Lawrencium chemistry: no evidence for oxidation states lower than 3+ in aqueous solution

Lawrencium (3-min 260Lr) together with other actinides, was produced in the bombardment of a 249Bk target with 18O ions. There was no sign of a reduction of Lr3+ in dilute hydrochloric acid by V2+ or Cr2+, although in the same experiments, Md3+ was reduced to Md2+ (E°=−0.2 V). The resulting limit for the reduction potential of the Lr3+/Lr1(2)+ couple is E° < −0.44 V.

The new nuclide225U

In the bombardment of a 270μg/cm 2 180Hf target with48 Ca projectiles at a primary beam energy of E/A=4.24 MeV/u the new nuclide225U was produced. The experiment was performed at the velocity filter SHIP. 225U was found to decay by α emission with Eα=(7880 ±20) keV (≈90%), (7830±20) keV (≈10%) and has a half-life ofT 1/2=(80 −20 +40 ms).

Extraction behavior of rutherfordium into tributylphosphate from hydrochloric acid

The extraction behavior of rutherfordium (Rf) into tributylphosphate (TBP) from hydrochloric acid (HCl) has been studied together with those of the lighter group-4 elements Zr and Hf. The extractability of261Rf,169Hf, and85Zr into TBP was investigated under identical conditions in 7.2–8.0 M HCl by on-line reversed-phase extraction chromatography. The percent extractions of Rf, Hf, and Zr into the TBP resin increase steeply with increasing HCl concentration, and the order of extraction is Zr &gt; Hf ≈ Rf. By considering the order of chloride complexation among these elements, it is suggested that the stability of the TBP complex of Rf tetrachloride is lower than those of Zr and Hf.

An EC-branch in the decay of 27-s 263Db: Evidence for the isotope 263Rf

Summary 27-s 263Db was produced in the 249Bk ( 18O, 4n) reaction at 93 MeV. The activity was transported by a He/KCl-jet to the laboratory where it was collected for 15 min and then subjected to a chemical separation specific for group-4 elements. The activity was dissolved in 0.5 M unbuffered α-HiB and eluted from a cation-exchange column. The effluent was made 9 M in HCl and group-4 tetrachlorides were extracted into TBP/Cyclohexane which was evaporated to dryness on a Ta disc. The Ta discs were assayed for α and SF activity. A SF activity with a half life on the order of 20 min was observed and assigned to the nuclide 263Rf. It is formed by electron-capture decay of 263Db with a decay br…

The recoil transfer chamber—An interface to connect the physical preseparator TASCA with chemistry and counting setups

Performing experiments with transactinide elements demands highly sensitive detection methods due to the extremely low production rates (one-atom-at-a-time conditions). Preseparation with a physical recoil separator is a powerful method to significantly reduce the background in experiments with sufficiently long-lived isotopes (t1/2≥0.5 s). In the last years, the new gas-filled TransActinide Separator and Chemistry Apparatus (TASCA) was installed and successfully commissioned at GSI. Here, we report on the design and performance of a Recoil Transfer Chamber (RTC) for TASCA—an interface to connect various chemistry and counting setups with the separator. Nuclear reaction products recoiling o…

First Aqueous Chemistry with Seaborgium (Element 106)

Fluoride Complexation of Element 104, Rutherfordium (Rf), Investigated by Cation-exchange Chromatography

We report on new and much more precise cation-exchange data of element 104, rutherfordium (Rf), in the fluoride ion concentration [F−] range of 5.29×10−5−1.04×10−3 M. The result based on one-atom-a...

Extraction of the fluoride-, chloride- and bromide complexes of the elements Nb, Ta, Pa, and 105 into aliphatic amines

Abstract Previous studies of the halide complex formation of element 105 in HCl–HF mixtures and extractions into triisooctyl amine (TIOA) have been performed with the Automated Rapid Chemistry Apparatus, ARCA II. Element 105 was shown to be absorbed on the column from 12 M HCl–0.02 M HF together with its lighter homologues Nb, Ta and the pseudohomologue Pa. In elutions with 10 M HCl–0.025 M HF, 4 M HCl–0.02 M HF, and 0.5 M HCl–0.01 M HF, the extraction sequence Ta>Nb>105>Pa was observed and element 105 behaved very differently from its closest homologue Ta. As it is not possible within reasonable effort to model the many presumably mixed fluoride–chloride complexes involved in these studies…

