction model for describing the proton-induced deuteron knockout reaction (p,pd) to investigate the nature of deuteron and clustering inside nuclei. Recently, the proton-induced alpha knockout reaction (p,pα) has attracted increasing interest as an essential probe for detecting alpha clustering in atomic nuclei. Similarly, the (p,pd) reaction is expected to be an efficient probe for deuteron formation inside nuclei. However, in contrast to an alpha cluster, a deuteron is a weakly-bound system, whose breakup and couplings with continuum states cannot be neglected. In the thesis, Dr. Chazono attacked this problem, developing 1) a combination of the distorted-wave impulse approximation (DWIA) with a microscopic pn-pair wave function based on a pn hole-hole random phase approximation (RPA); 2) a reaction theory for p-d elastic scattering and a deuteron breakup d(p,p)pn reactions, which are elementary processes in the (p,pd) reaction; and 3) a new theoretical framework, named the continuum-discretized coupled-channel impulse approximation (CDCCIA), which takes into account the breakup effects of a weakly-bound deuteron. With the newly developed theory, Dr. Chazono demonstrated the importance of reformations (or "back couplings") of a broken deuteron that can hinder elastic (p,pd) cross sections. In particular, his work is behind the birth of the ONOKORO project, where the nature of light clusters, such as deuteron, triton, 3He, and alpha particles, is explored via knockout reactions. His model is the key to upcoming cluster knockout experiments at RIBF. With best regards, Yuichi Ichikawa (RIBF-UEC chair) Hiroyoshi Sakurai (Director of Nishina Center for Accelerator-Based Science)

2022

We are pleased to announce that the RIBF Users Group Thesis Awards for 2022 will be presented to Dr. Toshitaka Niwase (KEK) and Dr. Tomoya Naito (RIKEN) for their outstanding contributions to the physics at RIBF. Dr. Toshitaka Niwase receives the award for his PhD thesis, titled “First direct mass measurement of superheavy nuclide via MRTOF mass spectrograph equipped with an alpha-TOF detector” (Kyushu University, 2021). In this work, Dr. Niwase presents the development of an alpha-TOF detector to equip the MRTOF mass spectrograph in order to suppress background events by tagging the alpha decay of the nucleus of interest, which enables precise mass measurements of nuclei with an extremely low production rate, such as superheavy nuclei. Dr. Niwase played a major role in developing the detector and conducting the experiments. The detector has been applied to the mass measurement of 206,207Ra and 257Db - the latter one is the first direct mass measurement of a superheavy nucleus. The results are in agreement with existing data measured with indirect methods, providing an experimental demonstration of the capability of the new technique to perform a Z and A identification with a single event only. Further, the thesis includes numerical simulations for 263Sg and 267Hs, and discusses future mass measurements of various superheavy elements and a possible application to beta decays which will be useful to search for neutron-rich superheavy elements. The technique developed in the thesis has paramount importance in future superheavy element research. Dr. Tomoya Naito receives the award for his PhD thesis, titled “On isospin symmetry breaking in nuclear density functional theory” (The University of Tokyo, 2022). In this work, Dr. Naito discusses systematically the effects of the isospin symmetry breaking (ISB) on ground states of atomic nuclei by incorporating those effects from electromagnetic and nuclear interactions as much as possible to the nuclear energy density functional (EDF) approach. In the first part of the thesis, Dr. Naito adopted the so-called generalized gradient approximation for the Coulomb EDF to handle the electromagnetic interaction more accurately, which has been used in condensed matter physics, but not in nuclear physics community. Furthermore, Dr. Naito included the nucleon finite-size effect, the vacuum polarization effect, the effect from the electromagnetic spin-orbit interaction, and the finite-light-speed correction. It is found that the nucleon finite-size effect and the vacuum polarization effect are on the order of a few MeV in 208Pb and are crucial for accurately describing nuclear masses. In the latter part of the thesis, Dr. Naito discusses the ISB effects from the nuclear interaction on ground-state properties of doubly-magic nuclei, dividing the nuclear interaction into two parts: charge symmetry breaking (CSB) and charge independence breaking (CIB) terms. It is found that the CSB component is more sensitive to the mirror energy difference and neutron-skin thickness than the CIB component. In the thesis, Dr. Naito proposed a new way to determine the CSB strength in a nuclear EDF in combination with ab initio calculations, which is an important step towards constructing an ab-initio-based EDF. Dr. Naito's work offers methodologies for quantifying the ISB effects and is of substantial importance in future developments of more reliable EDFs. With best regards, Yuichi Ichikawa (RIBF-UEC chair) Hiroyoshi Sakurai (Director of Nishina Center for Accelerator-Based Science)

