[en] In this report, we offer our most recent results concerning the decay properties for five new isotopes of Md, No, Lr, and for ^258mMd. In additions to these successful experiments, we have also conducted searches for ²⁶³[105], ²⁶⁴[105], ²⁷²[109], and superheavy elements from bombardments of ²⁵⁴Es with heavy ions. ² An exciting finding in the course of this work is a new fission phenomenon, which we have termed ''bidmodal fission''. This is described in a subsequent section. The final part summarizes our conclusions based on the unexpectedly long half-lives and surprising fission properties of the heaviest nuclei. 27 refs., 19 figs

[en] Heavy ion bombardments of ²⁵⁴Es have produced abundant experimental results in the past several years. These results include the discovery of bimodal fission in the neutron-rich Fm and Md isotopes, the discovery of several new isotopes, improved knowledge of the decay properties of the heavy actinides, and even information on the chemical behavior of lawrencium. They will present information on the decay properties of the new isotopes ²⁶⁰Md (31.8-d half-life), ²⁶¹Lr (39-m half-life) and ²⁶²Lr (216-m half-life). The observed spontaneous fission activity associated with ²⁶²Lr is most likely from its decay daughter, ²⁶²No, which would be the first isotope with N = 160. The relatively long half-lives for these neutron-rich nuclides and their production rates from ²⁵⁴Es targets make it possible to study their spontaneous fission properties in detail and indicate that new longer-lived isotopes may be discovered near the predicted 162 neutron shell at Z = 104 to 110

[en] A multicomponent decay curve analysis technique has been developed and incorporated into the decay curve fitting computer code, MLDS (maximum likelihood decay by the simplex method). The fitting criteria are based on the maximum likelihood technique for decay curves made up of time binned events. The probabilities used in the likelihood functions are based on the Poisson distribution, so decay curves constructed from a small number of events are treated correctly. A simple utility is included which allows the use of discrete event times, rather than time-binned data, to make maximum use of the decay information. The search for the maximum in the multidimensional likelihood surface for multi-component fits is performed by the simplex method, which makes the success of the iterative fits extremely insensitive to the initial values of the fit parameters and eliminates the problems of divergence. The simplex method also avoids the problem of programming the partial derivatives of the decay curves with respect to all the variable parameters, which makes the implementation of new types of decay curves straightforward. Any of the decay curve parameters can be fixed or allowed to vary. Asymmetric error limits for each of the free parameters, which do not consider the covariance of the other free parameters, are determined. A procedure is presented for determining the error limits which contain the associated covariances. The curve fitting procedure in MLDS can easily be adapted for fits to other curves with any functional form. (orig.)

[en] The chemical and nuclear properties of Lr and Ha have been studied, using 3-minute ²⁶⁰Lr and 35-second ²⁶²Ha. The crystal ionic radius of Lr³⁺ was determined by comparing its elution position from a cation-exchange resin column with those of lanthanide elements having known ionic radii. Comparisons are made to the ionic radii of the heavy actinides, Am³⁺ through Es³⁺, obtained by x-ray diffraction methods, and to Md³⁺ and Fm³⁺ which were determined in the same manner as Lr³⁺. The hydration enthalpy of -3622 kJ/mol was calculated from the crystal ionic radius using an empirical form of the Born equation. Comparisons to the spacings between the ionic radii of the heaviest members of the lanthanide series show that the 2Z spacing between Lr³⁺ and Md³⁺ is anomalously small, as the ionic radius of Lr³⁺ of 0.0886 nm is significantly smaller than had been expected. The chemical properties of Ha were determined relative to the lighter homologs in group 5, Nb and Ta. Group 4 and group 5 tracer activities, as well as Ha, were absorbed onto glass surfaces as a first step toward the determination of the chemical properties of Ha. Ha was found to adsorb on surfaces, a chemical property unique to the group 5 elements, and as such demonstrates that Ha has the chemical properties of a group 5 element. A solvent extraction procedure was adapted for use as a micro-scale chemical procedure to examine whether or not Ha displays eka-Ta-like chemical under conditions where Ta will be extracted into the organic phase and Nb will not. Under the conditions of this experiment Ha did not extract, and does not show eka-Ta-like chemical properties

[en] Progress in nuclear research has resulted in the discovery of many new and very heavy nuclides during the past few years. From bombardments of ²⁵⁴Es targets with heavy ions, the isotopes ²⁶⁰Md, ²⁶¹Lr, ²⁶²Lr, and ²⁶²No have been produced and identified. The nuclide ²⁶²No contains more neutrons than any previously known isotope. At the Gesellschaft fuer Schwerionenforschung, Darmstadt, the new elements 108 and 109 have been found, while at the Dubna Laboratory in the USSR, tentative evidence for the discovery of elements 110 as ²⁷²[110] has been presented. A major outcome of these discoveries is that, in defiance of all expectations, spontaneous fission is no greater a factor in limiting their nuclear stability than it is for lighter transplutonium model. Theorists now estimate that increasing nuclear stability will be found for nuclei with neutrons numbers ≤ 163 because of a strong neutron subshell at N = 162. Experiments have been made to detect this anticipated gain in stability by searching for ²⁷²[109] and ²⁶⁶[107] but, so far, without success. In other advances, spontaneous fission proceeding by two separate modes (bimodal) has been observed for five of the heaviest isotopes of Fm, Md, and No. Shell structure in both the fissioning species and the fragments appears to account for this unique phenomenon. This discovery has brought about renewed interest in the physics of nuclear fission because of new insights it has brought into understanding of the fission process

[en] Fast automated on-line and quasi-on-line radiochemical techniques are applied to search for new isotopes, to measure their decay characteristics and to study the cross sections of the heaviest most neutron-rich actinide isotopes in reactions of ^16,18O and ²²Ne projectiles with ²⁵⁴Es as a target. The measured yields for isotopes up to lawrencium-260 are three or more orders of magnitude higher than in any other reactions used so far. A comparison with data for similar transfers from ²⁴⁸Cm targets is made. Transfer cross sections are extrapolated for the production of unknown, neutron-rich isotopes of elements 101 through 105, and the unique potential of ²⁵⁴Es as a target to make these exotic nuclei accessible is demonstrated. (orig.)