Cross section limits for theCm248(Mg25,4n−5n)Hs268,269reactions

We report on an attempt to produce and detect $^{268}\mathrm{Hs}$ and $^{269}\mathrm{Hs}$ in the nuclear fusion reaction $^{25}\mathrm{Mg}+^{248}\mathrm{Cm}$ using the gas phase chemistry apparatus COMPACT. No decay chains attributable to the decay of hassium isotopes were observed during the course of this experiment. From the nonobservation of $^{269}\mathrm{Hs}$ we derive a cross section limit of 0.4 pb (63% confidence limit) for the reaction $^{248}\mathrm{Cm}(^{25}\mathrm{Mg},4n)^{269}\mathrm{Hs}$ at a center-of-target beam energy of 140 MeV. The evaluated cross section limit for the $^{248}\mathrm{Cm}(^{25}\mathrm{Mg},5n)^{268}\mathrm{Hs}$ reaction depends on the assumed half-life of …

Production and Decay of Element 114: High Cross Sections and the New NucleusHs277

The fusion-evaporation reaction Pu-244(Ca-48, 3-4n)(288,289)114 was studied at the new gas-filled recoil separator TASCA. Thirteen correlated decay chains were observed and assigned to the production and decay of (288, 289)114. At a compound nucleus excitation energy of E* = 39.8-43.9 MeV, the 4n evaporation channel cross section was 9.8(-3.1)(+3.9) pb. At E* = 36.1-39.5 MeV, that of the 3n evaporation channel was 8.0-(+7.4)(4.5) pb. In one of the 3n evaporation channel decay chains, a previously unobserved alpha branch in (281)Ds was observed ( probability to be of random origin from background: 0.1%). This alpha decay populated the new nucleus (277)Hs, which decayed by spontaneous fission…

Search for Superheavy Elements in theU238+U238Reaction

A search was made for spontaneously fissioning superheavy elements in damped collisions of two uranium nuclei. Different techniques were applied covering the elements 108 to 118 and approx. =126, and a half-life range from 1 ms to more than 1 yr. No evidence for superheavy elements was found at upper cross-section limits of 10/sup -32/, 10/sup -33/, and 10/sup -35/ cm/sup 2/ for half-lives from 1 to 100 ms, 100 ms to 1 d, and 1 d to 1 yr, respectively.

Isotope Distributions in the Reaction ofU238withU238

Radiochemically determined cross sections $\ensuremath{\sigma}(Z, A)$ were used to construct charge and mass distributions for the reaction of 1785-MeV $^{238}\mathrm{U}$ ions with thick $^{238}\mathrm{U}$ targets. Fission of the colliding nuclei is found to dominate. For the surviving uraniumlike fragments an enhancement of yields compared to the Kr + U and Xe + U reactions is observed. The formation of heavy actinides is shown to be associated with the low-energy tails of the broad excitation energy distributions in damped collisions.

Nuclear reactions in collisions of very heavy ions at energies below and near the barrier

Cross sections for nuclear reactions in collisions of238U+238U and238U +197Au near and below the coulomb barrier

Cross sections for nuclear reactions at beam energies near and below the spherical Coulomb barrier V c were measured in the very heavy collision systems238U +238U and238U +197Au. The most probable reaction channel with mass transfer is the one-neutron transfer. Its excitation function is understood in terms of Rutherford trajectories together with the quantal process of neutron tunnelling over large distances. In addition, the exchange of up to 15 nucleons is observed down to 0.90 V c . The excitation functions for the multi-nucleon transfer products have much steeper slopes than that for one-neutron transfer, and are steeper for238U +197Au than for238U +238U, suggesting that nuclear contac…

Determination of the partial electron capture- and spontaneous-fission half-lives of254No

The isotope254No was produced in the fusion reaction48Ca +208Pb. Using the velocity filter SHIP and radiochemical techniques it was found that the nuclide254No with a half-life of 55 s decays byα, EC, and spontaneous-fission. Deduced partial half-lives are (61±2) s forα-decay, (550−160+370) s for EC and [2.2−1.0+2.0]×104 s for spontaneous fission.

