Irena Dragojevic

Gas chemical investigation of hafnium and zirconium complexes with hexafluoroacetylacetone using preseparated short-lived radioisotopes

Abstract Volatile metal complexes of the group 4 elements Zr and Hf with hexafluoroacetylacetonate (hfa) have been studied using short-lived radioisotopes of the metals. The new technique of physical preseparation has been employed where reaction products from heavy-ion induced fusion reactions are isolated in a physical recoil separator – the Berkeley Gas-filled Separator in our work – and made available for chemistry experiments. Formation and decomposition of M(hfa)4 (M=Zr, Hf) has been observed and the interaction strength with a fluorinated ethylene propylene (FEP) Teflon surface has been studied. From the results of isothermal chromatography experiments, an adsorption enthalpy of -ΔHa=(57±3) kJ/mol was deduced. In optimization experiments, the time for formation of the complex and its transport to a counting setup installed outside of the irradiation cave was minimized and values of roughly one minute have been reached. The half-life of 165Hf, for which conflicting values appear in the literature, was measured to be (73.9±0.8) s. Provided that samples suitable for α-spectroscopy can be prepared, the investigation of rutherfordium (Rf), the transactinide member of group 4, appears possible. In the future, based on the studies presented here, it appears possible to investigate short-lived single atoms produced with low rates ( e.g. , transactinide isotopes) in completely new chemical systems, e.g. , as metal complexes with organic ligands as used here or as organometallic compounds.

Excitation Function for the 74Se(18O,p3n) Reaction

Abstract The 74Se( 18O,p3n)88gNb excitation function was measured and a maximum cross section of 495±5 mb was observed at and 18O energy of 74.0 MeV. Experimental cross sections were compared to theoretical calculations using the computer code ALICE-91 and the values were found to be in good agreement. The half-life of 88gNb was determined to be around 14.56±0.11 min.

Nuclear spectroscopy of the heaviest elements: studies of 254No, 257Rf, and 261Sg

Recently it has become possible to perform detailed spectroscopy on nuclei beyond Z = 100 with the aim of understanding the underlying single-particle structure of superheavy elements. A number of such experiments have been performed at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory using the Berkeley Gas-filled Separator (BGS), coupled with delayed γ-ray and electron-decay spectroscopy. Experiments have been performed on 254No (Z = 102), 257Rf (Z = 104), and 261Sg (Z = 106). The results provide new information on the properties of transactinide nuclei, which is important for testing models of the heaviest elements.

Electromagnetic decays of excited states in {sup 261}Sg (Z=106) and {sup 257}Rf (Z=104)

An isomeric one-quasineutron state, likely based on the [725]11/2{sup -} Nilsson level, was identified in {sup 261}Sg by its decay via internal conversion electrons. The state has an excitation energy of approx =200 keV and a half-life of 9.0{sub -1.5}{sup +2.0} mus. {sup 261}Sg has the highest Z and A of any nucleus in which the electromagnetic decay of an isomeric state was observed to date. A separate experiment was performed on the alpha daughter nucleus of {sup 261}Sg, namely {sup 257}Rf. Spectroscopy of delayed gamma rays and converted electrons from {sup 257}Rf resulted in the identification of a K isomer at an excitation energy of approx =1125 keV with a half-life of 134.9 +- 7.7 mus. The spin of the isomeric state is tentatively assigned I=21/2,23/2 and the state likely decays to a rotational band built on the [725]11/2{sup -} Nilsson level via a DELTAK=5 or 6 transition. The present results provide new information on excited states in the transactinide region, which is important for testing models of the heaviest elements.

