Radioactive decay of 217Pa

The radioactive decay of 217Pa was investigated by means of α-γ-spectroscopy. Fine structure in the ground-state α-decay was established. Ambiguities in the fine structure of the α-decay of the previously known isomeric state could be clarified by α-γ-coincidence measurements. A previously unknown α-transition of Eα = (8306 ± 5) keV was detected and identified by means of delayed α-α- and α-γ-γ-coincidence measurements. A second isomeric state decaying by α-emission was not observed. The quality of the previously reported data of the α-decay fine structure of 217Th was improved.

Measurement of Evaporation Residue and Fission Cross Sections of the Reaction 30Si + 238U at Subbarrier Energies

Effects of the prolate deformation of 238 U on fusion were studied in the reaction 30 Si + 238 U at bombarding energies close to the Coulomb barrier. The fission (capture) cross sections were measured at the JAEA tandem accelerator to see the enhancement of the cross sections in the subbarrier energy due to the lower Coulomb barrier in the collisions of projectile at the polar sides of 238 U. In order to obtain the direct evidence for complete fusion, evaporation residue cross sections were measured at UNILAC of GSI. At the subbarrier energy of Ec.m. = 133.0 MeV, where only polar collisions to 238 U occur, we measured three spontaneously fissioning nuclei which we assigned to the isotope 26…

Study of Superheavy Elements at the GSI-SHIP

Decay studies of new isomeric states in 255No

The decay of excited states in 255No was investigated by applying the evaporation-residue–conversion-electron correlation technique. Two new isomeric states were observed in 255No together with the previously known one. Excitation energies of the isomeric states were estimated based on the energies of conversion electrons and γ rays from correlation chains. These results were in accord with theoretical calculations based on the mean-field models. A tentative decay scheme of isomeric states in 255No is proposed, and their Nilsson configurations are discussed. The energy decrease of the 11/2−[725] Nilsson level for heavy N=153 isotones as a function of increasing proton number is confirmed. p…

STUDIES OF SUPERHEAVY ELEMENTS AT SHIP

An overview of present experimental investigation of superheavy elements is given. The data are compared with theoretical descriptions. Results are reported from an experiment to confirm production of element 112 isotopes in irradiation of 238 UF 4 with 48 Ca . One spontaneous fission event was measured, which agrees with three events of previously measured data which had been assigned to the decay of 283112. However, more experimental work is needed in order to obtain an independent and unambiguous confirmation of previous results.

The new element 112

The new element 112 was produced and identified unambiguously in an experiment at SHIP, GSI Darmstadt. Two decay chains of the isotope277112 were observed in irradiations of208Pb targets with70Zn projectiles of 344 MeV kinetic energy. The isotope decays by emission of α particles with a half-life of (240 −90 +430 )µs. Two different α energies of (11,649±20) keV and (11,454±20) keV were measured for the two observed decays. The cross-section measured in three weeks of irradiations is (1.0 −0.4 +1.8 ) pb.

The new element 111

The new element 111 was produced and unambiguously identified in an experiment at SHIP, GSI Darmstadt. Three nuclei of the isotope272111 were observed in irradiations of209Bi targets with64Ni projectiles of 318 MeV and 320 MeV energy. The cross-sections are (1.7 −1.4 +3.3 ) pb and (3.5 −2.3 +4.6 ) pb, respectively. The nuclei decay by a emission into the new and so far the heaviest isotopes of the elements 109 and 107 with mass numbers A=268 and A=264. Theα-decay chains were followed down to the known nuclei260105 and256Lr.

Studies of SHE at SHIP

An overview of present experimental investigation of superheavy elements is given. The data are compared with theoretical descriptions. Results are reported from an experiment to confirm production of element 112 isotopes in irradiation of 238UF4 with 48Ca. One spontaneous fission event was measured, which agrees with three events of previously measured data which had been assigned to the decay of 283112. However, more experimental work is needed in order to obtain an independent and unambiguous confirmation of previous results.