Backings and targets for chemical and nuclear studies of transactinides with TASCA

Abstract At GSI the gas-filled separator TASCA (TransActinide Separator and Chemistry Apparatus) was set up to investigate the chemical and physical properties of the heaviest elements making use of the highest beam intensities available [ www.gsi.de/tasca ; M. Schadel, D. Ackermann, A. Semchenkov, A. Turler, GSI Scientific Report 2005, GSI Report 2006-1, p. 262]. Appropriate backings and targets have to be developed. Conceivable backing materials are aluminium, titanium, and carbon. Aluminium backings and titanium backings in different thickness and from different companies are produced by cold rolling. Deposition by resistance heating is applied for carbon backings. For experiments in a c…

Superheavy element flerovium (element 114) is a volatile metal.

The electron shell structure of superheavy elements, i.e., elements with atomic number Z ≥ 104, is influenced by strong relativistic effects caused by the high Z. Early atomic calculations on element 112 (copernicium, Cn) and element 114 (flerovium, Fl) having closed and quasi-closed electron shell configurations of 6d(10)7s(2) and 6d(10)7s(2)7p1/2(2), respectively, predicted them to be noble-gas-like due to very strong relativistic effects on the 7s and 7p1/2 valence orbitals. Recent fully relativistic calculations studying Cn and Fl in different environments suggest them to be less reactive compared to their lighter homologues in the groups, but still exhibiting a metallic character. Expe…

New nuclides: Neptunium-243 and neptunium-244

We have observedγ-rays following the β−-decay of243Np and244Np after chemical isolation of neptunium isotopes from the products of the reaction of 835 MeV136Xe with244Pu. The ground-state of 1.85-min243Np hasJ=5/2. The decay of 2.29-min244Np (probableJπ=7−) populates high-spin members of the ground state rotational band in244Pu.

Actinide Production in Collisions ofU238withCm248

Cross sections for the production of heavy actinides in damped collisions of $^{238}\mathrm{U}$ ions with $^{248}\mathrm{Cm}$ targets are reported and compared with similar data for other projectiles. The relatively small differences in the formation rates of a given isotope made by different projectiles indicate a balance between increased mass transfer probability with increasing projectile mass and a concurrent decrease in survivability because of an increase in excitation energy.

Evidence for quasi-fission in40Ar+208Pb collisions near the coulomb-barrier

Fission-fragment angular distributions were measured in the reaction of40Ar with208Pb near the fusion barrier. For nearly symmetric mass-/charge splits we find angular distributions symmetric around θ=90 degrees, however, with unusually large anisotropies. These develop gradually into forward-backward asymmetric distributions as one moves away from mass-/charge symmetry. This indicates that non-compound fission (‘quasi-fission’) competes with true fusion-fission. The relative contribution of quasi-fission to the total fission cross section is somewhere between 51 and 85%. In the framework of the extra-push model this is equivalent to an extra-extra push energy for compound-nucleus formation…

Preparation of targets for the gas-filled recoil separator TASCA by electrochemical deposition and design of the TASCA target wheel assembly

Abstract The Transactinide Separator and Chemistry Apparatus (TASCA) is a recoil separator with maximized transmission designed for performing advanced chemical studies as well as nuclear reaction and structure investigations of the transactinide elements ( Z >103) on a one-atom-at-a-time basis. TASCA will provide a very clean transactinide fraction with negligible contamination of lighter elements from nuclear side reactions in the target. For TASCA a new target chamber was designed and built at GSI including the rotating target wheel assembly ARTESIA for beam intensities up to 2 μA (particle). For the production of longer-lived isotopes of neutron-rich heavier actinide and transactinide e…

Unusual excitation-energy division in quasi-fission reactions between50Ti,54Cr and58Fe with207,208Pb at the barrier

Angular distributions in quasi-fission reactions

Abstract Angular distributions for fission-like fragments were measured in the systems 50Ti, 56Fe + 208Pb by applying an off-line K X-ray activation technique. The distributions d2σ/dθdZ exhibit forward-backward asymmetries that are strongly Z-dependent. They result from a process (quasi-fission) which yields nearly symmetric masses in times comparable to the rotational period of the composite system. A method for obtaining the variance of the tilting angular momentum, K02, from these skewed, differential angular distributions is described. The results indicate that the tilting mode is not fully excited in quasi-fission reactions. The results are compared to the sum of the variances of all …

Fission in238U+238U collisions below the Coulomb barrier

Integral fission cross sections in the system238U+238U were measured at beam energies below the interaction barrierVC. Scattering angle dependent probabilities and integral cross sections for Coulomb fission were calculated. It is concluded that earlier observed discrepancies between measured and calculated angular distributions for the one-neutron transfer product239U cannot be explained by sequential fission. Multi-nucleon transfer induced fission is observed down to energies (0.90±0.02)×VC.

