O. Constantinescu

Experimental Studies of the Formation and Radioactive Decay of Isotopes with Z = 104—109

B y Y U . T s . O G A N E S S I A N , M. H U S S O N N O I S * , A . G . D E M I N , Y U . P . K H A R I T O N O V , H . B R U C H E R T S E I F E R * * , O . C O N S T A N T I N E S C U * * * , Y U . S . K O R O T K I N , S . P. T R E T Y A K O V A , V . K . U T Y O N K O V , I. V . S H I R O K O V S K Y , and J. ESTEVEZ, Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, Dubna, USSR

Production, chemical and isotopic separations of the long-lived isomer 178Hfm2 (T1/2=31 years)

The nucleus 178Hf, with its long-lived (T1/2=31 y) high-spin isomeric state Ipi =16+, is a challenge for new and exotic nuclear physics studies. The authors describe the first experiments they have performed in order to produce a reasonable microweight quantity of this hafnium isomer with an isomeric to ground-state ratio as high as possible (here 5%). The reaction 176Yb (4He, 2n) using an enriched target has been studied by measuring the excitation functions and the isomeric to ground-state ratio. About 3*1014 isomeric atoms have been produced up to now in irradiations with high-intensity beams ( approximately=100 mu A) at the U-200 cyclotron in Dubna. Chemical separation methods could be checked using about 1013 atoms of the isomer. Isotopic separation experiments have been performed in Orsay and preliminary results are given for the separation efficiency.

Chemical isolation of dubnium (element 105) in fluoride media

Summary The isotope 262Db was produced by irradiation of a 248Cm target with 106-MeV 19F ions at the 15 MV MP Tandem accelerator of Orsay (France). The reaction products were continuously and rapidly transported with a KCl aerosol helium jet system to the chemistry setup. They were dissolved in HF medium and the solution was passed through three successive ion exchange columns, allowing a continuous high level purification of Db from actinides on a first cation exchange column, the isolation of Db on an anion exchange one, and the retention of the long-lived decay products of 262Db (3.24 h–254Fm) on a second cation exchange column. Just after the end of irradiation, the decay products were desorbed from this column, purified and α-sources were prepared on carbon foil by electrospray. In 13 effective irradiation hours, 22 events corresponding to the α-decay of 254Fm were recorded. Almost 70 atoms of 262Db, produced in the reaction 248Cm + 19F were isolated in dilute HF medium. Like its homologues/analogues Nb, Ta and Pa, dubnium forms, with fluoride ions, negatively charged complexes which are strongly retained on anion exchanger.

On-line separation of short-lived tungsten isotopes from tantalum, hafnium and lutetium by adsorption on ion exchangers from aqueous ammonia solution

The title goal was achieved using a DOWEX 50Wx8 cation exchange column saturated with La(OH)3 and ammonia solution as eluent. Hf, Ta and Lu were adsorbed on this column, whereas W remained in the solution. This chemical system may be used for fast on-line separations of element 106.

Search for chemical separation of element 106 homologues in HF and HF-HCl media

Abstract In order to study the chemical properties of element 106 in aqueous media, fast, efficient and reproducible chromatographic separations were tested on its assumed homologues: Mo, W and U. Corroborative static and dynamic off-line experiments have shown that after fixation of these three elements on an anion-exchange resin in HF medium, selective elution could be achieved by using suitable concentrations of HCl-HF and HCl solutions. Separations of short-lived W isotopes, produced through heavy ion irradiation were also performed on-line.

On line ion exchange separation of short-lived Zr, Hf, Mo, Ta and W isotopes as homologs of transactinide elements

The short-lived isotopes of W and their descendants have been separated from nuclear reaction products arising in the bombardment of samarium target with24Mg ions at the U-400 cyclotron of JINR-Dubna. Chemical separations were performed using previously elaborated fast and continuous ion exchange. In similar way products of235U fission, obtained at the microtron MT-25 during irradiation of235U target with photoneutrons were separated.