Spectroscopy of 253No and its daughters

Abstract New high-statistics data have been obtained on the decay properties of 253 No and its daughters using the reaction 207 Pb( 48 Ca, 2n) 253 No. This was made possible thanks to an improved transmission of fusion–evaporation residues through the VASSILISSA recoil separator and an increased efficiency of the GABRIELA detector setup. The decay schemes of 253 No and 249 Fm have been revisited. The known level scheme of 249 Fm has been confirmed, including a new level at 669 keV excitation energy. The observation of L X-rays in coincidence with the α decay of 249 Fm gives additional support to the ground-state configuration of 1 / 2 + [ 631 ] instead of 5 / 2 + [ 622 ] for 245 Cf. In both…

New results on elements 111 and 112

Experiments on the synthesis and identification of the nuclei 272111 and 277112 were performed in order to confirm previous results. Three additional decay chains were measured in the reaction 64Ni + 209Bi →273111*. The study revealed considerably improved data on the decay chain originating from 272111. One additional chain was measured in the reaction 70Zn + 208Pb →278112*. The decay properties of the chain starting at 277112 are in excellent agreement with the second chain of the first experiment down to 265Sg, where the new chain ends by a previously unknown spontaneous-fission branch. A re-analysis of all the data on elements 110, 111, and 112 measured at GSI since 1994 (a total of 34 …

Production and decay of269110

In an experiment carried out to identify element 110, we have observed anα-decay chain, that can be unambiguously assigned to269110. In a scries of preexperiments the excitation functions of the fusion reactions50Ti +208Pb→258104* and58Fe +208Pb→266108* were measured with high precision in order to get the optimum projectile energies for the production of these heavy elements. The cross-section maxima of the 1n evaporation channels were observed at excitation energies of 15.6 MeV and 13.4 MeV, respectively. These data result in an optimum excitation energy of 12.3 MeV of the compound nucleus for the production of269110 in the reaction62Ni +208Pb→269110 + 1n. In irradiations at the correspon…

Decay properties of neutron-deficient isotopes 256, 257Db, 255Rf, 252, 253Lr

Isotopes of dubnium (element 105) with mass numbers A = 256, 257, and 258 were produced by the reaction 209Bi(50Ti,xn) 259-xDb (x = 1, 2, 3) at projectile energies of (4.59-5.08) AMeV. Excitation functions were measured for the 1n, 2n and 3n evaporation channels. The same position of the excitation function was observed for the 1n channel as for the previously measured 1n channel of the reaction 208Pb(50Ti,1n)257Rf. The measured α-decay data of 257Db and its daughter products resulted in the identification of α-decaying isomeric states in 257Db and 253Lr. Two new isotopes, 256Db and 252Lr, were produced at the highest bombarding energies of 4.97 AMeV and 5.08 AMeV. They were identified by d…

Properties of heavy nuclei measured at the GSI SHIP

Abstract The nuclear shell model predicts that the next doubly magic shell-closure beyond 208 Pb is at a proton number Z = 114, 120, or 126 and at a neutron number N = 172 or 184. The outstanding aim of experimental investigations is the exploration of this region of spherical ‘Super-Heavy Elements’ (SHEs). The measured decay data reveal that for the heaviest elements, the dominant decay mode is α emission, not fission. Decay properties as well as reaction cross-sections are compared with results of theoretical investigations. Finally, plans are presented for the further development of the experimental set-up and the application of new techniques.At a higher sensitivity, the exploration of …

Population of nuclides with Z≥98 in multi-nucleon transfer reactions of 48Ca + 248Cm

The results for nuclei above curium, produced in multi-nucleon transfer reactions of 48Ca + 248Cm at the velocity filter SHIP of GSI Darmstadt, are presented. Spontaneous fission and α-activities have been used to study the population of nuclei with lifetimes ranging from few milliseconds to several days. We observed several, relatively neutron-rich isotopes with atomic numbers Z≥98; among them a weak 224 millisecond activity which we tentatively attributed to 260No. The measured cross-sections of the observed nuclei give hope that multi-nucleon transfer reactions are a way to reach new neutron-rich heavy and superheavy nuclei, which are not accessible in other reactions. We compare our res…