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The reaction of 249Bk with 48Ca have been reinvestigated to provide new evidence for the discovery of element 117 on a larger number of events. The experiments were performed at five projectile energies and with a total beam dose of 48Ca of about 4.6×1019. Two isotopes 293,294117 were synthesized in the 249Bk+48Ca reaction, providing excitation functions and α-decay spectra of the produced isotopes that establishes these nuclei to be the products of the 4n- and 3n-evaporation channels, respectively. Decay properties of 293,294117 and of all the daughter products agree with the data of the experiment in which these nuclei were synthesized for the first time in 2010. The new 289115 events, populated by α decay of 293117, demonstrate the same decay properties as those observed for 289115 produced in the 243Am(48Ca,2n) reaction thus providing crossbombardment evidence. In addition, a single decay of 294118 was observed from the reaction with 249Cf – a result of the in-growth of 249Cf in the 249Bk target.

The neutron activation of gold is the basis of an implosion performance diagnostic at the National Ignition Facility at Lawrence Livermore National Laboratory. In support of this diagnostic, a series of γ-ray spectrometric measurements of the decay of 196Aum2 (Jπ = 12−) was performed to improve the currently accepted literature values of the nuclear data associated with its half-life, γ-ray energies, and γ-ray intensities. It was determined that 196Aum2 decays with a half-life of 9.603 h ± 0.23%. The relative intensities of the γ rays emitted during its decay were also measured, and an absolute decay branch of 0.3352 ± 2.9% was determined for the emission of the 188.2 keV photon, which arises from a nuclear transition whose multipolarity is predominantly M1. Properties of other products arising in the reaction of 197Au with fast neutrons were measured, as were selected production cross sections. The 196Aum2/196Aug isomer ratio measured in the 197Au(n, 2n) reaction at 14.1 MeV was found to be 0.0731 ± 2.6%. [ABSTRACT FROM AUTHOR]

The neutron-capture cross section for 238Pu was measured by using the detector for advanced neutron-capture experiments (DANCE) array, which is a highly segmented and highly efficient 4Π γ-ray calorimeter. The neutron-capture events were recognized by the total γ-ray energy deposited in DANCE, which is equal to the reaction Q value plus the incident neutron energy. The absolute neutron-capture cross section was derived as a function of incident neutron energy from thermal to about 30 keV. The measured cross section for incident neutron energy below 18 eV was performed for the first time by using the direct method and does not support the most recently adopted changes in endf/b-vii.1 where the neutron-capture cross section was lowered by as much as a factor of ∼3 in the neighborhood of 0.3 eV from those evaluated in ENDF/B-VII.0. [ABSTRACT FROM AUTHOR]

Studies of superheavy nuclei produced in the 249Bk + 48Ca reaction were performed using the Dubna Gas Filled Recoil Separator. The cross section for the production of 293117 and 294117 isotopes was measured at five excitation energies of the 297117 compound nucleus ranging from 30 to 48 MeV and yielding maximum values of 1.1-0.6+1.2 pb for the 3n and 2.4-1.4+3.3pb for the 4n reaction channels. Alpha emission from 281Rg competing with spontaneous fission (ɑ/SF decay probability 1:9) was observed for the first time leading to the identification of the new isotope 277Mt (TSF ~ 5 ms). The measured decay properties are in good agreement with those expected based on the properties of neighboring even-Z and odd-Z nuclei. The ɑ energies and half-lives of odd-Z isotopes observed in the 293117 and 294117 decay chains together with results obtained for lower-Z superheavy nuclei demonstrate enhanced stability with increasing neutron number toward the predicted new magic number N = 184. [ABSTRACT FROM AUTHOR]

The prompt γ-ray energy and multiplicity distributions were measured for the neutron-induced fission in 235U and 239,241Pu by using a highly segmented 4π γ-ray calorimeter in coincidence with the detection of fission fragments by a gas-filled parallel-plate avalanche counter. Both distributions were unfolded according to the detector response, which was simulated numerically by using a model validated with the γ-ray calibration sources. The mean value and the width of the γ-ray multiplicity distribution show a systematic increase with increasing mass of fissile nucleus, whereas, the energy distribution shows the dependence of γ-ray energy above 5 MeV on the species of fissile nuclei. The correlations between γ-ray energy and multiplicity were studied by comparing the mean value and the width of the total γ-ray energy between measurement and simulation by using an assembly with elements selected by random sampling of their unfolded distributions. The detector response was taken into account in the simulation. These results together with the detailed description of the experiment and analysis are presented. [ABSTRACT FROM AUTHOR]