Chemical studies on rutherfordium (Rf) at JAERI

SummaryChemical studies on element 104, rutherfordium (Rf), at JAERI (Japan Atomic Energy Research Institute) are reviewed. The transactinide nuclide261Rf has been produced in the reaction248Cm(18O, 5n) at the JAERI tandem accelerator with the production cross section of about 13 nb. On-line anion-exchange experiments on Rf together with the lighter homologues, group-4 elements Zr and Hf, in acidic solutions have been conducted with a rapid ion-exchange separation apparatus. From the systematic study of the anion-exchange behavior of Rf, it has been found that the properties of Rf in HCl and HNO3solutions are quite similar to those of Zr and Hf, definitely confirming that Rf is a member of …

Continuous separation and identification of neutronrich neptunium isotopes from heavy-ion reactions by means of the centrifuge system “sisak”

Abstract Neutron-rich neptunium isotopes formed in direct transfer reactions between 136 Xe projectiles and targets of 244 Pu have been separated from other reaction products with the on-line liquid-liquid extraction system SISAK. A four-detector delay method was used to determine the half-lives of the previously unknown isotopes 243 Np and 244 Np. From the decay of the 287.4 keV γ-line a half-life of 1.8 ± 0.3 min was determined for 243 Np. From the decay curves of the γ-lines at 162.7 and 216.7 keV, which fit into the known level scheme of 244 Pu, a half-life of 2.5 ± 0.3 min results for 244 Np.

Anion-exchange Behavior of Rf in HCl and HNO3 Solutions

H. Haba,∗,a K. Tsukada,a M. Asai,a S. Goto,a,b A. Toyoshima,a,c I. Nishinaka,a K. Akiyama,a M. Hirata,a S. Ichikawa,a Y. Nagame,a Y. Shoji,c M. Shigekawa,c T. Koike,c M. Iwasaki,c A. Shinohara,c T. Kaneko,b T. Maruyama,b S. Ono,b H. Kudo,b Y. Oura,d K. Sueki,d H. Nakahara,a,d M. Sakama,e A. Yokoyama,f J. V. Kratz,g M. Schadel,h and W. Bruchleh Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 3191195, Japan Department of Chemistry, Faculty of Science, Niigata University, Niigata-shi, Niigata 950-2181, Japan Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka-shi, Osaka 5600043, Japan Department of Chemistry, Graduate Sc…

Search for superheavy elements in damped collisions betweenU238andCm248

Negative results for the production of superheavy elements in damped collisions between $^{238}\mathrm{U}$ projectiles and $^{248}\mathrm{Cm}$ targets are reported. This reaction was believed to permit a closer and more widespread approach to the predicted island of stability near Z=114 and N=184 than any practical fusion reaction. Aqueous and gas phase chemistry techniques were used to isolate superheavy element fractions. The fractions were counted for spontaneous fission activity, fragment kinetic energies, and neutron multiplicities. Cross-section limits for half-lives from hours to several years are g4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}35}$ ${\mathrm{cm}}^{…

Excitation functions for production of heavy actinides from interactions ofCa40andCa48ions withCm248

Excitation functions have been measured for production of isotopes of Bk through Fm in bombardments of /sup 248/Cm with 234- to 294-MeV /sup 40/Ca ions and with 239- to 318-MeV /sup 48/Ca ions. The maxima of the isotopic distributions for these elements occur at only 2 to 3 mass numbers larger for /sup 48/Ca than for /sup 40/Ca reactions. The shapes of the distributions and the half-widths of about 2.5 mass numbers are quite similar to those observed previously for reactions of /sup 16/O, /sup 18/O, /sup 20/Ne, and /sup 22/Ne with /sup 248/Cm. In general, the excitation functions for /sup 40/Ca show maxima near the Coulomb barrier while those for /sup 48/Ca are about 20 MeV above the barrie…

Fission fragment angular distributions in the capture reactions 50Ti, 56Fe + 208Pb

The analysis of %-ray mult ip l ic i t ies measured I in coincidence with capture fission events in reactions of 26Mg through 54Cr, 58Fe, and 64Ni with 208pb in terms of Ko and~eff revealed1 that the resulting values of ~ /~e f f were rather independent of for a given system and, in a compound nucleus picture, were more in agreement with the 'shape of the non-rotating saddle than with the actually expected rotating saddle I f non-compound systenmhave to be considered these values of ~/J~ff__ shapes. suggested at least that the captured systems near the turning point from the inward radial motion to symmetric fragmentation had reached a remarkable compactness.