2021

We are pleased to announce that the RIBF Users Group Thesis Awards for 2021 will be presented to Dr. Oscar Hall (School of Physics and Astronomy, University of Edinburgh) and Dr. Masaomi Tanaka (RIKEN Nishina Center) for their outstanding contributions to the physics at RIBF. Dr. Oscar Hall receives the award for his PhD thesis “beta-delayed neutron emission from r-process nuclei along the N=82 shell closure’’ (University of Edinburgh, 2020). In this work, as part of the BRIKEN collaboration, Dr. Hall investigated the beta-delayed neutron emission (Pn) values of 31 neutron-rich nuclei in the region of N=82, including 14 isotopes studied for the first time – these isotopes play an important role in defining the second peak of the r-process. Dr. Hall played a key role in the preparations and execution of this experimental work, including commissioning of the AIDA detector at RIBF and development of the analysis tools required for extraction of the Pn values and half-lives. In addition, Dr. Hall went beyond the experimental results to investigate the impact of the new nuclear data on r-process abundances using reaction network calculations. This work is a key result from the BRIKEN effort and highlights the importance of continued measurements of this type to continue to constrain the r-process path. Dr. Masaomi Tanaka receives the award for his PhD ‘’Determination of nuclear matter radii of Ca isotopes across the neutron magic number N=28 via interaction cross-section measurements’’ (Osaka University, 2018). In his thesis work, Dr. Tanaka measured the interaction cross-sections of the neutron-rich Ca, K, and Sc isotopes, then using the Glauber model to extract matter radii for these nuclei. Dr.Tanaka played a central role in preparation of the experiment itself, including the detector and data acquisition systems. He also took the primarily role in the analysis of the experimental data, in addition to performing many of the Glauber and related calculations. Dr.Tanaka’s work has had a significant impact in establishing the trend of matter radii beyond N=28, opening questions related to the ‘swelling’ of the 48Ca core in the heavier Ca isotopes. This is a topic which has garnered much interest in the community and will continue to do so as more neutron-rich isotopes become experimentally accessible. RIBF Thesis Award 2021 Selection Committee Members T. Abe, A. Corsi, H. Crawford(co-chair), N. Hinohara, Y. Ichikawa, S. Naimi, S. Nishimura, K. Sekizawa, N. Shimizu(co-chair), D. Suzuki, J. Zenihiro With best regards, Nobuo Hinohara (Chair of RIBF Users Group Executive Committee) Hiroyoshi Sakurai (Director of Nishina Center for Accelerator-Based Science)

2020

We are pleased to announce that the RIBF Users Group Thesis Award for 2020 will be presented to Dr. Nancy Paul (Kastler–Brossel Laboratory) for her outstanding contributions to the physics research at RIBF. Dr. Paul obtained her Ph.D. degree at Paris-Saclay University in December, 2018. Dr. Nancy Paul receives the award for her Ph.D. thesis "First spectroscopy of 110Zr with MINOS" (Paris-Saclay University in 2018). Her thesis describes the spectroscopy of 110Zr produced from the (p,2p) experiment in the SEASTAR campaign at the RIBF, RIKEN. The low-lying level structure of 110Zr was studied by the proton removal reaction using the MINOS target system at RIKEN and its deexcitation gamma rays were detected with the DALI2 scintillator array. The level structure exhibits the rotational behavior and does not support a harmonic oscillator shell stabilization scenario at Z=40 and N=70. 110Zr is one of the most exotic nuclei for which excited states are now known, and this work has inspired future experiments in this region towards the understanding of the r-process nucleosynthesis. She also performed the careful, systematic studies of the inclusive cross section and found the systematic differences between the even-Z and odd-Z projectiles, which is interpreted as due to pairing correlations in the daughter nuclei. The improved understanding of the reaction mechanism modeling not only allows for a more realistic estimation of unknown cross sections, but also provides insights into the nuclear dynamics. RIBF Thesis Award 2020 Selection Committee Members H. Crawford(co-chair), N. Hinohara, W. Horiuchi, T. Matsumoto, K. Miki, S. Nishimura, S. Paschalis, S. Sakaguchi, N. Shimizu(co-chair), D. Suzuki, Y. Watanabe, K. Wimmer With best regards, Nobuo Hinohara (Chair of RIBF Users Group Executive Committee) Hiroyoshi Sakurai (Director of Nishina Center for Accelerator-Based Science)

