Abstract: The heaviest isotopes of elements Z = 117 to Z = 105, (294)117, (293)117, (290)115, (289)115, (286)113, (285)113, (282)Rg, (281)Rg, (278)Mt, (274)Bh, and (270)Db, were identified by means of the Dubna gas-filled recoil separator among the products of the Bk-249 + Ca-48 reaction. The details of the observed six decay chains, indicating the production and decay of isotopes (293)117 and (294)117, are presented and discussed. The decay energies and resulting half-lives of these new nuclei show a strong rise of stability with increasing neutron number, validating the concept of the island of enhanced stability for superheavy nuclei [Oganessian et al., Phys. Rev. Lett. 104, 142502 (2010)].
Abstract: Recently, the chemical investigation of element 112 revealed a highly volatile, noble metallic behaviour, as expected for the last group 12 member of the periodic table. The observed volatility and chemical inertness were ascribed to the growing influence of relativistic effects on the chemical properties of the heaviest elements with increasing nuclear charge. Here, we report for the first time on gas phase chemical experiments aiming at a determination of element 114 properties. This element was investigated using its isotopes (287)114 and (288)114 produced in the nuclear fusion reactions of Ca-48 with (242)pn and Pu-244 respectively. Identification of three atoms of element 114 in thermochromatography experiments and their deposition pattern on a gold surface indicates that this element is at least as volatile as simultaneously investigated elements Hg, At, and element 112. This behaviour is rather unexpected for a typical metal of group 14.
Abstract: The discovery of a new chemical element with atomic number Z = 117 is reported. The isotopes (293)117 and (294)117 were produced in fusion reactions between Ca-48 and Bk-249. Decay chains involving 11 new nuclei were identified by means of the Dubna gas-filled recoil separator. The measured decay properties show a strong rise of stability for heavier isotopes with Z >= 111, validating the concept of the long sought island of enhanced stability for superheavy nuclei.
Abstract: Currently, gas phase chemistry experiments with heaviest elements are usually performed with the gas-jet technique with the disadvantage that all reaction products are collected in a gas-filled thermalisation chamber adjacent to the target. The incorporation of a physical preseparation device between target and collection chamber opens up the perspective to perform new chemical studies. But this approach requires detailed knowledge of the stopping force (STF) of the heaviest elements in various materials. Measurements of the energy loss of mercury (Hg), radon (Rn), and nobelium (No) in Mylar and argon (Ar) were performed at low kinetic energies of around (40-270) keV per nucleon. The experimentally obtained values were compared with STF calculations of the commonly used program for calculating stopping and ranges of ions in matter (SRIM). Using the obtained data points an extrapolation of the STF up to element 114, eka-lead, in the same stopping media was carried out. These estimations were applied to design and to perform a first chemical experiment with a superheavy element behind a physical preseparator using the nuclear fusion reaction Pu-244(Ca-48; 3n)(289)114. One decay chain assigned to an atom of (285)112, the alpha-decay product of (289)114, was observed. (c) 2009 Elsevier B.V. All rights reserved.
Abstract: An experiment aimed at the synthesis of isotopes of element 120 has been performed using the Pu-244(Fe-58,xn)(302-x)120 reaction. No decay chains consistent with fusion-evaporation reaction products were observed during an irradiation with a beam dose of 7.1x10(18) 330-MeV Fe-58 projectiles. The sensitivity of the experiment corresponds to a cross section of 0.4 pb for the detection of one decay.
Abstract: This paper highlights some of the current basic nuclear physics research at Lawrence Livermore National Laboratory (LLNL). The work at LLNL concentrates on investigating nuclei at the extremes. The Experimental Nuclear Physics Group performs research to improve our understanding of nuclei, nuclear reactions, nuclear decay processes and nuclear astrophysics; an expertise utilized for important laboratory national security programs and for world-class peer-reviewed basic research. The bibliography includes 42 references.
