{"id":791,"date":"2025-01-20T06:54:34","date_gmt":"2025-01-20T06:54:34","guid":{"rendered":"https:\/\/www.cns.s.u-tokyo.ac.jp\/wp-homepage\/?page_id=791"},"modified":"2025-10-15T23:19:38","modified_gmt":"2025-10-15T23:19:38","slug":"structure-dynamics","status":"publish","type":"page","link":"https:\/\/www.cns.s.u-tokyo.ac.jp\/cns\/en\/research\/structure-dynamics\/","title":{"rendered":"Nuclear Structure and Dynamics"},"content":{"rendered":"
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Nuclear Structure<\/p>\n\n\n\n

Dynamics<\/p>\n<\/div><\/div>\n\n\n\n

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Nuclear Structure<\/p>\n\n\n\n

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\u539f\u5b50\u6838\u306f\u967d\u5b50\u6570\u3084\u4e2d\u6027\u5b50\u6570\u304c\u5909\u5316\u3055\u305b\u305f\u6642\u306b\u3001\u5f93\u6765\u306e\u5e38\u8b58\u3092\u8986\u3059\u73fe\u8c61\u304c\u767a\u73fe\u3057\u307e\u3059\u3002 \u81ea\u7136\u306b\u5b89\u5b9a\u306b\u5b58\u5728\u3059\u308b\u539f\u5b50\u6838\u3067\u306f\u3001\u30b9\u30d4\u30f3\u8ecc\u9053\u89d2\u904b\u52d5\u91cf\u76f8\u4e92\u4f5c\u7528\u306b\u3088\u3063\u3066\u4f5c\u3089\u308c\u308b\u79e9\u5e8f\u304c \u967d\u5b50\u6570\u3084\u4e2d\u6027\u5b50\u6570\u304c\u30a2\u30f3\u30d0\u30e9\u30f3\u30b9\u306a\u539f\u5b50\u6838\u3067\u306f\u3001\u30c6\u30f3\u30bd\u30eb\u529b\u3084\u3001\u56db\u91cd\u6975\u76f8\u95a2\u304c\u50cd\u304d\u3001\u79e9\u5e8f\u306e\u69d8\u76f8\u304c\u5909\u308f\u3063\u3066\u304d\u307e\u3059\u3002\u6700\u8fd1\u3067\u306f\u3001\u5909\u5f62\u7403\u5f62\u306e\u5f62\u72b6\u76f8\u8ee2\u79fb\u3068\u3044\u3063\u305f\u73fe\u8c61\u3082\u5831\u544a\u3055\u308c\u3066\u304d\u307e\u3057\u305f\u3002 \u9ad8\u7cbe\u5ea6\u306e\u8cea\u91cf\u6e2c\u5b9a\u3001\u30a4\u30f3\u30d3\u30fc\u30e0\u30ac\u30f3\u30de\u7dda\u6838\u5206\u5149\u3001\u967d\u5b50\u5171\u9cf4\u6563\u4e71\u7b49\u3001\u69d8\u3005\u306a\u30d7\u30ed\u30fc\u30d6\u3092\u7528\u3044\u3066\u3001 \u591a\u9762\u7684\u306b\u3001\u539f\u5b50\u6838\u3092\u69cb\u6210\u3059\u308b\u79e9\u5e8f\u3092\u53f8\u308b\u7269\u7406\u306b\u8feb\u308a\u307e\u3059\u3002\uff08\u9752\u4e95\u7814\u7a76\u5ba4\u3001(Imai Lab.<\/a>\uff09


\u3000Quantum Phase Transition in Zirconium Nuclei<\/strong>\u3000<\/mark>\u3000

In neutron-rich zirconium (Zr, Z = 40), it has long been known that when the mass number increases from A = 98 to A = 100, the ground-state shape suddenly undergoes aquantum phase transition from spherical to \u201clemon\u201d-shaped.<\/strong>Recently, the CNS Theory Group proposed a shape-coexistence model in which even the excited states of the stable Zr isotopes already exhibit a lemon shape,and this configuration crosses<\/strong>with the spherical one precisely at A = 99.

