Separator for Heavy ELement Spectroscopy - velocity filter SHELS

The SHELS velocity filter originated upon reconstruction of the VASSILISSA electrostatic separator used for investigations of heavy nuclei produced in complete fusion reactions. The goals of this modernization were to increase the transmission of products of asymmetric reactions and to extend the region of reactions to be investigated up to symmetric combinations. The first tests of the set-up were performed with the beams of accelerated 22Ne, 40Ar, 48Ca, and 50Ti ions.status: publishe

Fine structure in the alpha decay of U-224

224U nuclei were populated in fusion-evaporation reactions using a 206Pb target and an intense 22Ne beam. Fusion-evaporation residues were separated by the new separator SHELS at the FLNR, Dubna and implanted into a large-area double-sided silicon strip detector. Position- and time-correlated alpha decays were used to identify evaporation residues. A new α-decay line at 8095(11) keV was observed in this work and assigned as the decay from 224U to the first excited 2+ in the daughter nucleus 220Th. Coincident photons were also observed allowing to unambiguously determine the excitation energy of the first excited 2+ state in 220Th to be 386.5(1) keV and not 373.3(1)keV as previously reported. The half-life of 224U was measured to be 396(17)μs.status: publishe

FIRST EXPERIMENTAL TESTS OF SHELS: A NEW HEAVY ION SEPARATOR AT THE JINR

A new Separator for Heavy ELement Spectroscopy (SHELS) has been recently installed at the U-400 accelerator at FLNR, JINR in Dubna, Russia. The details of the upgrade as well as the results from some of the commissioning runs are discussed. In particular, transmission tests with the 208Pb(40Ar, 2–3n)245−6Fm reaction, as well as isomer spectroscopy results for 210Ra produced in 164Dy(50Ti, 3–5n)209−11Ra reaction are presented.status: publishe

Influence of octupole vibration on the low-lying structure of 251Fm^{251}\mathrm{Fm} and other heavy N=151N=151 isotones

© 2018 American Physical Society. The structure of low-lying excited states in Fm251, populated by the α decay of No255, has been investigated by means of combined γ and internal conversion electron spectroscopy. The values for the internal conversion coefficients for the 1/2+→5/2+ and 5/2+→9/2- transitions have been measured. The determined M2/E3 mixing ratio and lifetime for the 5/2+ decay to the ground state allowed to determine the corresponding reduced transitions strengths of B(E3)=18(6) W.u. and B(M2)=3.0(6)×10-3 W.u. These results, as well as the results of previous studies in N=151 isotopes, are compared to theoretical calculations beyond the mean-field approach, including the first QRPA calculations using the Gogny D1M parametrization for such heavy odd-N nuclei. The comparison points to the importance of accounting for the octupole vibrations for a proper understanding of the low-lying nuclear structure of some of the heaviest elements.status: Published onlin

Prompt Neutrons from Spontaneous 254Rf Fission

The neutron yield from the spontaneous fission of the short-lived neutron-deficient 254Rf nucleus produced in the complete fusion reaction with the beam of multiply charged ions from the U400 accelerator at the Flerov Laboratory of Nuclear Reactions (FLNR), Joint Institute for Nuclear Research (JINR), is measured using the combined detection system of the SHELS velocity filter. The half-life and the branching ratio of 254Rf are also measured. The data on the multiplicity of prompt neutrons from spontaneous fission of 256Rf ($$\bar {\nu }$$ = 3.87 ± 0.34) are presented for the first time.The neutron yield from the spontaneous fission of the short-lived neutron-deficient 254Rf nucleus produced in the complete fusion reaction with the beam of multiply charged ions from the U400 accelerator at the Flerov Laboratory of Nuclear Reactions (FLNR), Joint Institute for Nuclear Research (JINR), is measured using the combined detection system of the SHELS velocity filter. The half-life and the branching ratio of 254Rf are also measured. The data on the multiplicity of prompt neutrons from spontaneous fission of 256Rf ($$\bar {\nu }$$ = 3.87 ± 0.34) are presented for the first time.status: publishe

Measurement of evaporation residue cross-sections of the reaction Si-30+U-238 at subbarrier energies

The reaction 30Si + 238U ! 268Sg\ua4 was studied at beam energies close to the Coulomb barrier. At a center-of-mass energy of Ec:m: = 144:0MeV for reactions at half thickness of the target we measured three decay chains of 263Sg produced by evaporation of \uafve neutrons. The cross-section was (67+67 \ua137) pb. At Ec:m: = 133:0MeV we measured three spontaneously \uafssioning nuclei which we assigned to the isotope 264Sg. The production cross-section was (10+10 \ua1 6 ) pb and a half-life of (120+126 \ua1 44 ) ms was determined. This half-life is a factor of twenty shorter than theoretical predictions. At Ec:m: = 128:0MeV an upper cross- section limit of 15 pb was measured. The cross-section data reveal a strong in\ub0uence of the orientation of the deformed target nucleus on the production yield. Compared to excitation functions measured for the lighter system 16O + 238U ! 254Fm\ua4, a reduction of the fusion probability was observed at low beam energies indicating increasing competition from quasi\uafssion processes

Experimental tests of the modernized VASSILISSA separator (SHELS) with the use of accelerated 50Ti ions

A high intensity ion beam of 50Ti ions was obtained using the ECR ion source on the U400 cyclotron. The experimental tests using accelerated 50Ti ions were performed with a modernized VASSILISSA separator (SHELS). Data has been obtained on the transmission coefficients of recoil nuclei synthesized in complete fusion reactions. Estimates from ion optical calculations performed in the design phase of the project of modernizing the separator are completely confirmed.status: publishe

Short-Lived Isotopes of Transfermium Elements: Studying Characteristics of Spontaneous Fissioning

© 2018, Allerton Press, Inc. series of experiments performed in 2014–2015 using the SHELS separator and a beam of multicharged ions from the U-400 accelerator at the Flerov Laboratory of Nuclear Reactions (Joint Institute for Nuclear Research) in Dubna is described. In preparation for a large program of experiments with a beam of highly charged ions of 50Ti, an experiment is performed to study the spontaneous fissioning (SF) of short-lived neutron-deficient isotope 256Rf formed in complete fusion reaction50Ti + 208Pb = 256Rf + 2n. The half-life of 256Rf is measured along with the probability of its SF decay. The average number of neutrons per one event of 256Rf SF (v¯ = 4.47 ± 0.09) is obtained for the first time. The results from experiments to study the SF of short-lived neutron-deficient isotopes synthesized in complete fusion reaction 48Ca + 204Pb → 252No* are presented as well. Two short-lived activities attributed to the SF of the ground and isomeric states of 250No are revealed. The half-life, total kinetic energies of fission fragments, and neutron multiplicities are measured for the obtained short-lived nuclei. The average numbers of neutrons per one SF event are v¯ = 4.38 ± 0.13 and v¯ = 3.90 ± 0.20, respectively, for SF activity with t1/2 = 5.1 ± 0.3 and t1/2 = 36 ± 3 μs.status: publishe