N. Aoi

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.

Identification of New Isotopes 125 Pd and 126 Pd produced by In-flight Fission of 345 MeV/nucleon 238 U: First Results from the RIKEN RI Beam Factory

A search for new isotopes using in-flight fission of a 345 MeV/nucleon 238 U beam has been carried out in the commissioning experiment of the next-generation in-flight radioactive isotope beam separator BigRIPS at the RI Beam Factory at the RIKEN Nishina Center. Two neutron-rich palladium isotopes 125 Pd and 126 Pd were observed for the first time, which demonstrates the great potential of the RIKEN RI beam factory.

DALI2: A NaI(Tl) detector array for measurements of

A NaI(Tl) detector array called DALI2 (Detector Array for Lo w Intensity radiation 2) has been constructed for in-beam γ-ray spectroscopy experiments with fast radioactive isotope (R I) beams. It consists typically of 186 NaI(Tl) scintillator s covering polar angles from∼15◦ to ∼160◦ with an average angular resolution of 6 ◦ in full width at half maximum. Its high granularity (good angular resolution) enables Doppler-shift correcti ons that result in, for example, 10% energy resolution and 20 % full-energy photopeak e fficiency for 1-MeVγ rays emitted from fast-moving nuclei (velocities of v/c ≃ 0.6). DALI2 has been employed successfully in numerous experiments using fast RI beams wi th velocities ofv/c = 0.3 − 0.6 provided by the RIKEN RI Beam Factory.

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Excited states in (38,40,42) Si nuclei have been studied via in-beam γ-ray spectroscopy with multinucleon removal reactions. Intense radioactive beams of ^{40}S and (44)S provided at the new facility of the RIKEN Radioactive Isotope Beam Factory enabled γ-γ coincidence measurements. A prominent γ line observed with an energy of 742(8) keV in (42) Si confirms the 2(+) state reported in an earlier study. Among the γ lines observed in coincidence with the 2^{+} → 0+ transition, the most probable candidate for the transition from the yrast 4(+) state was identified, leading to a 4(1)+) energy of 2173(14) keV. The energy ratio of 2.93(5) between the 2(1)+ and 4(1)(+) states indicates well-developed deformation in (42) Si at N = 28 and Z = 14. Also for 38,40)Si energy ratios with values of 2.09(5) and 2.56(5) were obtained. Together with the ratio for (42)Si, the results show a rapid deformation development of Si isotopes from N = 24 to N = 28.

rays from fast nuclei

The electric dipole strength distribution in 120Sn between 5 and 22 MeV has been determined at RCNP Osaka from a polarization transfer analysis of proton inelastic scattering at E_0 = 295 MeV and forward angles including 0{deg}. Combined with photoabsorption data an electric dipole polarizability alpha_D(120Sn) = 8.93(36) fm^3 is extracted. The dipole polarizability as isovector observable par excellence carries direct information on the nuclear symmetry energy and its density dependence. The correlation of the new value with the well established alpha_D(208Pb) serves as a test of its prediction by nuclear energy density functionals (EDFs). Models based on modern Skyrme interactions describe the data fairly well while most calculations based on relativistic Hamiltonians cannot.

Well Developed Deformation in 42Si

Abstract First experimental evidence of the population of the first 2 − state in 16 C above the neutron threshold is obtained by neutron knockout from 17 C on a hydrogen target. The invariant mass method combined with in-beam γ -ray detection is used to locate the state at 5.45(1) MeV. Comparison of its populating cross section and parallel momentum distribution with a Glauber model calculation utilizing the shell-model spectroscopic factor confirms the core-neutron removal nature of this state. Additionally, a previously known unbound state at 6.11 MeV and a new state at 6.28(2) MeV are observed. The position of the first 2 − state, which belongs to a member of the lowest-lying p – sd cross shell transition, is reasonably well described by the shell-model calculation using the WBT interaction.

Dipole polarizability of 120 Sn and nuclear energy density functionals

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.

One-neutron knockout reaction of 17C on a hydrogen target at 70 MeV/nucleon

The change in the configuration of valence protons between the initial and final states in the neutrinoless double-

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

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Change of nuclear configurations in the neutrinoless double ß decay of 130Te→130Xe and 136Xe→136Ba

decay of