Abstract: A chemical separation method based on reversed-phase chromatography has been developed to separate the group five elements from the reaction products produced in the bombardment of Am-243 with Ca-48 ions. Decontamination factors on the order of 10(6) were achieved for group-three elements such as americium, and 10(7) or more for selected reaction products as measured by gamma-ray spectrometry pre- and post-chemistry. Details of the chemical separation scheme are discussed and compared to previously reported results.
Abstract: The recognition criterion for discovery of a new chemical element includes two aspects, the characterization properties and the assignment properties. In this paper, we will discuss the status of element 115 experiments that have been performed in Dubna, Russia, highlighting the characterization and assignment properties as they specifically relate to a recent experiment. After discussing the status of what is known about the decay properties of element 115 [1], observed previously using the Dubna Gas-Filled Recoil Separator, we will discuss the prior chemical studies that have been performed on the Db descendant of element 115 [2]. Following the success of that experiment, some additional chemical information was desired. Two separation chemistries were then developed at LLNL and JINR. LLNL utilized reversed phase chromatography and JINR utilized anion exchange chromotography to perform not only +4/+5 separations, but also intra-group separations, where Nb-like and Ta-like fractions were eluted. The results from an experiment using these chemistries for the first time during December 2005 in Dubna, Russia, will be compared with prior chemical results. We will conclude with a discussion of possible enhancements to the work already performed and the current status of the future experimental plans.
Abstract: The decay properties of the new isotope (282)113 and its daughter nuclei have been measured in the Np-237(Ca-48, 3n)(282)113 reaction. During an irradiation with a beam dose of 1.1x10(19)244-MeV Ca-48 projectiles, two decay chains originating from the odd-odd isotope (282)113 (E-alpha=10.63 +/- 0.08 MeV,T-alpha=73(-29)(+134) ms) were produced in the complete fusion reaction with a cross section of 0.9(-0.6)(+1.6) pb; these properties are all in agreement with expectations based on the results of previous experiments.
Abstract: The accurate measurement of neutron capture cross sections of the Eu isotopes is important for many reasons including nuclear astrophysics and nuclear diagnostics. Neutron capture excitation functions of Eu-151.153 targets were measured recently using a 4 pi gamma-ray calorimeter array DANCE located at the Los Alamos Neutron Science Center for E-n = 0.1-100 keV. The progress on the data analysis efforts is given in the present paper. The gamma-ray multiplicity distributions for the Eu targets and Be backing are significantly different. The gamma-ray multiplicity distribution is found to be the same for different neutron energies for both Eu-153 and Eu-153. The statistical simulation to model the gamma-ray decay cascade is summarized. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: Two experiments aiming at the chemical investigation of element 112 produced in the heavy ion induced nuclear fusion reaction of Ca-48 with U-238 were performed at the Geselischaft fur 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 degrees C and of Rn at about - 180 degrees C. Reports from the Flerov Laboratory for Nuclear Reactions (FLNR), Dubna, Russia claim production of a 5-min spontaneous fission (SF) activity assigned to 2111 12 for the U-238 (Ca-48,3n) (283)112 reaction. Hence, Experiment I was designed to detect spontaneously fissioning (SF) isotopes of elernent 112 with half-lives (t(1/2)) longer than about 20 s. 11 high-energy events were detected. 7 events exhibit a deposlition pattern resembling a chromatographic peak in the vicinity of Rn deposition. However, the energy of the events observed in Experiment I was lower than expected for a SF-decay of (283)112. Therefore, these events could not be unambiguously attributed to the decay of (283)112. In contradiction with earlier publications newer reports from FLNR Dubna claim that (283)12 decays by a-particle ernission (E-alpha = 9.5 MeV) with t(1/2) = 4 s followed by a SF-decay of (279)Ds (t(1/2) = 0.2 s). Therefore, Experiment II was designed to be sensitive to both claimed decay properties of (283)112. However, during this experiment neither short alpha-SF correlations nor SF coincidences were detected. The conclusion is that (283)112 was not unambiguously detected, neither in Experiment I nor in Experiment II.