2019

We are pleased to announce that the RIBF Users Group Thesis Awards for 2019 will be presented to Dr. Ryo Taniuchi (University of York) for his outstanding contributions to the physics research at RIBF. Dr. Taniuchi obtained his Ph.D. degree at the University of Tokyo in March, 2019. Dr. Taniuchi receives the award for his Ph.D. thesis “In-beam gamma-ray spctroscopy of 78Ni” (University of Tokyo, 2018). His thesis describes the eperimental study of in-beam gamma-ray spectroscopy of 78Ni carried out at BigRIPS using the MINOS and DALI2 systems in the SEASTAR collaboration. The work was motivated to reveal the magicity of N=50 for the most neutron-rich nucleus (78Ni) that has been accessible so far. The thesis presents the first spectroscopy of the excited states of 78Ni. The measured gamma-ray strengths clearly show the robustness of the N=50 magicity for 78Ni, and the indication of the possible shape coexistence in its spectrum is also obtained. This experimental study calls for further discussions towards the understanding of the nature of the evolution of the shell gap in neutron-rich unstable nuclei around/beyond N=50. RIBF Thesis Award 2019 Selection Committee Members H. Crawford, N. Hinohara, W. Horiuchi (Chair), T. Matsumoto, K. Miki, S. Nishimura, S. Paschalis, S. Sakaguchi, N. Shimizu, D. Suzuki, Y. Watanabe, K. Wimmer, With best regards, Kathrin Wimmer (Chair of RIBF Users Group Executive Committee) Hideto En’yo (Director of Nishina Center for Accelerator-Based Science

2018

We are pleased to announce that the RIBF Users Group Thesis Awards for 2018 will be presented to Dr. Takahiro Nishi (RIKEN) for his outstanding contributions to the physics research at RIBF. Dr. Nishi obtained his Ph.D. degree at the University of Tokyo in 2017. Dr. Takahiro Nishi receives the award for his Ph.D. thesis "Precision spectroscopy of deeply bound pionic states in 121,116Sn” (University of Tokyo, 2017). His thesis describes the experimental study of precise spectroscopy of pionic nuclear bound states at BigRIPS. The results are directly related to chiral symmetry breaking in nuclear medium. He successfully observed the atomic 1s and 2p states of pionic-121, 116Sn, and determined precisely the “b1” parameter of the pion-nucleus optical potential. The thesis presents the first result of the “πAF” project and also describes his contributions to all the aspects of the project, a detailed design and tuning of the dispersion-matched ion optics for the Sn(d,3He) reaction. This was paramount in order to achieve the required spectral resolution. This experimental study is a milestone towards systematic study of deeply bound pionic states. Further measurements along the Sn isotope chain will reveal the nuclear density dependence of the chiral symmetry breaking. RIBF Thesis Award 2018 Selection Committee Members M. Asai, J. Gibelin, W. Horiuchi (Chair), T. Isobe, Y. Kanada-En’yo, T. Matsumoto, T. Nakamura, S. Ota, S. Sakaguchi, M. Wakasugi, Y. Watanabe, K. Wimmer, K. Yoshida, J. Zenihiro, With best regards, Kathrin Wimmer (Chair of RIBF Users Group Executive Committee) Hideto En’yo (Director of Nishina Center for Accelerator-Based Science)

