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Masato Asai Last modified date:2021.10.30

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Research Group for Heavy Element Nuclear Science, Advanced Science Research Center, Japan Atomic Energy Agency .
Academic Degree
Country of degree conferring institution (Overseas)
Field of Specialization
Nuclear Physics (Experiment)
Total Priod of education and research career in the foreign country
Outline Activities
1. Nuclear structure studies for heavy and superheavy nuclei
Nuclear structure of heavy and superheavy nuclei are studied experimentally with alpha-decay spectroscopy and gamma-ray spectroscopy at JAEA Tandem Accelerator Facility using an on-line isotope separator (ISOL) and a gas-jet transport technique, and also using the gas-filled recoil ion separator GARIS at RIKEN Nishina Center.

2. Studies on chemical properties of superheavy elements
Chemical properties of superheavy elements are studied experimentally. Compound formation of elements 104 (Rf), 105 (Db), and 106 (Sg) have been studied at JAEA Tandem Accelerator and at RIKEN Nishina Center, and adsorption properties of elements 112 (Cn), 113 (Nh), 114 (Fl), and 115 (Mc) are being measured at GSI in Germany. The first ionization potential of element 103 (Lr) was measured for the first time with the newly developed surface ionization technique.

3. Studies on nuclear fission mechanism for superheavy nuclei
The abrupt change of fission fragment mass distribution from typical asymmetric mass distribution to sharp symmetric one observed in the region of neutron-rich Fm isotopes is investigated to clarify the mechanism of fission.
Research Interests
  • Nuclear structure studies for heavy and superheavy nuclei
    keyword : Superheavy Nuclei, Heavy Actinide Nuclei, Nuclear Structure, Nuclear Spectroscopy
  • Experimental studies on chemical properties of superheavy elements
    keyword : Superheavy Elements, Periodic Table of the Elements, Chemical Properties, Ionization Energy
  • Studies on nuclear fission mechanism for superheavy nuclei
    keyword : Nuclear Fission, Symmetric Fission, Superheavy Nuclei
Academic Activities
1. M. Asai, F. P. Hassberger, A. Lopez-Martens, Nuclear structure of elements with 100 <= Z <= 109 from alpha spectroscopy, NUCLEAR PHYSICS A, 10.1016/j.nuclphysa.2015.06.011, 944, 308-332, 2015.12, Significant technical progress concerning the availability of intense heavy-ion beams and highly-efficient and sophisticated detection devices has made nuclear-structure investigations possible in the region of superheavy nuclei. Exciting new results have been obtained by applying alpha spectroscopy as well as alpha-gamma and internal-conversion-electron coincidence spectroscopy. The present review article gives an overview of the experimental techniques and methods with specific attention to the recent developments of digital signal and data processing giving access to half-life ranges of less than a few microseconds. The presentation of the experimental results and the physics discussion will be focused on nuclear structure systematics in even-Z nuclei along the N = 151, 153, and 155 isotonic lines, where most progress has been achieved in the last 10 years. (C) 2015 Elsevier B.V. All rights reserved..
