T. Nakao

Identification of 45 New Neutron-Rich Isotopes Produced by In-Flight Fission of a 238U Beam at 345 MeV/nucleon

A search for new isotopes using in-flight fission of a 345 MeV/nucleon 238 U beam has been carried out at the RI Beam Factory at the RIKEN Nishina Center. Fission fragments were analyzed and identi...

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.

Analyzing power in elastic scattering of 6He from a polarized proton target at 71 MeV/nucleon

The vector analyzing power has been measured for the elastic scattering of neutron-rich {sup 6}He from polarized protons at 71 MeV/nucleon making use of a newly constructed solid polarized proton target operated in a low magnetic field and at high temperature. Two approaches based on local one-body potentials were applied to investigate the spin-orbit interaction between a proton and a {sup 6}He nucleus. An optical model analysis revealed that the spin-orbit potential for {sup 6}He is characterized by a shallow and long-ranged shape compared with the global systematics of stable nuclei. A semimicroscopic analysis with an {alpha}+n+n cluster folding model suggests that the interaction between a proton and the {alpha} core is essentially important in describing the p+{sup 6}He elastic scattering. The data are also compared with fully microscopic analyses using nonlocal optical potentials based on nucleon-nucleon g matrices.

Analyzing power for proton elastic scattering from the neutron-rich He6 nucleus

Vector analyzing power for the proton-{sup 6}He elastic scattering at 71 MeV/nucleon has been measured for the first time, with a newly developed polarized proton solid target, which works at a low magnetic field of 0.09 T. The results are found to be incompatible with a t-matrix folding model prediction. Comparisons of the data with g-matrix folding analyses clearly show that the vector analyzing power is sensitive to the nuclear structure model used in the reaction analysis. The {alpha}-core distribution in {sup 6}He is suggested to be a possible key for understanding the nuclear structure sensitivity.

Lifetime measurements of first excited states in C 16 , 18

The electric quadrupole transition from the first 2{sup +} state to the ground 0{sup +} state in {sup 18}C was studied through a lifetime measurement by an upgraded recoil shadow method applied to inelastically scattered radioactive {sup 18}C nuclei. The measured mean lifetime is 18.9{+-}0.9(stat){+-}4.4(syst) ps, corresponding to a B(E2;2{sub 1}{sup +}{yields}0{sub g.s.}{sup +}) value of 4.3{+-}0.2{+-}1.0 e{sup 2} fm{sup 4}, or about 1.5 Weisskopf units. The mean lifetime of the first 2{sup +} state in {sup 16}C was remeasured to be 18.3{+-}1.4{+-}4.8 ps, about four times shorter than the value reported previously. The discrepancy between the two results was explained by incorporating the {gamma}-ray angular distribution measured in this work into the previous measurement. These transition strengths are hindered compared to the empirical transition strengths, indicating that the anomalous hindrance observed in {sup 16}C persists in {sup 18}C.

Shallow and diffuse spin-orbit potential for proton elastic scattering from neutron-rich helium isotopes at 71 MeV/nucleon

Vector analyzing powers for proton elastic scattering from 8He at 71 MeV/nucleon have been measured using a solid polarized proton target operated in a low magnetic field of 0.1 T. The spin-orbit potential obtained from a phenomenological optical model analysis is found to be significantly shallower and more diffuse than the global systematics of stable nuclei, which is an indication that the spin-orbit potential is modified for scattering involving neutron-rich nuclei. A close similarity between the matter radius and the root-mean-square radius of the spin-orbit potential is also identified.

Experimental investigation of a linear-chain structure in the nucleus

Abstract It is a well-known fact that a cluster of nucleons can be formed in the interior of an atomic nucleus, and such clusters may occupy molecular-like orbitals, showing characteristics similar to normal molecules consisting of atoms. Chemical molecules having a linear alignment are commonly seen in nature, such as carbon dioxide. A similar linear alignment of the nuclear clusters, referred to as linear-chain cluster state (LCCS), has been studied since the 1950s, however, up to now there is no clear experimental evidence demonstrating the existence of such a state. Recently, it was proposed that an excess of neutrons may offer just such a stabilizing mechanism, revitalizing interest in the nuclear LCCS, specifically with predictions for their emergence in neutron-rich carbon isotopes. Here we present the experimental observation of α-cluster states in the radioactive 14C nucleus. Using the Be 10 + α resonant scattering method with a radioactive beam, we observed a series of levels which completely agree with theoretically predicted levels having an explicit linear-chain cluster configuration. We regard this as the first strong indication of the linear-chain clustered nucleus.

^{14}

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...

C

The O-14(alpha, p)F-17 reaction is one of the key reactions involved in the breakout from the hot-CNO cycle to the rp-process in type-I x-ray bursts (XRBs). The resonant properties in the compound nucleus Ne-18 have been investigated through resonant elastic scattering of F-17 + p. The radioactive F-17 beam was separated by the Center for Nuclear Study radioactive ion beam separator (CRIB) and bombarded a thick H-2 gas target at 3.6 MeV/nucleon. The recoiling light particles were measured by three Delta E-E silicon telescopes at laboratory angles of theta(lab) approximate to 3 degrees, 10 degrees, and 18 degrees. Five resonances at E-x = 6.15, 6.28, 6.35, 6.85, and 7.05 MeV were observed in the excitation functions, and their spin-parities have been determined based on an R-matrix analysis. In particular, J(pi) = 1(-) was firmly assigned to the 6.15-MeV state which dominates the thermonuclear O-14(alpha, p)F-17 rate below 2 GK. As well, a possible new excited state in Ne-18 was observed at E-x = 6.85 +/- 0.11 MeV with tentative J = 0 assignment. This state could be the analog state of the 6.880 MeV (0(-)) level in the mirror nucleus O-18, or a bandhead state (0(+)) of the six-particle four-hole (6p-4h) band. A new thermonuclear O-14(alpha, p)F-17 rate has been determined, and the astrophysical impact of multiple recent rates has been examined using an XRB model. Contrary to previous expectations, we find only a modest impact on predicted nuclear energy generation rates from using reaction rates differing by up to several orders of magnitude.

Active target studies of the αp-process at CRIB

In order to investigate the characteristic properties of proton-rich nuclei, β-decay spectroscopy was carried out on 24Si. The Gamow-Teller transition strength B(GT) of 24Si to low-lying states in 24Al was determined. Considering in particular the breaking of mirror symmetry for energy levels and B(GT), we discuss the behavior of a weakly-bound s-orbital proton by incorporating a recent experimental result.