Narrow Results

The phenomenon of Euclidean resonance (a strong enhancement of quantum tunneling through a nonstationary potential barrier) is applied to disintegration of atoms and molecules through tunnel barriers formed by applied constant and time-dependent electric fields. There are two different channels for such disintegration, electronic and ionic. The electronic mechanism is associated with the ionization of a molecule into an electron and a positive ion. The required frequencies are in a wide range between 100 MHZ and infrared. This mechanism may constitute a method of selective destruction of chemical bonds. The ionic mechanism consists of dissociation of a molecule into two ions. Since an ion is more massive than an electron, the necessary frequency is about 1 MHZ. This provides a theoretical possibility of a different method of isotope separation by radio frequency waves. The small sub-barrier tunneling probability of nuclear processes can be dramatically enhanced by collision with incident charged particles. Semiclassical methods of theory of complex trajectories have been applied to nuclear tunneling, and conditions for the effect have been obtained. The enhancement of α particle decay by incident proton with energy of about 0.25 MeV has been demonstrated. The general features of this process are common for other sub-barrier nuclear processes and can be applied to nuclear fission.

The nuclear structure effects on α decay and cluster emission are investigated in the case of even–even rare earth nuclei ^150–160Dy, ^150–160Er, ^150–160Yb, ^{158,162,166–176}Hf, ^{160,164–178}W and ^{162,166,170–180}Os. The role of shape and deformation of parent nuclei in the decay rate is studied by taking the Coulomb and proximity potentials as the interacting barrier for the post scission configuration. The quadrupole deformation of parent nuclei causes a slight change in the half-life of α emissions, but it affects the rate of heavy cluster emissions significantly. Prolate deformation of parents enhances cluster emission, while an oblate deformation slows down the decay. Shape and deformation of parent nuclei causes change in the branching ratio also. A prolate deformation increases the branching ratio, whereas an oblate deformation reduces it. Highest branching ratio is predicted at N ~ 90.

In this work, we systematically study the $\alpha$ decay half-lives of 170 even–even nuclei with $60\le Z\le 118$ within the two-potential approach while the $\alpha$ decay preformation factor $P_{\alpha }$ is obtained by the cluster-formation model. The calculated results can well reproduce the experimental data. In addition, we extend this model to predict the $\alpha$ decay half-lives of 64 even–even nuclei with $104\le Z\le 128$ whose $\alpha$ decay is energetically allowed or observed but not yet quantified. For comparison, the two famous models i.e., SemFIS proposed by Poenaru et al. [Europhys. Lett. 77 (2007) 62001] and UDL proposed by Qi et al. [Phys. Rev. Lett. 103 (2009) 072501] are used. The predicted results of these models are basically consistent. At the same time, through analyzing the changing trend of $\alpha$ decay energy $Q_{\alpha }$ of $Z=118,120,122,124,126$ and 128 isotopes nuclei with the increasing of neutron number N and that of $\alpha$ decay preformation factor $P_{\alpha }$ of those isotopes even–even nuclei with the increasing of neutron number N, $N=178$ may be a new neutron magic number.

Three new decay chains of ²⁸⁵Fl were observed in the 3n-evaporation channel of the ²⁴⁰Pu + ⁴⁸Ca reaction [1]. The irradiation was performed at the Dubna Gas Filled Recoil Separator (DGFRS) and using 250 MeV ⁴⁸Ca beam accelerated at the U400 cyclotron of the Flerov Laboratory of Nuclear Reactions, JINR. The decay properties of the observed nuclei are mostly in agreement with a chain identified at the BGS in the ²⁴²Pu(⁴⁸Ca,5n) reaction [2] and three chain previously observed at the DGFRS in the same ²⁴⁰Pu(⁴⁸Ca,3n) reaction at lower energy of 245 MeV [3]. The lifetime of ²⁶⁹Sg observed in one chain exceeds that derived from other five decays by a factor of 33, which might indicate the observation of transitions through different levels in ²⁸⁵Fl and descendants. The cross section of the reaction was measured to be ${0.58}_{-0.33}^{+0.60}$ pb, which is lower by a factor of 4-5 than the value measured at 245 MeV ⁴⁸Ca and is in agreement with expectations for the 3n-evaporation channel.

