D. Nishimura

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.

Single-Molecule Imaging of Rotaxanes Immobilized on Glass Substrates: Observation of Rotary Movement†

Even smallerartificialmolecularmotorsareextremelydifficulttosee,andthus, additional visualization techniques are necessary toobserve rotational behavior. Rotaxanes have a suitablestructure for the construction of molecular motors. We havepreviously investigated the rotational motion of a cyclo-dextrinrelativetoanaxismoleculeusingNMRtechniques.

Relative Rotational Motion between α-Cyclodextrin Derivatives and a Stiff Axle Molecule

Novel [2]rotaxanes bearing alpha-cyclodextrin (alpha-CD) derivatives and a diphenylacetylene axis molecule with trinitrobenzene as a bulky stopper have been prepared to investigate the relative rotary movement of a ring relative to an axis molecule and that of an axis molecule in a ring by NMR techniques. [2]Rotaxanes 2 and 3 were composed of alpha-CD derivatives (2: 6-phenyl-amide-alpha-CD; 3: 6-stilbene-amide-alpha-CD). The protons of alpha-CDs in rotaxanes were thoroughly assigned by the two-dimensional NMR techniques (TOCSY, COSY, ROESY, HMQC, and HMBC). The protons of alpha-CD in rotaxane 1 did not show splitting, whereas the resonance peak shifts and splitting for the corresponding protons of alpha-CD derivatives in rotaxanes 2 and 3 were observed by the shielding and deshielding effects from a diphenylacetylene axis molecule. The splitting of resonance peaks was closely related to the rotary movements of alpha-CDs and an axis molecule. We supposed that alpha-CD in rotaxane 1 rotates freely around a diphenylacetylene axis molecule, and vice versa, whereas the rotary movement of alpha-CD derivatives and the axis molecules of rotaxanes 2 and 3 were restricted by the steric repulsion between the substituent group of alpha-CD and the stopper group of an axis molecule. To estimate the relative rotary movement of CDs and an axis molecule in rotaxanes, the rotational correlation time (tauc) of rotaxanes was measured by 13C NMR. The results indicate that the corresponding rotary movement of the modified alpha-CD and the axis molecules in rotaxanes 2 and 3 depends on the size of the substituent group.

MEASUREMENTS OF INTERACTION CROSS SECTIONS TOWARDS NEUTRON-RICH Ne ISOTOPES AT RIBF

Interaction cross sections (σI) for Ne isotopes from the stability line to the vicinity of neutron-drip line have been measured using the RIBF facility at RIKEN. Measurements have been performed for 20–32Ne on C target at energies around 240 MeV/nucleon. A large enhancement of σI beyond the systematics of stable nuclei have been observed for neutron-rich Ne isotopes. The possible halo structures with lower orbital angular momentum for 29,31Ne are discussed by the preliminary analysis.

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.

Precise Branching Ratio of 24mAl Beta Decay

The branching ratio of the isomeric γ decay of 24mAl has been measured to be 69.6(7)% which is much smaller than the previously accepted value of 82.5(30)%. As a result, the branching ratio to the 24Mg ground state increases up to 24.1(7)% assuming that the other β-decay branching ratios are the previously accepted values. The half-life of 24mAl was also precisely determined to be 130.9(13) ms. The B(GT) value from the ground state of 24Mg to the 24mAl of 0.0577(16) deduced from the β decay is now in good agreement with that deduced from charge-exchange reactions.

Beta and gamma decays of J(pi)=1(+), Al-24m state beta and gamma branching ratios

A beta-gamma spectroscopy of the J(pi) = 1(+), 426 keV isomeric state of Al-24 ( Al-24m) has been carried out by using a Al-24 secondary beam with high purity and high isomeric ratio. From the absolute gamma-ray and beta-particle intensities observed in the decay of the isomeric state, the branching ratio R-B of the isomeric gamma-decay from Al-24m to the J(pi) = 4(+), ground state of Al-24 have been derived. The obtained R-B value of 69.6(7)% is much smaller than the previously accepted value of 82.5(30)%. The precise half-life for the isomer decay, T-1/2(m) = 130.9(13) ms, has been also determined in this experiment. Accordingly, the M3 gamma-decay strength B(M3) of the Al-24m decay becomes smaller and the total beta-decay branching ratio becomes larger. In particular, the beta-decay branching ratio to the ground state of Mg-24 becomes 24.3(9)%, which is 2.4 times larger than the previous value of 10.1(28)%. By combining the branching ratio and the half-life, the Gamow-Teller (GT) transition strength B(GT) of 0.0194(7) is deduced for the GT transition from Al-24m to the J(pi) = 0(+), ground state of Mg-24. This value is in good agreement with the values derived from charge-exchange reactions.

Beta and gamma decays of Jπ = 1+, 24mAl state

A beta-gamma spectroscopy of the J(pi) = 1(+), 426 keV isomeric state of Al-24 ( Al-24m) has been carried out by using a Al-24 secondary beam with high purity and high isomeric ratio. From the absolute gamma-ray and beta-particle intensities observed in the decay of the isomeric state, the branching ratio R-B of the isomeric gamma-decay from Al-24m to the J(pi) = 4(+), ground state of Al-24 have been derived. The obtained R-B value of 69.6(7)% is much smaller than the previously accepted value of 82.5(30)%. The precise half-life for the isomer decay, T-1/2(m) = 130.9(13) ms, has been also determined in this experiment. Accordingly, the M3 gamma-decay strength B(M3) of the Al-24m decay becomes smaller and the total beta-decay branching ratio becomes larger. In particular, the beta-decay branching ratio to the ground state of Mg-24 becomes 24.3(9)%, which is 2.4 times larger than the previous value of 10.1(28)%. By combining the branching ratio and the half-life, the Gamow-Teller (GT) transition strength B(GT) of 0.0194(7) is deduced for the GT transition from Al-24m to the J(pi) = 0(+), ground state of Mg-24. This value is in good agreement with the values derived from charge-exchange reactions.

Measurement of interaction cross-sections for neutron-rich Na isotopes

The interaction cross-sections (σI) of neutron-rich Na isotopes, 23-35Na, on C target have been measured at 250A MeV using the RI beam factory (RIBF) at RIKEN. Mass dependence of σI for 27-35Na suggests monotonic growth of the skin thickness. The root-mean-square nuclear matter radii (rm) of 23-35Na were deduced from observed σI via a Glauber-type calculation. These rm are in a good agreement with the theoretical prediction by relativistic mean field model (RMF). rm of 33-35Na were determined for the first time.

Reaction cross section studies at NIRS and RIBF

Reaction cross sections for stable nuclei at intermediate energies have been measured precisely and systematically. The data have been found to be reproduced nicely by the optical‐limit approximation of Glauber theory modified to include the nucleon multiple scattering effect and the Fermi‐motion effect. Applying this prescription, the nucleon density distribution of 17Ne has been studied. The surface structure of 8B and 11Be has been also studied using this prescription and hydrogen targets. Using the RIBF that has just started application to studies of exotic nuclei, neutron‐rich Ne isotopes around the Island of Inversion have been investigated through measurements of their interaction cross sections.