New Short-Lived Isotope ^{221}U and the Mass Surface Near N=126

Discovery of New Isotopes ^{160}Os and ^{156}W: Revealing Enhanced Stability of the N=82 Shell Closure on the Neutron-Deficient Side

Using the fusion-evaporation reaction ^{106}Cd(^{58}Ni,4n)^{160}Os and the gas-filled recoil separator SHANS, two new isotopes _{76}^{160}Os and _{74}^{156}W have been identified. The α decay of ^{160}Os, measured with an α-particle energy of 7080(26) keV and a half-life of 201_{-37}^{+58}  μs, is assigned to originate from the ground state. The daughter nucleus ^{156}W is a β^{+} emitter with a half-life of 291_{-61}^{+86}  ms. The newly measured α-decay data allow us to derive α-decay reduced widths (δ^{2}) for the N=84 isotones up to osmium (Z=76), which are found to decrease with increasing atomic number above Z=68. The reduction of δ^{2} is interpreted as evidence for the strengthening of the N=82 shell closure toward the proton drip line, supported by the increase of the neutron-shell gaps predicted in theoretical models.

Description for N = 126 Isotones 210Po and 212Rn with Particle-Hole Excited Nucleon-Pair Approximation and Realistic Effective Interaction

In this paper, we study yrast states of two N=126 isotones 210Po and 212Rn using the nucleon-pair approximation with particle–hole excitations and using a low-momentum interaction Vlow–k renormalized from the free CD-Bonn NN potential. An overall good agreement with experimental level structures, B(E2)s, and B(E3)s, is achieved. We also calculate the probabilities of neutron particle–hole excitations in these yrast states, with a focus on negative-parity states, which reflect the roles played by the neutron negative-parity configurations of one-particle-one-hole excitations across the N=126 shell gap and the negative-parity configurations of valence proton particles involving the 0i13/2 orbit. The N=126 shell gap is discussed in terms of energies of neutron one-particle-one-hole excitations.

New α-Emitting Isotope ^{214}U and Abnormal Enhancement of α-Particle Clustering in Lightest Uranium Isotopes

A new α-emitting isotope ^{214}U, produced by the fusion-evaporation reaction ^{182}W(^{36}Ar,4n)^{214}U, was identified by employing the gas-filled recoil separator SHANS and the recoil-α correlation technique. More precise α-decay properties of even-even nuclei ^{216,218}U were also measured in the reactions of ^{40}Ar, ^{40}Ca beams with ^{180,182,184}W targets. By combining the experimental data, improved α-decay reduced widths δ^{2} for the even-even Po-Pu nuclei in the vicinity of the magic neutron number N=126 are deduced. Their systematic trends are discussed in terms of the N_{p}N_{n} scheme in order to study the influence of proton-neutron interaction on α decay in this region of nuclei. It is strikingly found that the reduced widths of ^{214,216}U are significantly enhanced by a factor of two as compared with the N_{p}N_{n} systematics for the 84≤Z≤90 and N<126 even-even nuclei. The abnormal enhancement is interpreted by the strong monopole interaction between the valence protons and neutrons occupying the π1f_{7/2} and ν1f_{5/2} spin-orbit partner orbits, which is supported by the large-scale shell model calculation.

Search for Electron-Capture Delayed Fission in the New Isotope ^{244}Md

The electron-capture decay followed by a prompt fission process was searched for in the hitherto unknown most neutron-deficient Md isotope with mass number 244. Alpha decay with α-particle energies of 8.73-8.86 MeV and with a half-life of 0.30_{-0.09}^{+0.19}  s was assigned to ^{244}Md. No fission event with a similar half-life potentially originating from spontaneous fissioning of the short-lived electron-capture decay daughter ^{244}Fm was observed, which results in an upper limit of 0.14 for the electron-capture branching of ^{244}Md. Two groups of fission events with half-lives of 0.9_{-0.3}^{+0.6}  ms and 5_{-2}^{+3}  ms were observed. The 0.9_{-0.3}^{+0.6}  ms activity was assigned to originate from the decay of ^{245}Md. The origin of eight fission events resulting in a half-life of 5_{-2}^{+3}  ms could not be unambiguously identified within the present data while the possible explanation has to invoke previously unseen physics cases.

