S. Michimasa

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.

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.

21Na(p,gamma)22Mg reaction and oxygen-neon novae.

The 21Na(p,gamma)22Mg reaction is expected to play an important role in the nucleosynthesis of 22Na in oxygen-neon novae. The decay of 22Na leads to the emission of a characteristic 1.275 MeV gamma-ray line. This report provides the first direct measurement of the rate of this reaction using a radioactive 21Na beam, and discusses its astrophysical implications. The energy of the important state was measured to be E(c.m.)=205.7+/-0.5 keV with a resonance strength omegagamma=1.03+/-0.16(stat)+/-0.14(sys) meV.

Anomalously hindered E2 strength B(E2;2+(1)-->0+) in 16C.

The electric quadrupole transition from the first 2(+) state to the ground 0(+) state in 16C is studied through measurement of the lifetime by a recoil shadow method applied to inelastically scattered radioactive 16C nuclei. The measured mean lifetime is 77+/-14(stat)+/-19(syst) ps. The central value of mean lifetime corresponds to a B(E2;2+(1)-->0(+)) value of 0.63e(2) fm(4), or 0.26 Weisskopf units. The transition strength is found to be anomalously small compared to the empirically predicted value.

Well Developed Deformation in 42Si

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.

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 186  MeV/u. The energy of the state is 0.83±0.65(stat)±1.25(syst)  MeV 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.

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.

Active target studies of the αp-process at CRIB

The αp-process is a sequence of (α, p)(p, γ) reactions important to the nuclear trajectory to higher masses in type I X-ray bursts. Specifically, the αp-process is schematically pure helium-burning, and thus unlike pure hydrogen-burning processes, does not require slow β+ decays. Explosive helium burning is responsible for the observed short rise-times of X-ray bursts but ultimately gives way to the rp-process as the Coulomb barrier increases. Because the stellar reaction rates of these (α, p) reactions are poorly known over the relevant astrophysical energies, we performed systematic studies of the 18Ne(α,p), 22Mg(α,p) and 30S(α,p) reactions at the Center for Nuclear Study (CNS) low-energy radioactive ion beam separator, called CRIB. We produce the radioactive beams in-flight and scan the center-of-mass energy down into the Gamow Window using a thick target in inverse kinematics. The helium target gas also serves as part of the detector system, an active target, which was newly designed for these measure...

Experimental studies using a low-energy RI beam separator at CNS

Abstract Radioactive-ion (RI) beams of 10 C, 14 O, 12N and 11 C with energies low 10 A MeV were produced by using a low-energy in-flight RI beam separator newly constructed by CNS, University of Tokyo. Using the 12 N and 11 C beams, some resonance states were identified in the proton elastic scattering 12 N+p and 11 C+p, respectively.

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.