A search for the heaviest isotopes of fluorine, neon, and sodium was conducted by fragmentation of an intense ^{48}Ca beam at 345 MeV/nucleon with a 20-mm-thick beryllium target and identification ...of isotopes in the large-acceptance separator BigRIPS at the RIKEN Radioactive Isotope Beam Factory. No events were observed for ^{32,33}F, ^{35,36}Ne, and ^{38}Na and only one event for ^{39}Na after extensive running. Comparison with predicted yields excludes the existence of bound states of these unobserved isotopes with high confidence levels. The present work indicates that ^{31}F and ^{34}Ne are the heaviest bound isotopes of fluorine and neon, respectively. The neutron dripline has thus been experimentally confirmed up to neon for the first time since ^{24}O was confirmed to be the dripline nucleus nearly 20 years ago. These data provide new keys to understanding the nuclear stability at extremely neutron-rich conditions.
The β-decay half-lives of 55 neutron-rich nuclei 134−139Sn, 134−142Sb, 137−144Te, 140−146I, 142−148Xe, 145−151Cs, 148−153Ba, and 151−155La were measured at the Radioactive Isotope Beam Factory ...employing the projectile fission fragments of 238U. The nuclear level structure, which relates to deformation, has a large effect on the half-lives. The impact of newly measured half-lives on modeling the astrophysical origin of the heavy elements is studied in the context of r process nucleosynthesis. For a wide variety of astrophysical conditions, including those in which fission recycling occurs, the half-lives have an important local impact on the second (A ≈ 130) peak.
•We measured the production cross sections for a variety of radioactive isotopes.•They were produced from 124Xe, 48Ca, and 238U beams at 345MeV/nucleon using BigRIPS.•The measured production cross ...sections were compared with the EPAX formulae.•We discovered four new isotopes 85,86Ru and 81,82Mo produced by 124Xe+Be.•103Sb is particle unbound with an upper limit of 49ns for the half-life.
We have measured the production rates and production cross sections for a variety of radioactive isotopes which were produced from 124Xe, 48Ca, and 238U beams at an energy of 345MeV/nucleon using the BigRIPS separator at the RIKEN Nishina Center RI Beam Factory (RIBF). Proton-rich isotopes with atomic numbers Z=40–52 and neutron-rich isotopes with Z=5–16 were produced by projectile fragmentation of the 124Xe and 48Ca beam on Be targets, respectively. Neutron-rich isotopes with Z=20–59 were produced by in-flight fission of the 238U beam, in which both Be and Pb were used as production targets. The measured production rates and production cross sections were compared with those of the LISE++ calculations, and overall fairly good agreement has been obtained. Furthermore, in the measurements with the 124Xe beam, we have discovered four new isotopes on the proton-drip line, 85,86Ru and 81,82Mo, and obtained the clear evidence that 103Sb is particle unbound with an upper limit of 49ns for the half-life. The measurements of projectile-fragment momentum distributions have been also performed with the 124Xe beam, in which the low-momentum tails of the distributions have been measured for the first time at the energy of 345MeV/nucleon.
Unbound states in C17 were investigated via one-neutron removal from a C18 beam at an energy of 245 MeV/nucleon on a carbon target. The energy spectrum of C17, above the single-neutron decay ...threshold, was reconstructed using invariant mass spectroscopy from the measured momenta of the C16 fragment and neutron, and was found to exhibit resonances at Er=0.52(2), 0.77(2), 1.36(1), 1.91(1), 2.22(3) and 3.20(1) MeV. The resonance at Er=0.77(2) MeV Ex=1.51(3) MeV was provisionally assigned as the second 5/2+ state. The two resonances at Er=1.91(1) and 3.20(1) MeV Ex=2.65(2) and 3.94(2) MeV were identified, through comparison of the energies, cross sections and momentum distributions with shell-model and eikonal reaction calculations, as p-shell hole states with spin-parities 1/21− and 3/21−, respectively. A detailed comparison was made with the results obtained using a range of shell-model interactions. The YSOX shell-model Hamiltonian, the cross-shell part of which is based on the monopole-based universal interaction, was found to provide a very good description of the present results and those for the neighbouring odd-A carbon isotopes – in particular for the negative parity cross-shell states.
Structure of the neutron-rich N=86 isotope 140Xe, located northeast of a doubly-magic nucleus 132Sn, is investigated by β−γ spectroscopy. Two β-decay isomers in 140I are newly found in the study of ...two different β decays of 140I which were produced by two reactions (i) direct in-flight fission at a primary target and (ii) β decay of 140Te at an active stopper. Half-lives of the β decays of the ground state, the low-spin isomer, and the high-spin isomer are determined to be 0.38(2), 0.91(5), and 0.47(4) s, respectively. Decay schemes of the β decay of the high-spin isomer and of the mixed β decays of the ground state and the low-spin isomer in 140I to 140Xe are constructed using the information on γ-ray coincidence relation and γ-ray intensity. Nuclear structures of the low-lying states in 140Xe and 140I are discussed by comparing the experimental results to two theoretical calculations based on a large-scale shell model and the deformed Skyrme Hartree-Fock-Bogoliubov plus deformed quasiparticle-random-phase approximation. Possible candidates for (quasi-)γ-band members of 2+ and 4+ states and the octupole collective 1− state are proposed in 140Xe. Increase of quadrupole, triaxial, and octupole collectivities is discussed with the increase of neutron and proton numbers.