Short-Lived α-Emitting Isotope ^{222}Np and the Stability of the N=126 Magic Shell

A new, very short-lived neutron-deficient isotope ^{222}Np was produced in the complete-fusion reaction ^{187}Re(^{40}Ar,5n)^{222}Np, and observed at the gas-filled recoil separator SHANS. The new isotope ^{222}Np was identified by employing a recoil-α correlation measurement, and six α-decay chains were established for it. The decay properties of ^{222}Np with E_{α}=10016(33)  keV and T_{1/2}=380_{-110}^{+260}  ns were determined experimentally. The α-decay systematics of Np isotopes is improved by adding the new data for ^{222}Np, which validates the N=126 shell effect in Np isotopes. The evolution of the N=126 shell closure is discussed in the neutron-deficient nuclei up to Np within the framework of α-decay reduced width.

New Isotope ^{220}Np: Probing the Robustness of the N=126 Shell Closure in Neptunium

A new short-lived neutron-deficient isotope ^{220}Np was synthesized in the fusion-evaporation reaction ^{185}Re(^{40}Ar,5n)^{220}Np at the gas-filled recoil separator SHANS. Based on the measurement of the correlated α-decay chains, the decay properties of ^{220}Np with E_{α}=10040(18)  keV and T_{1/2}=25_{-7}^{+14}  μs were determined, which are in good agreement with theoretical predictions. From the new experimental results coupled with the recently reported α-decay data of ^{219,223}Np, the α-decay systematics for Np isotopes around N=126 was established, which allows us for the first time to test the robustness of the N=126 shell closure in Z=93 Np isotopes. The results also indicate that, in the region of nuclei with Z≥83, the proton drip line has been reached for all odd-Z isotopes up to Np.

Production and study of chemical properties of superheavy elements

Some highlight examples on the study of production and chemical properties of heaviest elements carried out mostly at GSI Darmstadt are presented. They focus on the production of some of the heaviest known elements (114Fl, 115Mc, and 117Mc), studies of non-fusion reactions, and on chemical studies of 114Fl. This is the heaviest element, for which chemical studies have been performed to date.

Online chemical adsorption studies of Hg, Tl, and Pb on SiO2 and Au surfaces in preparation for chemical investigations on Cn, Nh, and Fl at TASCA

Online gas-solid adsorption studies with single-atom quantities of Hg, Tl, and Pb, the lighter homologs of the superheavy elements (SHE) copernicium (Cn, Z=112), nihonium (Nh, Z=113), and flerovium (Fl, Z=114), were carried out using short-lived radioisotopes. The interaction with Au and SiO2 surfaces was studied and the overall chemical yield was determined. Suitable radioisotopes were produced in fusion-evaporation reactions, isolated in the gas-filled recoil separator TASCA, and flushed rapidly to an adjacent setup of two gas chromatography detector arrays covered with SiO2 (first array) and Au (second array). While Tl and Pb adsorbed on the SiO2 surface, Hg interacts only weakly and reached the Au-covered array. Our results contribute to elucidating the influence of relativistic effects on chemical properties of the heaviest elements by providing experimental data on these lighter homologs.

Towards high-resolution laser ionization spectroscopy of the heaviest elements in supersonic gas jet expansion

Resonant laser ionization and spectroscopy are widely used techniques at radioactive ion beam facilities to produce pure beams of exotic nuclei and measure the shape, size, spin and electromagnetic multipole moments of these nuclei. However, in such measurements it is difficult to combine a high efficiency with a high spectral resolution. Here we demonstrate the on-line application of atomic laser ionization spectroscopy in a supersonic gas jet, a technique suited for high-precision studies of the ground- and isomeric-state properties of nuclei located at the extremes of stability. The technique is characterized in a measurement on actinium isotopes around the N=126 neutron shell closure. A significant improvement in the spectral resolution by more than one order of magnitude is achieved in these experiments without loss in efficiency.