Hidetada Baba

Evidence for a new nuclear /`magic number/' from the level structure of 54Ca

Atomic nuclei are finite quantum systems composed of two distinct types of fermion—protons and neutrons. In a manner similar to that of electrons orbiting in an atom, protons and neutrons in a nucleus form shell structures. In the case of stable, naturally occurring nuclei, large energy gaps exist between shells that fill completely when the proton or neutron number is equal to 2, 8, 20, 28, 50, 82 or 126 (ref. 1). Away from stability, however, these so-called ‘magic numbers’ are known to evolve in systems with a large imbalance of protons and neutrons. Although some of the standard shell closures can disappear, new ones are known to appear. Studies aiming to identify and understand such behaviour are of major importance in the field of experimental and theoretical nuclear physics. Here we report a spectroscopic study of the neutron-rich nucleus 54Ca (a bound system composed of 20 protons and 34 neutrons) using proton knockout reactions involving fast radioactive projectiles. The results highlight the doubly magic nature of 54Ca and provide direct experimental evidence for the onset of a sizable subshell closure at neutron number 34 in isotopes far from stability.

The first excited state of 30Ne studied by proton inelastic scattering in reversed kinematics

Abstract The energy of the first excited state of the very neutron-rich nucleus 30 Ne was determined by proton inelastic scattering. Axa0liquid hydrogen target was bombarded by a beam of radioactive 30 Ne at 48xa0MeV/nucleon, and energies of the de-excitation γ rays emitted from the excited 30 Ne were measured. The observed transition is likely to be the one from the first 2 + state to the ground state of 30 Ne. The excitation energy, 791(26)xa0keV, is lower than that of 32 Mg, indicating enhancement of deformation as the neutron excess increases in the N =20 isotones. The results indicate that 30 Ne belongs to the ‘island of inversion’.

Candidate Resonant Tetraneutron State Populated by the He 4 (He 8, Be 8) Reaction

A candidate resonant tetraneutron state is found in the missing-mass spectrum obtained in the double-charge-exchange reaction ^{4}He(^{8}He,^{8}Be) at 186u2009u2009MeV/u. The energy of the state is 0.83±0.65(stat)±1.25(syst)u2009u2009MeV above the threshold of four-neutron decay with a significance level of 4.9σ. Utilizing the large positive Q value of the (^{8}He,^{8}Be) reaction, an almost recoilless condition of the four-neutron system was achieved so as to obtain a weakly interacting four-neutron system efficiently.

SπRIT: A time-projection chamber for symmetry-energy studies

A time-projection chamber (TPC) called the SAMURAI Pion-Reconstruction and Ion-Tracker (S pi RIT) has recently been constructed at Michigan State University as part of an international effort to constrain the symmetry-energy term in the nuclear Equation of State (EoS). The S pi RIT TPC will be used in conjunction with the SAMURAI spectrometer at the Radioactive Isotope Beam Factory (RIBF) at RIKEN to measure yield ratios for pions and other light isospin multiplets produced in central collisions of neutron-rich heavy ions, such as Sn-132+Sn-124. The S pi RIT TPC can function both as a TPC detector and as an active target. It has a vertical drift length of 50 cm, parallel to the magnetic field. Gas multiplication is achieved through the use of a multi-wire anode plane. Image charges, produced in the 12096 pads, are read out with the recently developed Generic Electronics for TPCs

Pulse shape simulation and analysis of segmented Ge detectors for position extraction

A simulation method, which can predict the relation between the pulse shapes and the interaction points of incident /spl gamma/-rays, has been established for the entire effective volume of a segmented Ge detector with planar geometry, and the pulse shapes shows good agreement with the experimental observation. We are using pulse shape analysis to gain position information with a granularity finer than the segment size, and we will discuss the expected position resolution based on the simulation results.

Collectivity evolution in the neutron-rich Pd isotopes toward the N=82 shell closure

The neutron-rich, even-even 122,124,126Pd isotopes has been studied via in-beam gamma-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. Excited states at 499(9), 590(11), and 686(17) keV were found in the three isotopes, which we assign to the respective 2+ -> 0+ decays. In addition, a candidate for the 4+ state at 1164(20) keV was observed in 122Pd. The resulting Ex(2+) systematics are essentially similar to those of the Xe (Z=54) isotopic chain and theoretical prediction by IBM-2, suggesting no serious shell quenching in the Pd isotopes in the vicinity of N=82.

Inelastic Scattering of 12Be with 4He

. Inelastic scattering of the neutron-rich nucleus 1 2 Be on a liquid-He target has been studied by measuring de-excitation γ rays in coincidence with scattered 1 2 Be. In the angular distribution, a difference between the 2 + 1 and the 1- state in 1 2 Be was cleary seen.

Identification of New Neutron-Rich Isotopes in the Rare-Earth Region Produced by 345 MeV/nucleon 238U

A search for new isotopes in the neutron-rich rare-earth region has been carried out using a 345 MeV/nucleon 238U beam at the RIKEN Nishina Center RI Beam Factory. Fragments produced were analyzed and identified using the BigRIPS in-flight separator. We observed a total of 29 new neutron-rich isotopes: 153Ba, 154,155,156La, 156,157,158Ce, 156,157,158,159,160,161Pr, 162,163Nd, 164,165Pm, 166,167Sm, 169Eu, 171Gd, 173,174Tb, 175,176Dy, 177,178Ho, and 179,180Er.

Coulomb Excitation of 24Si

We have measured Coulomb excitation of 2 4 Si to its first excited state to obtain the E2 transition probability. This is the first measurement of B(E2) for a T z = -2 nucleus, which enables us to compare their transitionprobabilities between the mirror pair of T = 2, namely 2 4 Si and 2 4 Ne.

Sharaq Spectrometer: High-resolution Spectroscopy Using Exotic Beams And Reactions

We introduce the recent development for increasing the performance of the SHARAQ spectrometer, which was operated at RI Beam Factory in RIKEN, Japan. The paper contains main achievements of our developments, which are detector developments, improvements of high-resolution performance at SHARAQ, and developments of new reaction probes to explore new kinds of nuclear responses. By using those new experimental technique, we recently performed various nuclear experiments at SHARAQ. We described here the preliminary results of the experiments together with their brief details.