Continuous Purification of 223Fr from its Decay Products on a Nickel Hexacyanoferrate(II) Composite Ion Exchanger

Continuous isolation and purification of 2 2 3 F r from 2 2 7 A c and its daughters has been performed. 2 2 3 F r was sorbed on a column filled with nickel hexacyanoferrate(II) composite ion exchanger and the decay products were continuously eluted with 0.01 M EDTA solution. For the first time, pure y-spectra of 2 2 3 Fr, without perturbing y-lines of its decay-products were recorded. 88 new y-transitions were put in evidence.

Cross sections of the (HI, αn) channel in the cold-fusion-type reactions 209Bi+40Ar and 208Pb+37Cl

Abstract By applying an off-line radiochemistry technique, cross sections for the production of the isotope 240 Cm (T 1 2 =27 d ) in the cold-fusion-type reactions 209 Bi+ 40 Ar and 208 Pb+ 37 Cl at the bombarding energy E lab ⩽230 MeV were determined to be 0.5±0.2 nb and 0.6±0.3 nb, respectively. The production of 240 Cm was attributed to the 1n-deexcitation channel of the composite systems 249 Md and 245 Es. At the same time, the measured 240 Cm production cross sections represent upper cross-section limits for the (HI, αn) channel of the reactions under study. These upper limits are about 100 times lower than the cross-section values reported recently by Nomura et al. for the ( 40 Ar , αx n ) channels with x =1,2 of the 209 Bi+ 40 Ar reaction at E lab =208 MeV. In the context of the above measurements, presented and discussed is the up-to-date summary of the available evidence on cross sections of the (HI, αx n) channels in the cold-fusion-type reactions induced by projectiles ranging from 37 Cl to 50 Ti on targets of 203,205 Tl, 208 Pb, and 209 Bi nuclei. Appreciable EC(β + )-delayed or/and spontaneous fission effects were detected in the 209 Bi+ 40 Ar, 208 Pb+ 37 Cl, and 206 Pb+ 37 Cl reactions. The obtained data point, in particular, to the EC(β + )-delayed fission occuring in the decay chains and .

Precise measurement of gamma-ray energies and gamma-ray emission probabilities in178m2Hf decay

Nowadays large interest is attached to the production of exotic beams and targets for nuclear structure and reaction studies. The nucleus178Hf, with is long-lived (T1/2=31 years) high-spin isomeric statelπ=16+ at a relatively low-excitation energy (2.446 MeV), is indeed a unique probe for studying phenomena in a new way. A wide research program has been underway in the framework of the “Hafnium Collaboration” which now concems around 80 scientists. Various targets have been prepared and adapted to different types of experiments. Irradiation processes with high-intensity beams, high-purity chemistry methods and isotopic separations have been developed. The obtaining of appreciable amounts of very high purity178m2Hf by isotopic separation, has made it possible for us to carry out γ-ray measurements with good statistics and high energy resolution with a γ-spectrometer, in order to use this isomer as a calibration standard in the 200 to 600 ke V energy range. For example, relative to the main γ-rays, energies have been determined with an absolute uncertainty of about 2 e V and photon emission probabilities with a relative uncertainty better than 1%.

Investigation of the 168Hf Electron Capture Decay Using Fast Radiochemical Separation

The (EC+/? ) decay of Hf, produced by the Gd ( , 0,4n) reaction, has been studied, with high efficiency HPGe coaxial detectors, using on-line measurements carried out with the RACHEL setup, built to investigate the chemical properties of transactinide elements. Off-line measurements were also carried out using catcher collection technique. Energies and intensities of 119 y-lines are reported among which only two were known in the earlier study of Chu and Reednick. The Lu level scheme built on the basis of single and γ—γ coincidence measurements, allows the interpretation of more than 79 y-transitions, between 38 excited states reported for the first time.