2. T. K. Sato, M. Asai, A. Borschevsky, T. Stora, N. Sato, Y. Kaneya, K. Tsukada, Ch E. Duellmann, K. Eberhardt, E. Eliav, S. Ichikawa, U. Kaldor, J. V. Kratz, S. Miyashita, Y. Nagame, K. Ooe, A. Osa, D. Renisch, J. Runke, M. Schaedel, P. Thoerle-Pospiech, A. Toyoshima, N. Trautmann, Measurement of the first ionization potential of lawrencium, element 103, NATURE, 10.1038/nature14342, 520, 7546, 209-U153, 2015.04, The chemical properties of an element are primarily governed by the configuration of electrons in the valence shell. Relativistic effects influence the electronic structure of heavy elements in the sixth row of the periodic table, and these effects increase dramatically in the seventh row including the actinides even affecting ground-state configurations(1,2). Atomic s and p(1/2) orbitals are stabilized by relativistic effects, whereas p(3/2), d and f orbitals are destabilized, so that ground-state configurations of heavy elements may differ from those of lighter elements in the same group. The first ionization potential (IP1) is a measure of the energy required to remove one valence electron from a neutral atom, and is an atomic property that reflects the outermost electronic configuration. Precise and accurate experimental determination of IP1 gives information on the binding energy of valence electrons, and also, therefore, on the degree of relativistic stabilization. However, such measurements are hampered by the difficulty in obtaining the heaviest elements on scales of more than one atom at a time(3-5). Here we report that the experimentally obtained IP1 of the heaviest actinide, lawrencium (Lr, atomic number 103), is 4.96(-0.07)(+0.08) electronvolts. The IP1 of Lr was measured with Lr-256. (half-life 27 seconds) using an efficient surface ion-source and a radioisotope detection system coupled to a mass separator. The measured IP1 is in excellent agreement with the value of 4.963(15) electronvolts predicted here by state-of-the-art relativistic calculations. The present work provides a reliable benchmark for theoretical calculations and also opens the way for IP1 measurements of superheavy elements (that is, transactinides) on an atom-at-a-time scale..
3. J. Even, A. Yakushev, Ch E. Duellmann, H. Haba, M. Asai, T. K. Sato, H. Brand, A. Di Nitto, R. Eichler, F. L. Fan, W. Hartmann, M. Huang, E. Jaeger, D. Kaji, J. Kanaya, Y. Kaneya, J. Khuyagbaatar, B. Kindler, J. V. Kratz, J. Krier, Y. Kudou, N. Kurz, B. Lommel, S. Miyashita, K. Morimoto, K. Morita, M. Murakami, Y. Nagame, H. Nitsche, K. Ooe, Z. Qin, M. Schaedel, J. Steiner, T. Sumita, M. Takeyama, K. Tanaka, A. Toyoshima, K. Tsukada, A. Tuerler, I. Usoltsev, Y. Wakabayashi, Y. Wang, N. Wiehl, S. Yamaki, Synthesis and detection of a seaborgium carbonyl complex, SCIENCE, 10.1126/science.1255720, 345, 6203, 1491-1493, 2014.09, Experimental investigations of transactinoide elements provide benchmark results for chemical theory and probe the predictive power of trends in the periodic table. So far, in gas-phase chemical reactions, simple inorganic compounds with the transactinoide in its highest oxidation state have been synthesized. Single-atom production rates, short half-lives, and harsh experimental conditions limited the number of experimentally accessible compounds. We applied a gas-phase carbonylation technique previously tested on short-lived molybdenum (Mo) and tungsten (W) isotopes to the preparation of a carbonyl complex of seaborgium, the 106th element. The volatile seaborgium complex showed the same volatility and reactivity with a silicon dioxide surface as those of the hexacarbonyl complexes of the lighter homologs Mo and W. Comparison of the product's adsorption enthalpy with theoretical predictions and data for the lighter congeners supported a Sg(CO)(6) formulation..
4. M. Asai, K. Tsukada, M. Sakama, H. Haba, T. Ichikawa, Y. Ishii, A. Toyoshima, T. Ishii, I. Nishinaka, Y. Nagame, Y. Kasamatsu, M. Shibata, Y. Kojima, H. Hayashi, Ground-state configuration of the N=157 nucleus No-259, PHYSICAL REVIEW C, 10.1103/PhysRevC.87.014332, 87, 1, 014332-1-6, 2013.01, The ground-state configuration of the N = 157 nucleus No-259 has been identified through alpha-gamma coincidence and alpha-singles measurements. Three gamma transitions were observed for the first time in the alpha decay of No-259, and its decay scheme was established. The neutron 9/2(+)[615] configuration was assigned to the ground state of No-259 as well as to the 231.4 keV level in Fm-255. Ground-state deformations and neutron single-particle energies in Z = 102 isotopes were calculated with the macroscopic-microscopic model. The 9/2(+)[615] orbital was calculated to be the highest among the five orbitals between the N = 152 and 162 deformed shell gaps, which is consistent with the experimental one-quasiparticle energies in N = 153 and 155 isotones, but is inconsistent with the present experimental result of the 9/2(+)[615] ground state at N = 157. To reproduce the 9/2(+)[615] ground state at N = 157, the order of the neutron orbitals should be different between the N = 153 and 157 isotones. DOI: 10.1103/PhysRevC.87.014332.