Abstract: The decay properties of (290)116 and (291)116, and the dependence of their production cross sections on the excitation energies of the compound nucleus, (293)116, have been measured in the Cm-245 (Ca-48, xn)(293-x)116 reaction. These isotopes of element 116 are the decay daughters of element 118 isotopes, which are produced via the Cf-249+Ca-48 reaction. We performed the element 118 experiment at two projectile energies, corresponding to (297)118 compound nucleus excitation energies of E-*=29.2 +/- 2.5 and 34.4 +/- 2.3 MeV. During an irradiation with a total beam dose of 4.1x10(19) Ca-48 projectiles, three similar decay chains consisting of two or three consecutive alpha decays and terminated by a spontaneous fission (SF) with high total kinetic energy of about 230 MeV were observed. The three decay chains originated from the even-even isotope (294)118 (E-alpha=11.65 +/- 0.06 MeV, T-alpha=0.89(-0.31)(+1.07) ms) produced in the 3n-evaporation channel of the Cf-249+Ca-48 reaction with a maximum cross section of 0.5(-0.3)(+1.6) pb.
Abstract: Manganese oxides, present as minor phases in the vadose zone, have been previously shown to sequester large quantities of plutonium under environmental conditions. We are now continuing these studies with Np(V). Sorption onto manganite (MnOOH) and hausmannite (Mn3O4) at solid-to-solution ratios of 2.5-3.3 mg/mL has been studied as a function of neptinium concentration and pH. The sorption of Np increased as a function of pH for both minerals, attaining a maximum at neutral pH, and then decreased with increasing alkalinity. X-ray absorption fine structure spectroscopy (XAFS), taken at the Np L-III- edge, has been used to determine the oxidation state of the sorbed Np. Our experimental results indicate reduction of the Np(V) because of interaction with the X-ray beam. These findings significantly impact the interpretation of results reported elsewhere on Np(V) investigated though the use of high-intensity X-ray beams.
Abstract: The recent discovery of the elements 115 and 113 in the reaction Ca-48 + Am-243 was confirmed by an independent radiochemical experiment based on the identification of the long-lived decay product dubnium.
Abstract: The fast liquid-liquid extraction system SISAK with continuous liquid scintillation detection was coupled to the Berkeley Gas-filled Separator to enable studies of the chemical properties of a-decaying transactinide elements. The preseparated transactinide (257)Rf (4.7 s), was transported via a He/KCl-jet directly into the scintillation solution. This permitted the first unambiguous detection of transactinide atoms by the means of the a-liquid scintillation detectors of the SISAK-system. During the experiment, 89 (257)Rf atoms were observed. The type of events observed correspond well with the expectations based on half-lives, decay branches and transport- and hold-up times. The successful result of this experiment demonstrates that the system is well fitted for count-rates of 1-2 atoms/h and half-lives of a few seconds. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: A new flexible prompt gamma-ray activation analysis (PGAA) station will be installed and operated at the FRM-II reactor in Garching at the beginning of the year 2006. The PGAA station has been moved from the Paul Scherrer Institute (PSI), Villigen, Switzerland and is now being re-designed for three different experimental applications: PGAA instrument, cold neutron tomography setup and a compact Ge-array. Simulations of the beam guide have been carried out in order to yield the best conditions for each of the instruments, and are discussed here. A number of the PGAA applications are listed here. A new project dealing with a systematic measurement of bulky composition of small amount of meteorites (similar to 100 mg) is proposed. Results of a test measurement of the Dhurmasala meteorite composition at the PGAA station at the Budapest Research Reactor are presented. Expected parameters of the PGAA facility at FRM-II are compared both with those at PSI and with the Budapest facility.
Abstract: A Monte Carlo random probability analysis developed at LLNL for heavy element research was performed for recent experiments aimed at the synthesis of nuclides with Z >= 112 and N >= 170, to estimate the probability that observed decay chains were a result of a random event. Low probabilities (< 10(-4)% for most decay chains) were found.