The β-decay scheme of 138Te and the level structure of 138I is reported for the first time. The experiment was performed at the Radioactive Isotope Beam Factory of RIKEN, as one of the EUROBALL-RIKEN ...Cluster Array campaigns. Secondary radioactive ions, including 138Te and 138Sb, were produced by the in-flight fission of a 238U beam with the energy of 345 MeV per nucleon. From the β decay of 138Te, the level scheme of 138I was supplemented with new spin and parity assignments, such as the low-lying negative-parity states and a positive-parity 1+ state. This 1+ state can be interpreted as being associated with the π0h11/2⊗ν0h9/2 partner orbital configuration populated by the Gamow-Teller transition between a neutron in the 0h9/2 orbital and a proton in the 0h11/2 orbital. Details of the structure of 138I are discussed in terms of the proton-neutron interactions and Gamow-Teller transition strength within the theoretical context of shell-model calculations.
BigRIPS is a powerful two-stage in-flight separator for the research with exotic nuclei studied in frontier experiments since more than a decade. The ion-optical system is very versatile due to the ...multi-stage structure of BigRIPS combined with the ZeroDegree spectrometer or the Superconducting Ring Cyclotron (SRC). Various optical modes can be flexibly realized according to the purpose of experiments. Two categories of developments are presented here. One is a new operating mode of BigRIPS aiming at higher ion-optical resolving power. BigRIPS itself has a two-stage structure. Spatial isotope separation is made at both the first and second stages. In the standard operating mode of BigRIPS, at the second stage the two spatial separations with energy degraders are subtractive in their resolving powers. Here, we present the additive mode. With the resulting increased spatial separation power, the isotopic background can be substantially reduced. Higher ion-optical resolving powers of the first and second BigRIPS degrader stages are also investigated with the goals to reduce further the background and to yield access to new isotopes of heavier elements. The other development is a dispersion-matched system with BigRIPS for high-resolution spectrometer experiments. The BigRIPS and ZeroDegree spectrometer are presently two independent, coupled achromatic systems. A new dispersion-matched mode of BigRIPS and ZeroDegree will enable novel experiments. For high-resolution spectroscopy experiments with high-intensity light projectiles, SRC and BigRIPS can be operated as a dispersion-matched system. The described different ion-optical developments are a base for a new category of experiments exploring exotic nuclei and mesic atoms. Characteristic future experiments with these new ion-optical developments are exemplified in this report.
The low-lying unbound level structure of the halo nucleus 19C has been investigated using single-neutron knockout from 20C on a carbon target at 280 MeV/nucleon. The invariant mass spectrum, derived ...from the momenta of the forward going beam velocity 18C fragment and neutrons, was found to be dominated by a very narrow near threshold (Erel=0.036(1) MeV) peak. Two less strongly populated resonance-like features were also observed at Erel=0.84(4) and 2.31(3) MeV, both of which exhibit characteristics consistent with neutron p-shell hole states. Comparisons of the energies, measured cross sections and parallel momentum distributions to the results of shell-model and eikonal reaction calculations lead to spin-parity assignments of 5/21+ and 1/21− for the levels at Ex=0.62(9) and 2.89(10) MeV with Sn=0.58(9) MeV. Spectroscopic factors were also deduced and found to be in reasonable accord with shell-model calculations. The valence neutron configuration of the 20C ground state is thus seen to include, in addition to the known 1s1/22 component, a significant 0d5/22 contribution. The level scheme of 19C, including significantly the 1/21− cross-shell state, is well accounted for by the YSOX shell-model interaction developed from the monopole-based universal interaction.
The interaction cross sections (σI) of the very neutron-rich carbon isotopes 19C, 20C and 22C have been measured on a carbon target at 307, 280, and 235 MeV/nucleon, respectively. A σI of ...1.280±0.023 b was obtained for 22C, significantly larger than for 19,20C, supporting the halo character of 22C. A 22C root-mean-squared matter radius of 3.44±0.08 fm was deduced using a four-body Glauber reaction model. This value is smaller than an earlier estimate (of 5.4±0.9 fm) derived from a σI measurement on a hydrogen target at 40 MeV/nucleon. These new, higher-precision σI data provide stronger constraints for assessing the consistency of theories describing weakly bound nuclei.
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