Engineering topics
Yasuda, M.
Publications and source records attributed to Yasuda, M..
First observation of 28 O
Subjecting a physical system to extreme conditions is one of the means often used to obtain a better understanding and deeper insight into its organization and structure. In the case of the atomic nucleus, one such approach is to investigate isotopes that have very different neutron-to-proton ($N/Z$) ratios than in stable nuclei. Light, neutron-rich isotopes exhibit the most asymmetric $N/Z$ ratios and those lying beyond the limits of binding, which undergo spontaneous neutron emission and exist only as very short-lived resonances (about 10 -21 s), provide the most stringent tests of modern nuclear-structure theories. Here we report on the first observation of 28 O and 27 O through their decay into 24 O and four and three neutrons, respectively. The 28 O nucleus is of particular interest as, with the $Z$ = 8 and $N$ = 20 magic numbers, it is expected in the standard shell-model picture of nuclear structure to be one of a relatively small number of so-called ‘doubly magic’ nuclei. Furthermore, both 27 O and 28 O were found to exist as narrow, low-lying resonances and their decay energies are compared here to the results of sophisticated theoretical modelling, including a large-scale shell-model calculation and a newly developed statistical approach. In both cases, the underlying nuclear interactions were derived from effective field theories of quantum chromodynamics. Finally, it is shown that the cross-section for the production of 28 O from a 29 F beam is consistent with it not exhibiting a closed $N$ = 20 shell structure.
Level structures of 56,58 Ca cast doubt on a doubly magic 60 Ca
Gamma decays were observed in 56 Ca and 58 Ca following quasi-free one-proton knockout reactions from 57,59 Sc beams at ≈200 MeV/nucleon. For 56 Ca, a γ ray transition was measured to be 1456(12) keV, while for 58 Ca an indication for a transition was observed at 1115(34) keV. Both transitions were tentatively assigned as the $2^{+}_{1}\rightarrow0^{+}_{gs}$ decays, and were compared to results from ab initio and conventional shell-model approaches. A shell-model calculation in a wide model space with a marginally modified effective nucleon-nucleon interaction depicts excellent agreement with experiment for $2^{+}_{1}$ level energies, two-neutron separation energies, and reaction cross sections, corroborating the formation of a new nuclear shell above the N = 34 shell. Its constituents, the 0$f$ 5/2 and 0$g$ 9/2 orbitals, are almost degenerate. This degeneracy precludes the possibility for a doubly magic 60 Ca and potentially drives the dripline of Ca isotopes to 70 Ca or even beyond.
Intruder configurations in 29 Ne at the transition into the island of inversion: Detailed structure study of 28 Ne
Detailed γ-ray spectroscopy of the exotic neon isotope 28 Ne has been performed for the first time using the one-neutron removal reaction from 29 Ne on a liquid hydrogen target at 240 MeV/nucleon. Based on an analysis of parallel momentum distributions, a level scheme with spin-parity assignments has been constructed for 28 Ne and the negative-parity states are identified for the first time. The measured partial cross sections and momentum distributions reveal a significant intruder p-wave strength providing evidence of the breakdown of the N=20 and N=28 shell gaps. Only a weak, possible f-wave strength was observed to bound final states. Large-scale shell-model calculations with different effective interactions do not reproduce the large p-wave and small f-wave strength observed experimentally, indicating an ongoing challenge for a complete theoretical description of the transition into the island of inversion along the Ne isotopic chain.
Observation of a correlated free four-neutron system
A long-standing question in nuclear physics is whether chargeless nuclear systems can exist. To our knowledge, only neutron stars represent near-pure neutron systems, where neutrons are squeezed together by the gravitational force to very high densities. The experimental search for isolated multi-neutron systems has been an ongoing quest for several decades, with a particular focus on the four-neutron system called the tetraneutron, resulting in only a few indications of its existence so far, leaving the tetraneutron an elusive nuclear system for six decades. Here we report on the observation of a resonance-like structure near threshold in the four-neutron system that is consistent with a quasi-bound tetraneutron state existing for a very short time. The measured energy and width of this state provide a key benchmark for our understanding of the nuclear force. The use of an experimental approach based on a knockout reaction at large momentum transfer with a radioactive high-energy 8 He beam was key.
Border of the island of inversion: Unbound states in Ne 29
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Investigation of the ground-state spin inversion in the neutron-rich Cl 47 , 49 isotopes
A first γ -ray study of Cl 47 , 49 spectroscopy was performed at the Radioactive Isotope Beam Factory with Ar 50 projectiles at 217 MeV/nucleon, impinging on the liquid hydrogen target of the MINOS device. Prompt deexcitation γ rays were measured with the NaI(Tl) array DALI2 + . Through the one-proton knockout reaction Ar 50 ( p , 2 p ) , a spin assignment could be determined for the low-lying states of Cl 49 from the momentum distribution obtained with the SAMURAI spectrometer. A spin-parity J π = 3 / 2 + is deduced for the ground state of Cl 49 , similar to the recently studied N = 32 isotope K 51 . The evolution of the energy difference E ( 1 / 2 1 + ) - E ( 3 / 2 1 + ) is compared to state-of-the-art theoretical predictions.