Chemical investigation of hassium (element 108).

The periodic table provides a classification of the chemical properties of the elements. But for the heaviest elements, the transactinides, this role of the periodic table reaches its limits because increasingly strong relativistic effects on the valence electron shells can induce deviations from known trends in chemical properties. In the case of the first two transactinides, elements 104 and 105, relativistic effects do indeed influence their chemical properties, whereas elements 106 and 107 both behave as expected from their position within the periodic table. Here we report the chemical separation and characterization of only seven detected atoms of element 108 (hassium, Hs), which were…

First superheavy element experiments at the GSI recoil separator TASCA: The production and decay of element 114 in thePu244(Ca48,3-4n) reaction

Experiments with the new recoil separator, Transactinide Separator and Chemistry Apparatus (TASCA), at the GSI were performed by using beams of Ca-48 to irradiate targets of Pb206-208, which led to the production of No252-254 isotopes. These studies allowed for evaluation of the performance of TASCA when coupled to a new detector and electronics system. By following these studies, the isotopes of element 114 ((288-291)114) were produced in irradiations of Pu-244 targets with Ca-48 beams at compound nucleus excitation energies around 41.7 and 37.5 MeV, demonstrating TASCA's ability to perform experiments with picobarn-level cross sections. A total of 15 decay chains were observed and were as…

Attempts to chemically investigate element 112

Summary Two experiments aiming at the chemical investigation of element 112 produced in the heavy ion induced nuclear fusion reaction of 48Ca with 238U were performed at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt, Germany. Both experiments were designed to determine the adsorption enthalpy of element 112 on a gold surface using a thermochromatography setup. The temperature range covered in the thermochromatography experiments allowed the adsorption of Hg at about 35 °C and of Rn at about -180 °C. Reports from the Flerov Laboratory for Nuclear Reactions (FLNR), Dubna, Russia claim production of a 5-min spontaneous fission (SF) activity assigned to 283112 for the 238U(48Ca,3n)…

Angular distributions in quasi-fission reactions: Evidence for incomplete relaxation of the tilting mode

Angular distributions of fission-like fragments have been measured for50Ti+208Pb and56Fe+208Pb collisions. Z-dependent asymmetries around Θincm= 90° preclude their interpretation in terms of compound nucleus fission with the transition state theory. Fits of the data with a simple ansatz for statistical angular momentum fluctuations (tilting) give evidence for an incomplete relaxation of the tilting mode in quasi fission reactions.

Fusion-fission in the reaction76Ge+170Er

Radiochemical yield measurements were performed to study mass- and charge distributions in the reaction of76Ge ions with170Er in the c.m. energy range from 222 MeV through 272 MeV. The magnitude of the resulting cross-section for fusion-fission is significantly lower than expected and is explained within the framework of Swiatecki's Extra-Push model.

Adsorption of Db and its homologues Nb and Ta, and the pseudo-homologue Pa on anion-exchange resin in HF solution

Anion-exchange chromatography of element 105, dubnim (Db), produced in the 206 Cm( 19 F, 5n) 262 Db reaction is investigated together with the homologues Nb and Ta, and the pseudo-homologue Pa in 13.9 M hydrofluoric acid (HF) solution. The distribution coefficient (K d ) of Db on an anion-exchange resin is successfully determined by running cycles of 1702 chromatographic column separations. The result clearly indicates that the adsorption of Db on the resin is significantly different from that of the homologues and that the adsorption of anionic fluoro complexes of these elements decreases in the sequence of Ta ≈ Nb > Db > Pa.

Reactions of40Ar with233U,235U, and238U at the barrier

Production cross sections for target-like transfer products in reactions of40Ar with233,235,238U at the barrier were determined using radiochemical techniques. The heaviest products detected are isotopes of californium (Z=98). In addition to the quasi-elastic component of the isotopic distributions observed in the vicinity of uranium, there are also relaxed contributions throughout the entire region. The peak positions of the isotope distributions of this component for fixed atomic number, after the transfer of > 3 charges, approach closely the minimum of the potential energy surfaces (PES). The experimental results thus indicate the cold formation of the reaction products. A search for lon…

Search for long-lived superheavy elements in the reaction of136Xe with238U

A search with radiochemical methods for long-lived superheavy elements in 238U targets bombarded with intense beams of136Xe ions produced negative results. A formation cross section of ≤1×10−35 cm2 is deduced at 95% confidence level for nuclides with half-lives between 1 and 200 d.

