Shigeru Kubono

Isomeric 0+ state in 12Be

An isomeric 0+ state at E x = 2.24 MeV in 12Be was found in the projectile fragmentation of 18O at 100 A MeV on a Be target. The excitation energy and the spin assignment were deduced by the measurements of the energies and the angular correlation function of the coincident two γ-rays corresponding to the 0+ → 2+ → 0+ decay, respectively.

Structural Evolution in the Neutron-Rich Nuclei ^{106}Zr and ^{108}Zr

The low-lying states in ¹⁰⁶Zr and ¹⁰⁸Zr have been investigated by means of β-γ and isomer spectroscopy at the radioactive isotope beam factory (RIBF), respectively. A new isomer with a half-life of 620 ± 150 ns has been identified in ¹⁰⁸Zr. For the sequence of even-even Zr isotopes, the excitation energies of the first 2⁺ states reach a minimum at N = 64 and gradually increase as the neutron number increases up to N = 68, suggesting a deformed subshell closure at N = 64. The deformed ground state of ¹⁰⁸Zr indicates that a spherical subshell gap predicted at N = 70 is not large enough to change the ground state of ¹⁰⁸Zr to the spherical shape. The possibility of a tetrahedral shape isomer in ¹⁰⁸Zr is also discussed.

Experimental determination of astrophysical reaction rates with radioactive nuclear beams

Nuclear astrophysics is one of the basic and indispensable fields for the science of the Universe. Nuclear reactions play an important role in the evolution of the Universe. The large amount of energy stored in atomic nuclei plays a decisive role in the evolution of the Universe [1–3]. The origin and the distribution of the elements are other important factors for understanding the Universe and the constituents of our world. Nuclear reactions cause synthesis of a variety of elements from light to very heavy ones in the Universe. The environmental conditions for nuclear burning are dependent on the stellar sites, which characterize the scenario of burning such as the CNO cycle in the hydrogen burning stage in massive stars. Recent progress is summarized for instance in Refs. [4–10]. Observation of elemental abundance, on the other hand, provides important clues for understanding not only various phenomena but also the evolution of the Universe. Detailed abundance ratios have been observed optically for several novae, where heavy elements such as Si and S were observed, which should have been produced in the explosive nucleosynthesis [11]. These allow us to investigate the nucleosynthesis in novae quantitatively. Investigation of isotopic anomalies in meteorites [12,13] also provides interesting information for understanding explosive nuclear burning. Recently,

Experimental determination of the Ne-19(p, gamma)Na-20 reaction rate and the breakout problem from the hot CNO cycle

The nuclear levels of Na-20 are studied experimentally using the (He-3, t) and (p, n) reactions on Ne-20. The spin parities and excitation energies are determined for the states near and above the p + Ne-19 threshold. The stellar reaction rate of the Ne-19(p, gamma) process is evaluated based on the data. The calculation shows a more than two orders of magnitude larger rate than the previous theoretical predictions. The finding of s-wave resonance just above the capture threshold may change the onset temperature of breakout from the CNO cycle and explains the strong enhancement of Ne and heavier elements in observations of nova V1370 Aql 1982. 16 refs.

Identification of the Lowest T =2, Jπ =0+ Isobaric Analog State in 52Co and Its Impact on the Understanding of β-Decay Properties of52Ni

Masses of ^{52g,52m}Co were measured for the first time with an accuracy of ∼10  keV, an unprecedented precision reached for short-lived nuclei in the isochronous mass spectrometry. Combining our results with the previous β-γ measurements of ^{52}Ni, the T=2, J^{π}=0^{+} isobaric analog state (IAS) in ^{52}Co was newly assigned, questioning the conventional identification of IASs from the β-delayed proton emissions. Using our energy of the IAS in ^{52}Co, the masses of the T=2 multiplet fit well into the isobaric multiplet mass equation. We find that the IAS in ^{52}Co decays predominantly via γ transitions while the proton emission is negligibly small. According to our large-scale shell model calculations, this phenomenon has been interpreted to be due to very low isospin mixing in the IAS.

Low-lying non-normal parity states in 8B measured by proton elastic scattering on 7Be

Abstract A new measurement of proton resonance scattering on 7Be was performed up to the center-of-mass energy of 6.7 MeV using the low-energy RI beam facility CRIB (CNS Radioactive Ion Beam separator) at the Center for Nuclear Study of the University of Tokyo. The excitation function of Be 7 + p elastic scattering above 3.5 MeV was measured successfully for the first time, providing important information about the resonance structure of the 8B nucleus. The resonances are related to the reaction rate of 7Be( p , γ )8B, which is the key reaction in solar 8B neutrino production. Evidence for the presence of two negative parity states is presented. One of them is a 2 − state observed as a broad s-wave resonance, the existence of which had been questionable. Its possible effects on the determination of the astrophysical S-factor of 7Be( p , γ )8B at solar energy are discussed. The other state had not been observed in previous measurements, and its spin and parity were determined as 1 − .

Nuclear Astrophysics with Radioactive Nuclear Beams

Nuclear reactions playa crucial role in the evolution of the universe. Various astronomical observations of nuclear effects provide important clues and tests for understanding of the mechanism of stellar events and the evolution of the universe. The explosive phenomena in the universe inevitably involve radioactive nuclei. Nucleosynthesis in big bang models and stellar models are discussed critically, in particular the key nuclear reactions that involve radioactive nuclei. Also discussed is how new facets recently attained in the nuclear physics of unstable nuclei alter the nucleosynthesis scenarios. The scope of the new research field developed with radioactive nuclear beams in nuclear astrophysics is also discussed.

Advanced focal plane detector with improved particle identification

Abstract A focal plane detector for a magnetic spectrograph was developed, which consists of two position counters, two energy-loss counters, and large-area silicon detectors. The energy, energy-loss and time-of-flight signals from this detector system depend on the particle incident angle into the counter and the position on the focal plane. Corrections to the angle and position dependence have improved the resolutions of these signals significantly and given unambiguous particle identification for studying reactions which have very small yields under intense backgrounds. The incident angle into the focal plane detector is found to be very useful to determine the scattering angle of reaction products.

New ISOL-based radioactive nuclear beam facility at INS

Abstract An ISOL-based radioactive nuclear beam facility is just about to come into operation at INS. The present status of the INS radioactive nuclear beam project is reported. The capability of the facility and possible experiments are also discussed, including research programs of nuclear physics and nuclear astrophysics.

A proposed direct measurement of cross section at Gamow window for key reaction 19F(p,α) 16O in Asymptotic Giant Branch stars with a planned accelerator in CJPL

In 2014, the National Natural Science Foundation of China (NSFC) approved the Jinping Underground Nuclear Astrophysics laboratory (JUNA) project, which aims at direct cross-section measurements of four key stellar nuclear reactions right down to the Gamow windows. In order to solve the observed fluorine overabundances in Asymptotic Giant Branch (AGB) stars, measuring the key 19F(p,α)16O reaction at effective burning energies (i.e., at Gamow window) is established as one of the scientific research sub-projects. The present paper describes this sub-project in details, including motivation, status, experimental setup, yield and background estimation, aboveground test, as well as other relevant reactions.