Abstract: The results of two experiments designed to synthesize element 115 isotopes in the Am-243+Ca-48 reaction are presented. Two new elements with atomic numbers 113 and 115 were observed for the first time. With 248-MeV Ca-48 projectiles, we observed three similar decay chains consisting of five consecutive alpha decays, all detected in a total time interval of 30 s. Each chain was terminated by a spontaneous fission (SF) with a high-energy release and a lifetime of about a day. With 253-MeV Ca-48 projectiles, we registered a different decay chain of consecutive alpha decays detected in a time interval of 0.5 s, also terminated by spontaneous fission, but after 1.8 h. The decay properties of the eleven new alpha- and SF-decaying nuclei are consistent with expectations for consecutive alpha decays originating from the parent isotopes (288)115 and (287)115, produced in the 3n- and 4n-evaporation channels, respectively. Support for the assignment of the atomic numbers of all of the nuclei in the (288)115 decay chain was obtained in an independent experiment in which a long-lived spontaneous fission activity, (268)Db (15 events), was found to be chemically consistent with the fifth group of the periodic table. The odd-odd isotope (288)115 was observed with largest cross section of about 4 pb. In the SF decay of (268)Db, a total kinetic energy of 230 MeV and a neutron multiplicity per fission of 4.2 were measured. The decay properties of the 11 new isotopes with Z=105-115 and the production cross sections are in agreement with modern concepts of the role of nuclear shells in the stability of superheavy nuclei. The experiments were carried out at the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research.
Abstract: We have studied the dependence of the production cross-sections of the isotopes (282,283)112 and (286-288)114 on the excitation energy of the compound nuclei (286)112 and (290)114. The maximum cross-sections of the xn-evaporation channels for the reaction U-238(Ca-48, xn)(286-x)112 were measured to be: sigma(3n) = 2.5(-1.1)(+1.8) pb and sigma(4n) = 0.6(-0.5)(+1.6) pb; for the reaction Pu-242(Ca-48,xn)(290-x)114: sigma(2n) similar to 0.5 pb, sigma 3n = 3.6(-1.7)(+3.4)pb and sigma(4n) = 4.5(-1.9)(+3.6)pb. In the reaction U-233(Ca-48, (2-4n))(277-279)112 we measured an upper cross-section limit of sigma(xn) <= 0.6 pb. An increase of sigma(ER) in the reactions of actinide targets Ca-48 can be due to the expected increase of the survivability of the excited compound nucleus upon closer approach to the closed neutron shell N = 184. The observed nuclear decay properties of the nuclides with Z = 104-118 are compared with theoretical nuclear mass calculations and the systematic trends of a.-decay properties. As a whole, they give a consistent pattern of decay of the 18 even-Z neutron-rich nuclides with Z = 104-118 and N = 163-177.
Abstract: We have studied the dependence of the production cross sections of the isotopes (282,283)112 and (286,287)114 on the excitation energy of the compound nuclei (286)112 and (290)114. The maximum cross section values of the xn-evaporation channels for the reaction U-238(Ca-48,xn)(286-x)112 were measured to be sigma(3n)=2.5(-1.1)(+1.8) pb and sigma(4n)=0.6(-0.5)(+1.6) pb; for the reaction Pu-242(Ca-48,xn)(290-x)114: sigma(2n)similar to0.5 pb, sigma(3n)=3.6(-1.7)(+3.4) pb, and sigma(4n)=4.5(-1.9)(+3.6) pb. In the reaction U-233(Ca-48,2-4n)(277-279)112 at E*=34.9=2.2 MeV we measured an upper cross section limit of sigma(xn)less than or equal to0.6 pb. The observed shift of the excitation energy associated with the maximum sum evaporation residue cross section sigma(ER)(E*) to values significantly higher than that associated with the calculated Coulomb barrier can be caused by the orientation of the deformed target nucleus in the entrance channel of the reaction. An increase of sigma(ER) in the reactions of actinide targets with Ca-48 is consistent with the expected increase of the survivability of the excited compound nucleus upon closer approach to the closed neutron shell N=184. In the present work we detected 33 decay chains arising in the decay of the known nuclei (282)112, (283)112, (286)114, (287)114, and (288)114. In the decay of (287)114(alpha)-->(283)112(alpha)-->(279)110(SF), in two cases out of 22, we observed decay chains of four and five sequential alpha transitions that end in spontaneous fission of (271)Sg (T-alpha/SF=2.4(-1.0)(+4.3) min) and (267)Rf (T(SF)similar to2.3 h), longer decay chains than reported previously. We observed the new nuclide (292)116 (T-alpha=18(-6)(+16) ms,E-alpha=10.66+/-0.07 MeV) in the irradiation of the Cm-248 target at a higher energy than in previous experiments. The observed nuclear decay properties of the nuclides with Z=104-118 are compared with theoretical nuclear mass calculations and the systematic trends of spontaneous fission properties. As a whole, they give a consistent pattern of decay of the 18 even-Z neutron-rich nuclides with Z=104-118 and N=163-177. The experiments were performed with the heavy-ion beam delivered by the U400 cyclotron of the FLNR (JINR, Dubna) employing the Dubna gas-filled recoil separator.