Fluoride Complexation of Element 104, Rutherfordium

Fluoride complexation of element 104, rutherfordium (Rf), produced in the 248Cm(18O,5n)261Rf reaction has been studied by anion-exchange chromatography on an atom-at-a-time scale. The anion-exchange chromatographic behavior of Rf was investigated in 1.9-13.9 M hydrofluoric acid together with those of the group-4 elements Zr and Hf produced in the 18O-induced reactions on Ge and Gd targets, respectively. It was found that the adsorption behavior of Rf on anion-exchange resin is quite different from those of Zr and Hf, suggesting the influence of relativistic effects on the fluoride complexation of Rf.

New nuclideHa263

A new nuclide $^{263}\mathrm{Ha}$ was produced in the bombardment of a $^{249}\mathrm{Bk}$ target with 93-MeV $^{18}\mathrm{O}$ ions. It was detected via spontaneous fission counting and was shown to have a half-life of about 0.5 min. This activity was also separated from the reaction products by automated rapid chemical separations using cation-exchange chromatography in 0.05M \ensuremath{\alpha}-hydroxyisobutyric acid. After chemical separation, $^{263}\mathrm{Ha}$ was found to decay by spontaneous fission (${57}_{\mathrm{\ensuremath{-}}15}^{+13}$%) and by \ensuremath{\alpha} emission (${\mathit{E}}_{\mathrm{\ensuremath{\alpha}}}$=8.35 MeV, 43%) with a half-life of ${27}_{\mathrm{\ensurem…

Gas phase chromatography of halides of elements 104 and 105

On-line isothermal gas phase chromatography was used to study halides of261104 (T1/2=65 s) and262,263105 (T1/2=34 s and 27 s) produced an atom-at-a time via the reactions248Cm(18O, 5n) and249Bk(18O, 5n, 4n), respectively. Using HBr and HCl gas as halogenating agents, we were able to produce volatile bromides and chlorides of the above mentioned elements and study their behavior compared to their lighter homologs in Groups 4 or 5 of the periodic table. Element 104 formed more volatile bromide than its homolog Hf. In contrast, element 105 bromides were found to be less volatile than the bromides of the group 5 elements Nb and Ta. Both 104 and Hf chlorides were observed to be more volatile tha…

Electrodeposition methods in superheavy element chemistry

To prepare electrodeposition experiments with superheavy elements (SHE), their homologs were investigated. In the experiments, various electrode materials and electrolytes were used. Critical potentials (E crit ) where the electrodeposition starts and potentials for the deposition of 50% of the atoms in solution (E 50% ) were determined. Underpotential deposition was observed in most cases. An electrolytic cell for a fast electrochemical deposition was developed and the time for the deposition of 50% of the atoms in solution (t 50% ) was determined. Short lived α-emitting isotopes were produced at Gesellschaft fur Schwerionenforschung (GSI), Darmstadt, transferred to the aqueous phase with …

Competition between binary reactions and fusion in heavy-ion collisions at the Coulomb barrier

Mass and charge distributions for binary reaction channels have been measured for the reactions86Kr with76Ge,104Ru and130Te at the Coulomb barrier using chemical separations andγ-ray spectroscopy. These systems span the region where dynamical hindrance to complete fusion sets in. The binary reactions can be subdivided into two components associated withi) reflection from the outer potential barrier (quasielastic), andii) reseparation after passing the barrier (complex reactions). The sum of complex-reaction channels and evaporation residues from complete fusion can be reproduced by a barrier passing calculation. The fraction of the barrier passing flux leading to reseparation increases from…

Hexafluoro complex of rutherfordium in mixed HF/HNO3 solutions

Formation of anionic fluoride-complexes of element 104, rutherfordium, produced in the 248 Cm( 18 O, 5n) 261 Rf reaction was studied by anion-exchange on an atom-at-a-time scale. It was found that the hexafluoro complex of Rf, [RfF 6 ] 2- , was formed in the studied fluoride ion concentrations of 0.0005-0.013 M. Formation of [RfF 6 ] 2- was significantly different from that of the homologues Zr and Hf, [ZrF 6 ] 2- and [HfF 6 ] 2- ; the evaluated formation constant of [RfF 6 ] 2- is at least one-order of magnitude smaller than those of [ZrF 6 ] 2- and [HfF 6 ] 2- .