Extraction of niobium and tantalum isotopes using organophosphorus compounds - Part I - Extraction of 'carrier-free' metal concentrations from HCl solutions

Abstract The extraction of niobium (Nb) and tantalum (Ta) from hydrochloric acid media by bis(2-ethylhexyl) hydrogen phosphate (HDEHP) and bis(2-ethylhexyl) hydrogen phosphite (BEHP) was studied. The goal of the experiments is to find a system that demonstrates selectivity between the members of group five of the Periodic Table and is also suitable for the study of dubnium (Db, Z=105). Experiments were performed at the trace level (10-16 M Nb or Ta) using hydrochloric acid with concentrations ranging from 1−11 M and short-lived isotopes of Nb and Ta produced in nuclear reactions. When HDEHP was used as the extractant, the Nb extraction yield decreased with increasing acid concentrations above 6 M, while the amount of Ta extracted remained over 75% for all acid concentrations studied. Tantalum was found to be extracted by BEHP at acid concentrations above 6 M, while niobium was not significantly extracted. The data obtained are used as the basis to discuss the speciation of Nb and Ta under the conditions studied and to evaluate possible extraction mechanisms.

Independent Verification of Element 114 Production in the {sup 48}Ca+{sup 242}Pu Reaction

Independent verification of the production of element 114 in the reaction of 244-MeV {sup 48}Ca with {sup 242}Pu is presented. Two chains of time- and position-correlated decays have been assigned to {sup 286}114 and {sup 287}114. The observed decay modes, half-lives, and decay energies agree with published results. The measured cross sections at a center-of-target energy of 244 MeV for the {sup 242}Pu({sup 48}Ca,3-4n){sup 287,286}114 reactions were 1.4{sub -1.2}{sup +3.2} pb each, which are lower than the reported values.

High-K multi-quasiparticle states and rotational bands in {sub 103}{sup 255}Lr

Two isomeric states have been identified in 255 Lr. The decay of the isomers populates rotational structures. Comparison with macroscopic-microscopic calculations suggests that the lowest observed sequence is built upon the [624]9/2+ Nilsson state. However, microscopic cranked relativistic Hartree-Bogoliubov (CRHB) calculations do not reproduce the moment of inertia within typical accuracy. This is a clear challenge to theories describing the heaviest elements.

Measurement of the {sup 208}Pb({sup 52}Cr,n){sup 259}Sg excitation function

The excitation function for the {sup 208}Pb({sup 52}Cr,n){sup 259}Sg reaction has been measured using the Berkeley Gas-filled Separator at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron. The maximum cross section of 320{sub -100}{sup +110} pb is observed at a center-of-target laboratory-frame energy of 253.0 MeV. In total, 25 decay chains originating from {sup 259}Sg were observed and the measured decay properties are in good agreement with previous reports. In addition, a partial excitation function for the {sup 208}Pb({sup 52}Cr,2n){sup 258}Sg reaction was obtained, and an improved {sup 258}Sg half-life of 2.6{sub -0.4}{sup +0.6} ms was calculated by combining all available experimental data.

New isotope {sup 263}Hs

A new isotope of Hs was produced in the reaction {sup 208}Pb({sup 56}Fe,n){sup 263}Hs at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory. Six genetically correlated nuclear decay chains have been observed and assigned to the new isotope {sup 263}Hs. The measured cross section was 21{sub -8.4}{sup +13} pb at 276.4 MeV lab frame center-of-target beam energy. {sup 263}Hs decays with a half-life of 0.74{sub -0.21}{sup +0.48} ms by {alpha}-decay and the measured {alpha}-particle energies are 10.57 {+-} 0.06, 10.72 {+-} 0.06, and 10.89 {+-} 0.06 MeV. The experimental cross section is compared to a theoretical prediction based on the Fusion by Diffusion model [W. J. SwiaPtecki et al., Phys. Rev. C 71, 014602 (2005)].

Lightest Isotope of Bh Produced via the {sup 209}Bi({sup 52}Cr,n){sup 260}Bh Reaction

The lightest isotope of Bh was produced in the new 209Bi(52Cr,n)260Bh reaction at the Lawrence Berkeley National Laboratorys 88-Inch Cyclotron. Positive identification was made by observation of eight correlated alpha particle decay chains in the focal plane detector of the Berkeley Gas-Filled Separator. 260Bh decays with a 35(-9)(+19) ms half-life by alpha particle emission mainly by a group at 10.16 MeV. The measured cross section of 59(-20)(+29) pb is compared to model predictions. The influence of the N=152 and Z=108 shells on alpha decay properties is discussed.