Abstract: We report the experimental confirmation of the production of element 110. In the bombardment of a Pb-208 target with a 309-MeV Ni-64 beam, we have observed two chains of time- and position-correlated events. Each chain consisted of the implantation of an evaporation residue followed by the emission of alpha particles. We attribute these two chains to the decay of (271)110 produced with a cross section of 8.3(-5.3)(+11) pb.
Abstract: 27-s (263)Db was produced in the Bk-249 (O-18, 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 alpha-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 a and SF activity. A SF activity with a half life on the order of 20 min was observed and assigned to the nuclide (263)Rf. It is formed by electron-capture decay of (263)Db with a decay branch of 3(-1)(+4)%.
Abstract: In April-May, 2001, the previously reported experiment to synthesize element 118 using the Pb-208(Kr-86,n)(293)118 reaction was repeated. No events corresponding to the synthesis of element 118 were observed with a total beam dose of 2.6 x 10(18) ions. The simple upper-limit cross-sections (1 event) were 0.9 and 0.6 pb for evaporation residue magnetic rigidities of 2.00 Tm and 2.12 Tm, respectively. A more detailed cross-section calculation, accounting for an assumed narrow excitation function, the energy loss of the beam in traversing the target and the uncertainty in the magnetic rigidity of the Z = 118 recoils is also presented. Re-analysis of the primary data files from the 1999 experiment showed the reported element 118 events are not in the original data. The current results put constraints on the production cross-section for synthesis of very heavy nuclei in cold-fusion reactions.
Abstract: Electron-capture delayed fission was observed in 37-s Es-244 produced via the Np-237(C-12,5n)Es-244 reaction at 81 MeV (on target) with a production cross section of 0.31+/-0.12 mub. The kinetic energies of coincident fission fragments were measured with our rotating wheel detection system and the average pre-neutron-emission total kinetic energy of the fragments was found to be 186+/-19 MeV. The mass-yield distribution of the fission fragments is predominantly asymmetric. Based on the ratio of the number of fission events to the measured number of alpha decays from the electron-capture daughter Cf-244 (100% alpha branch), the probability of delayed fission was determined to be (1.2+/-0.4) X 10(-4). This value for the delayed fission probability fits the experimentally observed trend of increasing delayed fission probability with increasing Q value for electron capture.
Abstract: The synthesis and the properties of the heaviest elements are the subject of this paper. All of the elements above seaborgium (Z=106) have been identified on the basis of single-atom decays after separation in-flight. These elements cover a region of deformed shell nuclei connecting the elements above seaborgium and the predicted superheavy elements located at the next spherical double shell closure above lead. Recent calculations of R. Smolanczuk indicated a dramatic increase in the production cross section for element 118 in the reaction Pb-208(Kr-86,1n)(293)118. A study of this reaction was carried out at the Berkeley Gas-Filled Separator at the 88-inch Cyclotron. Experimental results on the structure of the heaviest elements will be discussed in the frame of recent theoretical models together with new predictions for the location of the next double shell closure.
