R. Sudowe

Cross-section limits for the 208Pb(86Kr, n) 293118 reaction

Abstract.In April-May, 2001, the previously reported experiment to synthesize element 118 using the 208Pb(86Kr,n)293118 reaction was repeated. No events corresponding to the synthesis of element 118 were observed with a total beam dose of 2.6 x 1018 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.

Extraction of short-lived zirconium and hafnium isotopes using crown ethers: A model system for the study of rutherfordium

Summary The extraction of zirconium and hafnium from hydrochloric acid media was studied using the crown ethers dibenzo-18-crown-6 (DB18C6), dicyclohexano-18-crown-6 (DC18C6) and dicyclohexano-24-crown-8 (DC24C8) as extractants. The goal was to find an extraction system that exhibits a high selectivity between the members of group 4 of the periodic table and is suitable for the study of rutherfordium. It was found that Zr and Hf are both extracted using DB18C6, DC18C6 and DC24C8. The extraction yield increases with increasing acid concentration and increasing concentration of crown ether. The extracted species most likely consists of an ion-association complex formed between a Zr or Hf chloro complex and a hydronium crown ether complex. Conditions can be found for each extractant that provide for the separation of Zr from Hf. This selective separation between Zr and Hf makes the extraction with crown ethers from HCl well suited to study the extraction behaviour of Rf and compare it to the behaviour of Zr and Hf. These extraction systems can be used to determine whether the extraction behaviour of Rf is similar to Zr, similar to Hf or follows the trend established by the lighter homologs. The extraction kinetics are fast enough for the study of the 78-s isotope 261mRf.

Cross-section limits for the Pb-208 (Kr-86, n) 118-293 reaction

Abstract.In April-May, 2001, the previously reported experiment to synthesize element 118 using the 208Pb(86Kr,n)293118 reaction was repeated. No events corresponding to the synthesis of element 118 were observed with a total beam dose of 2.6 x 1018 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.

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 branch of 3+4-1%.

Neutron Capture Experiments on Unstable Nuclei

A primary objective of this project is to study neutron capture cross sections for various stable and unstable isotopes that will contribute to the Science Based Stockpile Stewardship (SBSS) program by providing improved data for modeling and interpretation of nuclear device performance. The information obtained will also be important in astrophysical modeling of nucleosynthesis. During this reporting period, the emphasis has been on preparing a radioactive target of {sup 155}Eu (half-life = 4.7 years), and several stable targets, including isotopically separated {sup 154}Sm, {sup 151}Eu, and {sup 153}Eu. Measurements of their neutron capture cross sections will be conducted in collaboration with researchers at the Los Alamos Neutron Science Center (LANSCE) facility using the Detector for Advanced Neutron Capture Experiments (DANCE). A suitable backing material (beryllium) for the targets has been selected after careful calculations of its contribution to the background of the measurements. In addition, a high voltage plating procedure has been developed and optimized. Stable targets of {sup 151}Eu and {sup 153}Eu and a target of natural Eu ({approx}50% {sup 151}Eu and {approx}50% {sup 153}Eu) have each been plated to a mass thickness of >1 mg/cm{sup 2} and delivered to the DANCE collaboration at Los Alamos National Laboratory (LANL). Natural Eu targets will be tested first to confirm that the target dimensions and backing are appropriate prior to performing measurements on the extremely valuable targets of separated isotopes. In order to prepare a target of the radioactive {sup 155}Eu, it must first be separated from the {sup 154}Sm target material that was irradiated in a very high neutron flux of 1.5x1015 neutrons/cm{sup 2}/s for 50 days. The reaction is {sup 154}Sm (n,f){sup 155}Sm (half-life = 22 minutes) {sup 155}Eu. Considerable progress has been made in developing a suitable high-yield and high-purity separation method for separating Eu from targets containing about twenty times as much Sm. An exhaustive review of the literature indicated that a multiprocess approach in which Eu(III) is reduced to Eu(II) prior to separation should provide an effective and efficient means of separation from the Sm(III). To date, three multiprocess methods have been developed and tested for their ability to meet the design requirements set forth by this project. These methods combine an initial reduction step using Zn(Hg) with either cation exchange resin in (1) column form or in (2) a batch reactor and hydroxyisobutyrate (?-HIB) as the eluant for trivalent lanthanides. Another multiprocess method uses solvent extraction with 0.1 M thenoyl trifluoroacetone (TTA) in benzene. Preliminary experiments indicate that: (a) A multiprocess approach using ?-HIB as a complexing agent for trivalent lanthanides is ineffective for separating Eu from Sm because ?-HIB stabilizes Eu(III) even in the presence of excess amounts of the reductant; (b) A multiprocess approach using solvent extraction shows promise, indicating that 0.1 M TTA in benzene favors extraction of trivalent over divalent metal ions by a factor of greater than 750. However, the reduction step using Zn(Hg), when combined with the TTA extraction, becomes less effective at reducing Eu during subsequent extractions and may also affect the stability of the TTA. Use of the amalgam also introduces Zn(II) contamination that must be separated from the Eu with additional solvent extraction steps. A PhD student from the group has visited the LANSCE facility, participated in several parameter checks of the DANCE, and acquainted himself with the data acquisition system. During these initial experiments, data were collected and brought back to UC Berkeley for analysis. A high purity P-type germanium detector was purchased, set up, and calibrated to assist with the determination of separation yields and efficiencies using ?-ray spectroscopy measurements of suitable radioactive tracers.

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.

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.

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.

Comparison of reactions for the production of {sup 258,257}Db: {sup 208}Pb({sup 51}V,xn) and {sup 209}Bi({sup 50}Ti,xn)

Excitation functions for the 1n and 2n exit channels of the 208Pb(51V,xn)259-xDb reaction were measured. A maximum cross section of the 1n exit channel of 2070+1100/-760 pb was measured at an excitation energy of 16.0 +- 1.8 MeV. For the 2n exit channel, a maximum cross section of 1660+450/-370 pb was measured at 22.0 +- 1.8 MeV excitation energy. The 1n excitation function for the 209Bi(50Ti,n)258Db reaction was remeasured, resulting in a cross section of 5480+1750/-1370 pb at an excitation energy of 16.0 +- 1.6 MeV, in agreement with previous values [F. P. Hebberger, et al., Eur. Phys. J. A 12, 57 (2001)]. Differences in cross section maxima are discussed in terms of the fusion probability below the barrier.

New isotope {sup 264}Sg and decay properties of {sup 262-264}Sg

New isotope, {sup 264}Sg, was identified using the {sup 238}U({sup 30}Si, xn){sup 268-x}Sg reaction and excitation functions for {sup 262-264}Sg were measured. {sup 264}Sg decays by spontaneous fission with a half-life of 37{sub -11}{sup +27} ms. The spontaneous fission branch for 0.9-s {sup 263}Sg was measured for the first time and found to be (13{+-}8)%. {sup 262}Sg decays by spontaneous fission with a 15{sub -3}{sup +5} ms half-life. Spontaneous fission partial half-life systematics are evaluated for even-even Sg isotopes from {sup 258}Sg through {sup 266}Sg, spanning the transition region between the N=152,Z=100 and N=162,Z=108 deformed shells.