Results from the production and decay properties of element 115 nuclei observed using the reaction 243 Am + 48Caat various beam energies between November 1,2010, and February 26,2012, at the Dubna Gas Filled Recoil Separator are presented. This long-running experiment with a total beam dose of 3.3 × 1019 and carried out in the excitation energy range E* = 31-47 MeV of the 291115 compound nucleus resulted in observation of three isotopes of element 115 with masses 287, 288, and 289. The 28 detected decay chains of 288115 show that this isotope is produced with the maximum probability at E* = 34.0-38.3 MeV with a corresponding cross section of σ3n = 8.5-3.7+6.4Pb- The four events attributed to the isotope 289115 that decays via a short ɑ --> ɑ -» SF chain could be detected only at the lowest excitation energy E* = 31-36 MeV, in accordance with what could be expected for the 2n-evaporation channel of the reaction. The decay characteristics of this nuclide were established earlier (2010) and more recently (2012) in the reaction 249Bk(48Ca,4n)239117 and following a decay to 289115. At the energy E* = 44.8 ± 2.3 MeV we observed only a single long chain of the isotope 287115. The decay properties of nuclei starting at 288115 and 287115 isotopes obtained in the present work reproduce in full the results of the first experiment of 2003 that reported the discovery of elements 115 and 113. The excitation functions of the production of the isotopes of element 115 and observation of the isotope 289115 in cross-bombardment reactions with the targets of 243Am and 249Bk provide additional evidence of the identification of the nuclei of elements 115 and 113. The experiments were carried out using the 48Ca beam of the U400 cyclotron of the Flerov Laboratory of Nuclear Reactions, JINR. [ABSTRACT FROM AUTHOR]

Two years after the discovery of element 117, we undertook a second campaign using the 249Bk + 48Ca reaction for further investigations of the production and decay properties of the isotopes of element 117 on a larger number of events. The experiments were started in the end of April 2012 and are still under way. This Letter presents the results obtained in 1200 hours of an experimental run with the beam dose of 48Ca of about 1.5 X 1019 particles. The 249Bk target was irradiated at two energies of 48Ca that correspond to the maximum probability of the reaction channels with evaporation of three and four neutrons from the excited 297117. In this experiment, two decay chains of 294117 (3M) and five decay chains of 293117 (4M) were detected. In the course of the long-term work, 249Cf--the product of decay of 249Bk (330 d)--is being accumulated in the target. Consequently, in the present experiment, we also detected a single decay of the known isotope 294118 that was produced during 2002-2005 in the reaction 249Cf(48Ca, 3M)294118. The obtained results are compared with the data from previous experiments. The experiments are carried out in the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, using the heavy-ion cyclotron U400. [ABSTRACT FROM AUTHOR]

The prompt &ggr;-ray energy and multiplicity distributions in the spontaneous fission of 252Cf have been measured using a highly segmented 4π &ggr;-ray calorimeter. Corrections were made for both distributions according to the detector response, which is simulated numerically using a model validated with the &ggr;-ray calibration sources. A comparison of the total &ggr;-ray energy distribution was made between the measurement and a simulation performed by random sampling of the corrected &ggr;-ray energy and multiplicity distributions and then transporting those &ggr; rays through the response of the detector array. The agreement between the measurement and simulation for the mean energy and width is markedly improved at higher multiplicities compared to the lower ones, illustrating the diminishing correlation between the &ggr;-ray and multiplicity energy and the stochastic aspect of the prompt &ggr; emission in spontaneous fission at higher multiplicities. [ABSTRACT FROM AUTHOR]

Results of a new series of experiments on the study of production cross sections and decay properties of the isotopes of element 115 in the reaction 243Am + 48Ca are presented. Twenty-one new decay chains originating from 288115 were established as the product of the 3n-evaporation channel by measuring the excitation function at three excitation energies of the compound nucleus 291115. The decay properties of all newly observed nuclei are in full agreement with those we measured in 2003. At the lowest excitation energy E* = 33 MeV, for the first time we registered the product of the 2n-evaporation channel, 289115, which was also observed previously in the reaction 249Bk + 48Ca as the daughter nucleus of the decay of 293117. The maximum cross section for the production of 288115 is found to be 8.5 pb at E* ≈ MeV. [ABSTRACT FROM AUTHOR]