5. M. Asai, K. Tsukada, H. Haba, Y. Ishii, T. Ichikawa, A. Toyoshima, T. Ishii, Y. Nagame, I. Nishinaka, Y. Kojima, K. Sueki, Neutron one-quasiparticle states in Fm-251(151) populated via the alpha decay of No-255, PHYSICAL REVIEW C, 10.1103/PhysRevC.83.014315, 83, 1, 014315-1-12, 2011.01, Excited states in Fm-251 populated via the alpha decay of No-255 are studied in detail through alpha-gamma coincidence and alpha fine-structure measurements. Five excited states reported previously in Fm-251 are firmly established through the alpha-gamma coincidence measurement, and rotational bands built on one-quasiparticle states are newly established through the alpha fine-structure measurement. Spin-parities and neutron configurations of the excited states in Fm-251 as well as the ground state of No-255 are definitely identified on the basis of deduced internal conversion coefficients, lifetimes of gamma transitions, rotational-band energies built on one-quasiparticle states, and hindrance factors of alpha transitions. It is found that the excitation energy of the 1/2(+)[620] state in N = 151 isotones increases with the atomic number, especially at Z >= 100, while that of the 1/2(+)[631] state decreases at Z = 100. Ground-state deformations and energies of neutron one-quasiparticle states in the N = 151 isotones are calculated using a macroscopic-microscopic model, and the energy systematics of the one-quasiparticle states in the isotones are discussed in terms of the evolution of nuclear deformation involving the hexadecapole (beta(4)) and hexacontatetrapole (beta(6)) deformations..
6. Asai M., Tsukada K., Sakama M., Ichikawa S., Ishii T., Nagame Y., Nishinaka I., Akiyama K., Osa A., Oura Y., Sueki K., Shibata M., Experimental Identification of Spin-Parities and Single-Particle Configurations in 257No and Its alpha-Decay Daughter 253Fm, PHYSICAL REVIEW LETTERS, 10.1103/PhysRevLett.95.102502, 95, 10, 102502-1-4, 2005.09.
7. Asai M, Sakama M, Tsukada K, Ichikawa S, Haba H, Nishinaka I, Nagame Y, Goto S, Kojima Y, Nakahara H, Oura Y, Shibata M, Kawade K, EC and alpha decays of 235Am, EUROPEAN PHYSICAL JOURNAL A, 10.1140/epja/i2004-10044-6, 22, 3, 411-416, 2004.12.
8. Asai M., Sakama M., Tsukada K., Ichikawa S., Haba H., Nishinaka I., Nagame Y., Goto S., Kojima Y., Oura Y., Nakahara H., Shibata M., Kawade K., Proton-neutron configurations in 236g,mAm and its EC-decay daughter 236Pu, EUROPEAN PHYSICAL JOURNAL A, 10.1140/epja/i2004-10096-6, 23, 3, 395-400, 2004.12.
9. Asai M., Sekine T., Osa A., Koizumi M., Kojima Y., Shibata M., Yamamoto H., Kawade K., Energy systematics of low-lying 0+ states in neutron-deficient Ba nuclei, PHYSICAL REVIEW C, 10.1103/PhysRevC.56.3045, 56, 6, 3045-3053, 1997.12.
10. Asai M., Ichikawa S., Tsukada K., Sakama M., Shibata M., Kojima Y., Osa A., Nishinaka I., Nagame Y., Kawade K., Tachibana T., Beta-decay half-lives of new neutron-rich isotopes 167,168Tb and levels in 167,168Dy, PHYSICAL REVIEW C, 10.1103/PhysRevC.59.3060, 59, 6, 3060-3065, 1999.06.
Membership in Academic Society
  • The Japan Society of Nuclear and Radiochemical Sciences
  • The Physical Society of Japan
  • Poster Title: Alpha Fine Structure Spectroscopy for Heavy- and Transactinide Nuclei