A new experimental complex, the SHE Factory at FLNR, including a high-current cyclotron DC280 and a gas-filled separator DGFRS-2, is planned to be commissioned by the end of 2018. The project of priority experiments which are planned to be performed for testing the capabilities of DGFRS-2 coupled with DC280 is presented. These include study of the ²⁴³Am+⁴⁸Ca reaction for measuring yields of Mc isotopes with large statistics at different beam energies. Investigation of chemical properties of elements Cn and Fl with use of the chemistry setup placed after DGFRS-2 is discussed as well. The first research experiments, including synthesis of new elements 119 and 120 in the reactions of ²⁴⁹Bk and ^249-251Cf with ⁵⁰Ti, are also considered.

We report on a high-statistics measurement of bremsstrahlung emitted in the α decay of ²¹⁰Po. The measured differential emission probabilities, which we could follow up to γ-energies of ~ 500 keV, allow for the first time for a serious test of various model calculations of bremsstrahlung accompanied a decay.¹ Taking into account the interference between the electric dipole and quadrupole amplitudes, which we calculated within the framework of a refined quasi-classical approximation² and which modifies the angular correlation between the α particle and the emitted photon, we find good agreement of the measured γ-emission probabilities with those calculated in our quasi-classical model as well as with the fully quantum mechanical prediction of Ref. 3.

A new spontaneously fissioning isotope ²⁸⁴Fl was produced for the first time in the ²⁴⁰Pu+⁴⁸Ca and ²³⁹Pu+⁴⁸Ca reactions studied at the DGFRS. The cross section of the ²³⁹Pu(⁴⁸Ca,3n)²⁸⁴Fl reaction channel was about 20 times lower than predicted by theoretical models and about 50 times lower than the maximum fusion-evaporation cross section for the 3n and 4n channels measured in the ²⁴⁴Pu+⁴⁸Ca reaction. In the ²⁴⁰Pu+⁴⁸Ca experiment, three decay chains of ²⁸⁵Fl were detected. The cross section exceeds that observed in the reaction with ²³⁹Pu by a factor of 10. The decay properties of the synthesized nuclei and their production cross sections indicate a rapid decrease of stability of superheavy nuclei as the neutron number decreases from the predicted magic neutron number N=184.

Review of discovery and investigation of isotopes of elements 113-118 produced in the reactions of ⁴⁸Ca with target nuclei ²³⁸U-²⁴⁹Cf is presented. The synthesis of the heaviest nuclei, their summary decay properties, and methods of identification are discussed. The radioactive properties of the new nuclei give evidence of the significant increase of the stability of the heavy nuclei with rise of their neutron number and approaching magic number N=184.

Review of discovery of isotopes of elements 113–118 produced in the ⁴⁸Ca-induced reactions with actinide target nuclei is presented. These superheavy nuclei were synthesized at DGFRS; the decay properties of the majority of these nuclei were subsequently reproduced in experiments carried out at SHIP, BGS, TASCA, and GARIS. The increased role of shell effects in the stability of superheavy nuclei with rise of their neutron number is demonstrated by comparison of the experimental data and results of theoretical calculations.

The α-decay half-lives of superheavy nuclei around the N = 152 shell gap have been systematically investigated using theoretical Q values. These theoretical Q values are extracted from the local formula of binding energies for heavy and superheavy nuclei. Exact calculations are performed in two ways: the unified formula of half-lives for α decay and cluster radioactivity, and the generalized density-dependent cluster model. It is found that the available experimental α-decay half-lives are well reproduced in this two ways. Based on this fact, some useful predictions of α-decay half-lives are made for still unknown superheavy nuclei, which may guide future experiments.

The fine structure in the α decay of neutron-deficient Ds, Cn, and 114 isotopes have been systematically predicted using the multi-channel cluster model (MCCM). The theoretical α-decay energy Q_α is deduced from the local formula of Q_α values for heavy and superheavy nuclei. The ground-state rotational states in a daughter nucleus are established based on the macroscopic- microscopic model with some improved ingredients. Exact five-channels microscopic calculations are performed, and the branching ratios to various daughter states and total α-decay half-lives are evaluated. Any adjustable parameter is not introduced in our calculations. It is expected that the present coupled- channel predictions would provide a reference for future structure researches of superheavy nuclei.