Decay properties of265Sg(Z=106)and266Sg(Z=106)

The presently known most neutron-rich isotopes of element 106 (seaborgium, Sg), {sup 265}Sg and {sup 266}Sg, were produced in the fusion reaction {sup 22}Ne+{sup 248}Cm at beam energies of 121 and 123 MeV. Using the On-Line Gas chemistry Apparatus OLGA, a continuous separation of Sg was achieved within a few seconds. Final products were assayed by {alpha}-particle and spontaneous fission (SF) spectrometry. {sup 265}Sg and {sup 266}Sg were identified by observing time correlated {alpha}-{alpha}-({alpha}) and {alpha}-SF decay chains. A total of 13 correlated decay chains of {sup 265}Sg (with an estimated number of 2.8 random correlations) and 3 decay chains of {sup 266}Sg (0.6 random correlat…

Doubly Magic NucleusHs162108270

Theoretical calculations predict $^{270}\mathrm{Hs}$ ($Z=108$, $N=162$) to be a doubly magic deformed nucleus, decaying mainly by $\ensuremath{\alpha}$-particle emission. In this work, based on a rapid chemical isolation of Hs isotopes produced in the $^{26}\mathrm{Mg}+^{248}\mathrm{Cm}$ reaction, we observed 15 genetically linked nuclear decay chains. Four chains were attributed to the new nuclide $^{270}\mathrm{Hs}$, which decays by $\ensuremath{\alpha}$-particle emission with ${Q}_{\ensuremath{\alpha}}=9.02\ifmmode\pm\else\textpm\fi{}0.03\text{ }\text{ }\mathrm{MeV}$ to $^{266}\mathrm{Sg}$ which undergoes spontaneous fission with a half-life of ${444}_{\ensuremath{-}148}^{+444}\text{ }\t…

Evidence for the formation of sodium hassate(VIII)

SummaryHassium, element 108, was produced in the fusion reaction between26Mg and248Cm. The hassium recoils were oxidizedin-situto a highly volatile oxide, presumably HsO4, and were transported in a mixture of He and O2to a deposition and detection system. The latter consisted of 16 silicon PIN-photodiodes facing a layer of NaOH, which served, in the presence of a certain partial pressure of water in the transport gas, as reactive surface for the deposition of the volatile tetroxides. Six correlated α-decay chains of Hs were detected in the first 5 detectors centred around detection position 3. In analogy to OsO4, which forms Na2[OsO4(OH)2], an osmate(VIII), with aqueous NaOH, HsO4presumably…

Fluoride complexation of rutherfordium (Rf, element 104)

The fluoride complexation of the group-4 elements Zr, Hf and Rf, and of the pseudo-homolog Th, has been investigated in mixed HNO3/HF solutions by studying Kd values on both cation exchange resins (CIX) and anion exchange resins (AIX) using the automated rapid chemistry apparatus ARCA. On the CIX, the four elements are strongly retained as cations below 10-3M HF. For Zr and Hf, the decrease of the Kd values due to the formation of fluoride complexes occurs between 10-3M HF and 10-2M HF. For Rf and Th, this decrease is observed at one order of magnitude higher HF concentrations. On the AIX, for Zr and Hf, a rise of the Kd values due to the formation of anionic fluoride complexes is observed …

The hydration enthalpies of Md3+ and Lr3+

Lawrencium (3-min 260Lr) and lighter actinides were produced in the bombardment of a 249Bk target with 18O ions and loaded onto a cation exchange column in 0.05 M α-hydroxyisobutyrate solution at pH=4.85, together with the radioactive lanthanide tracers 166Ho, 171Er and 171Tm. In elutions with 0.12 M α=hydroxyisobutyrate solution (pH=4.85), trivalent Lr was eluted exactly together with the Er tracer and Md was eluted close to Ho. Lr elutes much later than expected based on the known elution positions of the lighter actinides and the expected analogy to the elution positions of the homologous lanthanides. From the measured elution positions, ionic radii were calculated for Lr3+ and Md3+. Sem…

Observation of the3nEvaporation Channel in the Complete Hot-Fusion ReactionMg26+Cm248Leading to the New Superheavy NuclideHs271