Shape coexistence occurs when multiple mean-field vacua (distinct equilibrium shapes) appear as the nucleus deforms.\n A quantum phase transition takes place when the relative energies of these vacua shift as a function of nucleon number, causing one shape to become the new ground state. Although Zr exhibits two coexisting vacua, nuclei with three have also been identified\u2014yet the properties of each vacuum remain largely unexplored.

To probe these distinct vacua, our group proposed using proton-resonance scattering followed by decay spectroscopy.
As a proof-of-principle\u2014and to uncover the mechanism driving the quantum phase transition in Zr\u2014we performed a proton-resonance scattering experiment on \u2079\u2076Zr at Kyushu University\u2019s tandem accelerator facility96<\/sup>. We observed resonances with different spin-parity assignments, directly revealing the shapes of the corresponding excited states. This method opens the door to detailed investigations of even more neutron-rich Zr isotopes in the future.<\/p>\n\n\n\n

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\u3000Mass Measurements of Short-Lived Rare Nuclei<\/strong>\u3000<\/mark>\u3000

Themass of an atomic nucleus<\/strong>\u2014one of the most fundamental quantities describing its energy states\u2014is determined by the combined effects of all nucleon\u2013nucleon interactions, including the strong (nuclear) force and the Coulomb force. By measuring nuclear masses directly and mapping out their systematics (e.g. as functions of neutron and proton number), we gain critical insight into how nuclear energy structure evolves. We have developed a technique that pairs extremely neutron-rich or proton-rich beams from RIKEN\u2019s accelerator complex with our own high-resolution beamline and the SHARAQ magnetic spectrometer. This setup enables us to measure the masses of rare nuclei whose lifetimes are only a few milliseconds.Developments<\/a>)
Applying this method, we are now uncovering the masses and underlying structure of nuclei in regions of the nuclear chart that have never before been explored.<\/p>\n\n\n\n

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\u3000Zeptosecond Dynamics<\/strong>\u3000<\/mark>\u3000

When roughly twenty neutrons are added to nihonium-278 (278<\/sup>Nh; Z = 113; t\u2081\/\u2082 \u2248 2 ms), theory predicts the emergence of an\u201cisland of stability\u201d<\/strong>\u2014a region of nuclei whose half-lives exceed years. Discovering these long-lived superheavy nuclides would profoundly advance our understanding of nuclear quantum\u2010system stability. Because fusion of two stable nuclei cannot supply enough neutrons, one promising route is to use very neutron-rich radioactive isotopes.

To exploit heavy\u2010ion fusion effectively, it\u2019s essential to understand not only fission but also the competing process known asfusion-hindrance (or fusion suppression)<\/strong>. Experimental data to date suggest an exponential dependence of the fusion probability on the product of the target and projectile proton numbers, yet model uncertainties span roughly two orders of magnitude.
In the Nuclear Dynamics Laboratory (Imai Group), we are using the high-intensity136<\/sup>Xe beam at QST-HIMAC to produce isotopes from polonium (Po, Z = 84) through neptunium (Np, Z = 93). By measuring fusion yields and cross sections under well\u2010controlled conditions, we aim to quantify fusion-hindrance effects on a zeptosecond timescale.<\/p>\n\n\n\n

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Island of Stability (Wikipedia<\/figcaption><\/figure>\n<\/div>\n\n\n\n
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Mosaic detector for
charged particle detection
2025 School of Science Calendar
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Exotic deformation<\/p>\n\n\n\n