2017

We are pleased to announce that the RIBF Users Group Thesis Awards for 2017 will be presented to Dr. Kazuyuki Sekizawa (Faculty of Physics, Warsaw University of Technology) and Dr. Jin Wu (Argonne National Laboratory) for their outstanding contributions to physics at RIBF. Dr. Kazuyuki Sekizawa belonged to University of Tsukuba and JSPS as a DC2 Research Fellow, and received Ph.D. in 2015. Dr. Jin Wu belonged to Peking University and RIKEN as an International Program Associate, and received Ph.D. in 2016. Dr. Kazuyuki Sekizawa receives the award for his Ph.D. thesis entitled “Multinucleon Transfer Reactions and Quasifission Processes in Time-Dependent Hartree-Fock Theory” (University of Tsukuba in 2015). The thesis presents the first extensive application of the time-dependent Hartree-Fock (TDHF) theory to the multi-nucleon transfer and quasifission processes in the low-energy heavy-ion reactions. With the particle-number-projection method, the transfer probabilities and the cross sections are investigated. This method can explain existing experimental data reasonably well. Since the method is size extendable and has no adjustable parameter, the developed framework can be applied to production reactions of unknown neutron-rich nuclei as well as superheavy elements. Dr. Jin Wu receives the award for his Ph.D. thesis "beta-decay spectroscopy of Z=55∼67 neutron-rich nuclei" (Peking University in 2016). His thesis describes the experimental study of beta-decay half-lives of heavy mid-shell nuclei via beta-decay spectroscopy in combination of the germanium detector array EURICA and the double-sided silicon detector array WAS3ABi, aiming at understanding the production mechanism of the smaller and broader peak of rare-earth elements in the solar abundance. The beta-decay half-lives of 94 neutron-rich nuclei were measured precisely, and 57 of them are measured for the first time. Newly measured half-lives give a significant contribution to describe the solar abundance pattern of rare-earth elements, which makes a strong impact on the scenario of r-process nucleosynthesis. The thesis also discuss the large drop in the systematic trend of half-lives at N=97 and N=105. With Best Regards, Tadaaki Isobe (Chair of RIBF Users Group Executive Committee) Hideto En'yo (Director of Nishina Center for Accelerator-Based Science)

2016

We are pleased to announce that the RIBF Users Group Thesis Awards for 2016 will be presented to Dr. Kosho Minomo (Research Center for Nuclear Physics, Osaka University) and Dr. Clementine Santamaria (National Superconducting Cyclotron Laboratory, Michigan State University) for their outstanding contributions to the physics at RIBF. Dr. Kosho Minomo receives the award for his PhD thesis ``Determination of deformed and halo structure of unstable nuclei by fully microscopic theory'' (Kyushu University in 2013). In this thesis, Dr. Minomo constructed a new theoretical method to microscopically derive a nucleus-nucleus optical potential. With this method, the optical potential can be obtained using a fundamental interaction with no adjustable parameters, which is a significant advantage compared to other methods and essential to investigate reaction of exotic nuclei. Using this method, he successfully reproduced the reaction cross section for Ne isotopes and concluded that the observed large cross section of 31Ne comes from its halo structure with a deformed core. The reaction theory and physics of a deformed halo nucleus proposed in this thesis will have an impact in both theoretical and experimental studies. It will open the way for further developments of physics of general reaction phenomena with exotic nuclei. Dr. Clementine Santamaria receives the award for her PhD thesis ``Quest for new nuclear magic numbers with MINOS'' (Universite Paris Sud XI in 2015). Her thesis work is 1) on the development of MINOS (MagIc Numbers Off Stability), a unique hydrogen target surrounded by a TPC to reconstruct the reaction vertex point, and 2) on the first in-beam gamma ray spectroscopy experiment using the MINOS + DALI2 setup. Dr. Santamaria played a key role in the development, construction, and implementation of MINOS, and in particular the on-line and off-line commissioning experiments. For example, the geometry and gas to be used in MINOS were determined based on her analysis and she developed the primary tracking algorithm for the TPC. The MINOS device has now become widely used at RIBF and has significantly expanded the capability of spectroscopic studies of exotic nuclei far from the stability. With Best Regards, Tadaaki Isobe (Chair of RIBF Users Group Executive Committee) Hideto En'yo (Director of Nishina Center for Accelerator-Based Science)