Abstract: Subsecond Pa-224 (T-1/2 = 0.85 s) was produced via the Bi-209(O-18,3n)Pa-224 reaction at the 88 inch cyclotron at the Lawrence Berkeley National Laboratory. After production it was transported via a gas-jet system to the centrifuge system SISAK 3. Following on-line extraction with trioctylamine/scintillation solutions from 1M lactic acid, Pa-224 was detected applying on-line alpha -liquid scintillation counting. Unambiguous identification was achieved using time-correlated alpha-alpha -decay chain analysis. This constitutes the first chemical on-line separation and detection of a subsecond alpha -decaying nuclide, 0.85-s Pa-224 With the fast extraction system SISAK 3.
Abstract: Electron-capture delayed fission was observed in 26-min Es-248 and 7.7-min Es-246, which were produced at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron via Cf-249(p,2n) and Cf-249(p,4n) reactions at proton energies of 18 and 37 MeV on target, respectively. The delayed fission probabilities were determined to be (3.5+/-1.8) x 10(-6) for Es-248 and (3.7(-3.0)(+8.5)) x 10(-5) for Es-246. These values were determined in separate experiments using our automated rotating wheel online collection and measurement system. Collection and measurement times were optimized for the isotope being measured. The measured delayed fission probabilities are consistent with the experimentally observed trend of increasing delayed fission probability with increasing Q value for electron capture.
Abstract: The arrangement of the chemical elements in the periodic table highlights resemblances in chemical properties, which reflect the elements' electronic structure. For the heaviest elements, however, deviations in the periodicity of chemical properties are expected(1-3): electrons in orbitals with a high probability density near the nucleus are accelerated by the large nuclear charges to relativistic velocities, which increase their binding energies and cause orbital contraction. This leads to more efficient screening of the nuclear charge and corresponding destabilization of the outer d and f orbitals: it is these changes that can give rise to unexpected chemical properties. The synthesis of increasingly heavy elements(4-6), now including that of elements 114, 116 and 118, allows the investigation of this effect, provided sufficiently long-lived isotopes for chemical characterization are available(7). In the case of elements 104 and 105, for example, relativistic effects interrupt characteristic trends in the chemical properties of the elements constituting the corresponding columns of the periodic table(8), whereas element 106 behaves in accordance with the expected periodicity(9-12). Here we report the chemical separation and characterization of six atoms of element 107 (bohrium, Bh), in the form of its oxychloride. We find that this compound is less volatile than the oxychlorides of the lighter elements of group VII, thus confirming relativistic calculations(13) that predict the behaviour of bohrium, like that of element 106, to coincide with that expected on the basis of its position in the periodic table.
Abstract: The excitation function for the U-233(He-3,4n)Pu-232 reaction was measured. The maximum production cross section is 6.2+/-1.5 mu b at 32.5+/-2.2 MeV. The plutonium fraction was chemically separated from interfering activities. The Pu-232 half-life was determined to be 33.1+/-0.8 min using the interference-free data obtained from alpha-alpha correlations of daughter activities in equilibrium with the Pu-232 decay.
Abstract: New neutron rich istotopes (267)(107)Bh and (266)(107)Bh were produced in bombardments of a Bk-249 target with 117-MeV and 123-MeV Ne-22 ions at the Lawrence Berkeley :National Laboratory 88-Inch Cyclotron. Identification was made by observation of correlated alpha-particle decays between the Ph isotopes and their Db and Lr daughters using a rotating wheel system. (267)Bh was produced with a cross section of approximate to 70 pb and decays with a 17(-6)(+14) s half life by emission of alpha z particles with an average energy of 8.83 +/- 0.03 MeV. One atom of (266)Bh was observed, decaying within 1 s by emission of a 9.29-MeV alpha particle.
Abstract: Electron-capture delayed fission of Es-242 produced via the U-233(N-14,5n)Es-242 reaction at 87 MeV (on target) was observed to decay with a half-life of 11+/-3 s, consistent with the reported alpha-decay half-life of Es-242 of 16(-4)(+6) s. The mass-yield distribution of the fission fragments is highly asymmetric. The average pre-neutron emission total kinetic energy of the fragments was measured to be 183+/-18 MeV. Based on the ratio of the measured number of fission events to the measured number of cu decays from the electron-capture daughter Cf-242 (100% alpha branch), the probability of delayed fission was determined to be 0.006+/-0.002. This value for the delayed fission probability fits the experimental trend of increasing delayed fission probability with increasing Q value for electron capture.