The analysis of a large body of heavy ion fusion reaction data with medium-heavy projectiles ($6\ensuremath{\le}Z\ensuremath{\le}18$) and actinide targets suggests a disappearance of the $3n$ exit channel with increasing atomic number of the projectile. Here, we report a measurement of the excitation function of the reaction $^{248}\mathrm{Cm}(^{26}\mathrm{Mg},xn)^{274\mathrm{\text{\ensuremath{-}}}x}\mathrm{Hs}$ and the observation of the new nuclide $^{271}\mathrm{Hs}$ produced in the $3n$ evaporation channel at a beam energy well below the Bass fusion barrier with a cross section comparable to the maxima of the $4n$ and $5n$ channels. This indicates the possible discovery of new neutron-r…

Chemical Properties of Element 105 in Aqueous Solution: Halide Complex Formation and Anion Exchange into Triisoctyl Amine

Studies of the halide complexation of element 105 in aqueous solution were performed on 34-s 262Ha produced in the 249Bk(18-O,5n) reaction. The 262Ha was detected by measuring the fission and alpha activities associated with its decay and the alpha decays of its daughter, 4.3-s 258Lr. Time-correlated pairs of parent and daughter alpha particles provided a unique identification of the presence of 262Ha. About 1600 anion exchange separations of 262Ha from HCl and mixed HC1/HF solutions were performed on a one-minute time scale. Reversed-phase micro-chromatographic columns incorporating triisooctyl amine (TIOA) on an inert support were used in the computer-controlled liquid chromatography appa…

Charge-Asymmetry Equilibration in the Reaction ofXe129,132,136withAu197near the Interaction Barrier

Neutron-proton population ratios for quasielastic and inelastic processes in the reactions of 761-MeV $^{129}\mathrm{Xe}$, 769-MeV $^{132}\mathrm{Xe}$, and 795-MeV $^{136}\mathrm{Xe}$ ions with thick $^{197}\mathrm{Au}$ targets were determined radiochemically. Completely relaxed $\frac{N}{Z}$ ratios are observed for damped collisions involving the transfer of $\ensuremath{\Delta}Zg~1$ charge units. The limiting condition $\ensuremath{\Delta}Z\ensuremath{\approx}1$ corresponds to a characteristic time of the order of ${10}^{\ensuremath{-}22}$ sec which indicates the absence of dissipative forces in the equilibration of the charge-asymmetry mode.

Attempts to Produce Superheavy Elements by Fusion ofCa48withCm248in the Bombarding Energy Range of 4.5-5.2 MeV/u

A search for superheavy elements was made in bombardments of $^{248}\mathrm{Cm}$ with $^{48}\mathrm{Ca}$ ions performed at projectile energies close to the interaction barrier in order to keep the excitation energy of the compound nucleus $Z=116$, $A=296$ as low as possible. No evidence for superheavy nuclei was obtained in a half-life region from 1 \ensuremath{\mu}s to 10 yr with a production cross section greater than ${10}^{\ensuremath{-}34}$ to ${10}^{\ensuremath{-}35}$ ${\mathrm{cm}}^{2}$.

Electromagnetic and nuclear fission of238U in the reaction of 100, 500, and 1000 A�MeV208Pb with238U

The folding- and azimuthal-angle and velocity distributions for the238U fission fragments have been measured in reactions with 100, 500, and 1000 A·MeV208Pb. These distributions were used to decompose the fission cross section into its electromagnetic and nuclear components. The fraction of electromagnetic fission was found to be 0.16±0.07, 0.48±0.08, and 0.60±0.04, respectively. The electromagnetic fission cross section as a function of the208Pb nucleus energy is compared with theoretical predictions. The measured fission cross section from nuclear reactions (≈1.5 b) is approximately constant between 100 and 1000 A·MeV.

A rotating wheel system for the detection of spontaneously fissioning nuclides from heavy ion reactions

Abstract A rotating wheel system for the detection of spontaneously fissioning reaction products from heavy ion reactions was developed. In this system products recoiling from a rotating target wheel are stopped in a catcher foil stack which can be rotated at various velocities up to 80 rotations per second. All products emitted within a wide emission cone are stopped in the catcher foils and are rotated into shielded positions with stationary fission-track detectors positioned on both sides of each rotating foil. This technique allows a sensitive detection of spontaneously fissioning nuclides over a broad range of half-lives from 1 ms to about 1 d. By post-bombardment exposure of the catch…