2015

The RIBF Users Group executive committee (UEC) and the Director of RIKEN Nishina Center are proud to announce that the RIBF Users Group Thesis Awards for 2015 have been awarded to Dr. Zhengyu Xu (Department of Physics, the University of Hong Kong) for “Beta-decay spectroscopy on neutron-rich nuclei in a range of Z = 26~32” and to Dr. Yuta Ito (RIKE Nishina Center) for “A multi-reflection time-of-flight mass spectrograph for high-precision mass measurements of short-lived nuclei”. We would like to congratulate both winners for their outstanding work. Dr. Xu receives the award for his PhD thesis "Beta-decay spectroscopy on neutron-rich nuclei in a range of Z = 26?32" (School of Science, University of Tokyo in 2014). His thesis describes on an experimental work studying β-decay properties of neutron-rich nuclei in the vicinity of 78Ni. He was a member of the EURICA collaboration and was actively involved in the initial commissioning as well as all subsequent stages of the experiment. He was in charge of developing the double-sided silicon active stoppers. He contributed the setup of the EURICA gamma-ray detector system, and conducted a test experiment of EURICA and decay-spectroscopy experiments at RIBF. He was in charge of analysis for data obtained for Ni-78 and its vicinity. From this work, half-lives of 38 neutron-rich nuclei were measured, including 72?74Fe, 76,77Co, 79,80Ni, 81,82Cu, 84Zn, 87Ga and 87,88Ge, of which the half-lives are measured for the first time. In addition, ten β-delayed neutron emission probabilities (Pn) were investigated, and the Pn values of 78Ni, 80,81Cu, 83,84Zn, and 85Ga are newly deduced. Based on the new results, information was obtained on shell evolution at Z = 28 and N = 50 and these new data are used to used to suggest 78Ni is a good doubly-magic nucleus 78Ni. Dr. Ito receives the award for his PhD thesis "A multi-reflection time-of-flight mass spectrograph for high-precision mass measurements of short-lived nuclei" (Graduate School and Applied Science, University of Tsukuba in 2014). His thesis describes a novel online mass measurement method for radioactive nuclei, especially for short-lived ones. For a Multi-Reflection Time-Of-Flight mass spectrograph (MRTOF) experiments, he developed two interfaces between a gas catcher setup and the MRTOF. One is a tapered linear RF quadrupole structure trap that accumulates continuous ion beams from the gas catcher. The other is a flat-trap made of two printed circuit boards that bunches ions from the tapered trap before injecting to the MRTOF. This was a key improvement to make an online mass measurement of 8Li+ (T1/2 = 838 ms) successful. That was the first MRTOF mass measurement of radioactive ions produced in projectile fragmentation at RIPS. He has also developed a new data analysis method of a single reference method. The speed, precision, and accuracy of the first online measurement exemplify the potential for using this new type of mass spectrograph for precision measurements of short-lived nuclei. It will open a new era of precision mass measurements of short-lived nuclei, in particular of “difficult elements” which are not available at ISOL facilities and "superheavy elements".

2013

Dr. Nobuyuki Kobayashi (University of Tokyo)

Thesis title: "Spectroscopy of Neutron-Rich Nuclei via the Inclusive Breakup Reactions" His thesis established a novel spectroscopic method by using inclusive Coulomb and nuclear breakup reactions. With this method, he has systematically studied the ground-state properties of isotopes ranging from carbon (Z=6) to sulfur (Z=14) located in the vicinity of the neutron drip line. The observed large cross sections provide evidence of neutron halo structures in 22C, 31Ne, and 37Mg. Among them, 31Ne is likely to be the first case of a p-wave halo which appears in its ground state due to changes in the shell structure. This mechanism is of particular impact in that it could affect the structure of wide region of nuclei at the limit of existence resulting in possible modification in the location of the neutron drip line. Part of his work has been published in - "One- and two-neutron removal reactions from the most neutron-rich carbon isotopes", PRC86(2012)054604 - "Halo Structure of Island of Inversion Nucleus 31Ne", PRL103(2009)262501 Best regards, Ken Yako, on behalf of Thesis Award Selection Committee

2012

Dr. Kenjiro Miki (RCNP)

The thesis entitled "Study of the isovector spin monopole resonance via the (t,3He) reactions at 300 MeV/u". A part of this work was published as K. Mkiki et al., in Phys. Rev. Lett. 108, 262503 (2012).