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Nuclei are often depicted as perfect spheres in high-school textbooks\u2014but in reality, the momentum- and energy-space correlations among their constituent protons and neutrons can spontaneously break that symmetry and produce a variety of shapes. In quadrupole-deformed nuclei, for example, \u201csuperdeformed\u201d and even \u201chyperdeformed\u201d states appear. More exotic configurations\u2014such as octupole deformation, tetrahedral arrangements, linear-chain structures, and toroidal (\u201cdoughnut\u201d) shapes\u2014have also been predicted. These non-spherical shapes are understood as manifestations ofself-organization<\/strong>\u300d\u306e\u73fe\u308c\u3068\u3057\u3066\u7406\u89e3\u3055\u308c\u307e\u3059\u3002\u5909\u5f62\u72b6\u614b\u306e\u63a2\u7d22\u306b\u3088\u308a\u3001\u7834\u308c\u305f\u5bfe\u79f0\u6027\u304c\u3069\u306e\u3088\u3046\u306b\u56de\u5fa9\u3057\u3001\u96c6\u56e3\u904b\u52d5\u304c\u8a98\u8d77\u3055\u308c\u308b\u306e\u304b\u3092 \u8abf\u3079\u307e\u3059\u3002 (\u9752\u4e95\u7814\u7a76\u5ba4\u3001(Imai Lab.<\/a>)<\/p>\n\n\n\n

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Deformed nuclei
\uff08\u5f15\u7528\uff1aFrank, A., Jolie, J., Isacker, P.V. (2019). Symmetry in Nuclear Physics: The Shell Model. In: Symmetries in Atomic Nuclei. Springer Tracts in Modern Physics, vol 230. Springer, Cham. https:\/\/doi.org\/10.1007\/978-3-030-21931-4_2\uff09<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n\n\n\n

Collective Motion<\/p>\n\n\n\n

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By probing the nucleus via nuclear scattering, we can excite it into rotational and vibrational modes. This phenomenon\u2014called nuclear collective excitation\u2014reflects fundamental properties of the nucleus, such as its stiffness, and has long been a central topic in nuclear physics.\nThe Exotic Nuclear Reactions Group uses unstable beams from RIBF to study the interactions that give rise to new collective behaviors and novel nuclear phenomena. (Yako Lab.<\/a>)

\u3000Exploration of the Double Gamow\u2013Teller Giant Resonance<\/strong>\u3000<\/mark>\u3000

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Tetra Neutron<\/p>\n\n\n\n

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Neutron stars \u2014the final evolutionary stage of massive stars\u2014can be viewed as\u201cultra-giant nuclei\u201d<\/strong>composed almost entirely of neutrons, exhibiting extreme phenomena far beyond anything found on Earth. By creating the never-before-observed tetraneutron(an atomic nucleus of just four neutrons, Z = 0)<\/strong>in the laboratory, we can probe the exotic nuclear forces at play inside neutron stars. We probed tetra neutron system using a double charge-exchange reaction8<\/sup>He + 4<\/sup>He \u2192 8<\/sup>Be + <\/sup>4n, succeded in producing a four-neutron state with minimal recoil. This experiment, carried out with the SHARAQ spectrometer that CNS designed and built, revealed a clear resonance just above the four-neutron emission threshold\u2014an effect that cannot be explained by existing theories and points toward novel many-neutron correlations. These results establish the tetraneutron as the first step into a new research field\u2014\u201cmany-neutron nuclear physics\u201d\u2014with the promise of uncovering uncharted nuclear matter.

Building on our 4n observations, in 2023 a second experiment at RIKEN\u2019s SAMURAI spectrometer used a knock-out reaction8<\/sup>He + p \u2192 p + 4<\/sup>He + <\/sup>4n to produce tetraneutrons with much higher efficiency. The high-statistics data unambiguously confirmed the tetraneutron\u2019s existence, resolving a half-century of debate over whether a strongly correlated four-neutron state could exist. Also in 2023, in collaboration with Associate Professor Miki\u2019s group at Tohoku University, we used SHARAQ to study the three-neutron (3n) system, yielding new insights into neutron-rich matter. For pioneering the first experimental evidence of the tetraneutron, Professor S. Shimoura (University of Tokyo; now RIKEN ) was awarded the Nishina Memorial Prize for his work on the \u201cExperimental Study of the Four-Neutron State.\u201d(Imai Lab.<\/a>\/ \u65e7\u4e0b\u6d66\u7814\u7a76\u5ba4<\/a>)<\/p>\n<\/div>\n<\/div>\n<\/div>\n\n\n\n

<\/p>","protected":false},"excerpt":{"rendered":"

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