Abstract: The Heavy Element Volatility instrument (HEVI), an on-line isothermal gas chromatography system, has been used to separate the volatile bromide compounds of the group 4 elements Zr and Hf and the transactinide Rf according to their volatilities, and to provide data on the gas phase chemical properties of very short-lived isotopes in amounts as low as a few atoms. For these studies (261)Rf was produced via the Cm-248(O-18, 5n) reaction. Hf165-167 was produced via the reaction Eu-nat(F-19, xn), and Zr-85 was produced via the Cu-nat(Si-28, 3p3n) reaction. The hair-life for (261)Rf was measured to be 75 +/- 7 seconds. A Monte Carlo code was used to deduce the enthalpy of adsorption (DeltaH(a)) from the observed volatility and parameters of the chromatography system. The resulting adsorption enthalpies for the Zr, Hf, and Rf tetrabromides are: -108+/-5 kJ . mol(-1), -113 +/- 5 kJ . mol(-1) and -877 kJ . mol(-1), respectively. Volatilities of the group 4 bromides support the conclusion from previous results for the group 4 chlorides that Rf deviates from the trend expected by simple extrapolation of the properties of its lighter homologs in the periodic table. The group 4 bromides are also observed to be less volatile than their respective chlorides, as predicted by relativistic calculations.
Abstract: Following a prediction by Smolanczuk [Phys. Rev. C 59, 2634 (1999)], we searched for superheavy element formation in the bombardment of Pb-208 With 449-Mev Kr-86 ions. We have observed three decay chains, each consisting of an implanted heavy atom and six subsequent alpha decays, correlated in time and position. In these decay chains, a rapid (ms) sequence of high energy alpha particles (E-alpha greater than or equal to 10 MeV) indicates the decay of a new high-Z element. The observed chains are consistent with the formation of (293)118 and its decay by sequential alpha-particle emission to (289)116, (285)114, (281)112, (277)110, (273)Hs (Z = 108) and (269)Sg (Z = 106). The production cross section is 2.2(-0.8)(+2.6) pb.
Abstract: The new plutonium isotope, Pu-231, was produced by the U-233(He-3,5n)Pu-231 reaction. After chemical separation, the Pu-231 decay modes were studied using alpha-spectrometry. The isotope,Pu-231, was unequivocally identified by the alpha-decay of the chain members from its alpha- and electron-capture daughters using the alpha-alpha-correlation technique. The half-life of Pu-231 was determined to be (8.6+/-0.5) min. An cu-group with an energy of (6.72+/-0.03) MeV was assigned to Pu-231.
Abstract: We have measured the production cross section of 1.8-s (261)Ha from two different reactions. It was produced in the Cf-250(N-15,4n) reaction at 84 MeV and in the Am-243(Ne-22,4n) reaction at 116 MeV. Our rotating wheel system with a special parent-daughter stepping mode was used to detect alpha-alpha correlations between (261)Ha and Lr-257. We measured 13 and 9 correlations in the two reactions, respectively. Assuming a 100% alpha branch, we have determined the production cross section of (261)Ha to be 0.51+/-0.20 nb in the Cf-250(N-15,4n) reaction at 84 MeV, and 0.25+/-0.11 nb in the Am-243(Ne-22,4n) reaction at 116 MeV. Based on the number of fission events observed in the latter reaction, we have been able to set an upper limit of 18% for the spontaneous fission branch of (261)Ha. [S0556-2813(98)02211-0].
Abstract: Pa-224 was produced via the Bi-209(O-18,3n) reaction at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron and the half-life was determined to be 850+/-20 ms by measuring its alpha decay using our rotating wheel system. Our value is consistent with a previously reported half-life of 950+/-150 ms for Pa-224 produced via the Tl-205(Ne-22,3n) reaction, but its much more precise. The cross section for the Bi-209(O-18,3n)Pa-224 reaction was measured to be 0.5+/-0.1 mb for 87-89 MeV O-18(5+) projectiles incident on the target.
