D. N. Poenaru

Heavy-particle radioactivity of superheavy nuclei.

The concept of heavy-particle radioactivity (HPR) is changed to allow emitted particles with Z(e) > 28 from parents with Z > 110 and daughter around (208)Pb. Calculations for superheavy (SH) nuclei with Z = 104-124 are showing a trend toward shorter half-lives and larger branching ratio relative to α decay for heavier SHs. It is possible to find regions in which HPR is stronger than alpha decay. The new mass table AME11 and the theoretical KTUY05 and FRDM95 masses are used to determine the released energy. For 124 we found isotopes with half-lives in the range of ns to ps.

Nuclear lifetimes for cluster radioactivities

Abstract Calculated partial half-lives and Q values of the most probable decays by spontaneous cluster emission are listed for nuclides with Z = 52–122. Superheavy nuclei and nuclei far off the β-stability line are included; input mass values are from the mass tables published in 1988 in Atomic Data and Nuclear Data Tables (Vol. 39 , No. 2). Parent nuclei listed here are selected to meet the following criteria: partial lifetime for cluster emission 30 s and branching ratio relative to α decay >10 −18 . α-decay Q values and partial half-lives are given for all nuclei listed. A larger set of parent nuclei are presented in a T E X file, where half-lives 50 s and branching ratios >10 −30 are accepted provided the mass of the parent nucleus has been measured or estimated from systematics by Wapstra et al. The lifetimes for cluster emissions are calculated using the analytical superasymmetric fission model with even-odd effects taken into account; the lifetimes for α decay are experimental values or are deduced from a semiempirical formula.

Cluster preformation as barrier penetrability

It is shown that preformed-cluster models are equivalent with fission models, used to describe in a unified way cluster radioactivities and α decay. For the first time cluster preformation probability is interpreted as the penetrability of the prescission part of the barrier. A linearized universal curve of each kind of cluster radioactivity is derived in the range of mass numbers detected up to now. The corresponding formula describes well the general trend of experimental data. The need for shell effect correction is illustrated on α decay of 124 even-even emitters.

Valleys due to Pb and Sn on the potential energy surface of superheavy and lighter α-emitting nuclei

The strong shell effects of the magic nuclei 208Pb, 132Sn and 102Sn are the origin of valleys on the potential energy surfaces (PES) of the superheavy nucleus 294118 and of the alpha emitters 212Po and 106Te. We use the most advanced asymmetric two-centre shell model allowing us to obtain shell and pairing corrections which are added to the Yukawa plus exponential model deformation energy. For the first time the alpha valley of a nucleus (106Te) calculated by using the macroscopic–microscopic method is clearly seen on a PES. The α-decay half-lives calculated within fission theories (analytical superasymmetric fission model, universal curve, semiempirical formula based on fission theory) are very close to the experimental ones. The increased deviations in the neighbourhood of magic numbers of nucleons, present in other frequently used relationships (e.g. Viola–Seaborg formula), are smoothed out by the semiempirical formula based on fission theory.

Simple relationships for α-decay half-lives

The universal curve for α-decay and cluster radioactivities based on the fission approach of these decay modes is compared with the universal decay law derived using α-like R-matrix theory for a total of 534 α-emitters in four groups: even–even, even–odd, odd–even and odd–odd. The standard deviations of calculated half-lives from the experimental ones are comparable in the two cases. Large absolute values of deviations from experimental data which are found in the neighborhood of magic numbers of neutrons 126, 162 and 172, and magic numbers of protons 82 and 108 are explained by a part of shell effects which are not taken into account.

Complex fission phenomena

Abstract Complex fission phenomena are studied in a unified way. Very general reflection asymmetrical equilibrium (saddle point) nuclear shapes are obtained by solving an integro-differential equation without being necessary to specify a certain parametrization. The mass asymmetry in binary cold fission of Th and U isotopes is explained as the result of adding a phenomenological shell correction to the liquid drop model deformation energy. Applications to binary, ternary, and quaternary fission are outlined.

Alpha decay calculations with a new formula

A new semi-empirical formula for calculations of

Nuclear inertia and the decay modes of superheavy nuclei

\alpha

Fission decay of 282Cn studied using cranking inertia

~decay halflives is presented. It was derived from Royer relationship by introducing new parameters which are fixed by fit to a set of experimental data. We are using three sets: set A with 130 e-e (even-even), 119 e-o (even-odd), 109 o-e, and 96 o-o, set B with 188 e-e, 147 e-o, 131 o-e, and 114 o-o, and set C with 136 e-e, 84 e-o, 76 o-e, and 48 o-o alpha emitters. A comparison of results obtained with the new formula and the following well known relationships: semFIS (semiempirical based on fission theory), ASAF (analytical superAsymmetric fission) model, and UNIV (universal formula) is made in terms of rms standard deviation. We also introduced a weighted mean value of this quantity, allowing to compare the global properties of a given model. For the set B the order of the four models is the following: semFIS, UNIV, newF, and ASAF. Nevertheless for even-even alpha emitters UNIV gives the 2nd best result after semFIS, and for odd-even parents the 2nd is newF. Despite its simplicity in comparison with semFIS the new formula, presented in this article, behaves quite well, competing with the others well known relationships.

Liquid-drop stability of a superdeformed prolate semi-spheroidal atomic cluster

Superheavy nuclei produced up to now decay mainly by α emission and spontaneous fission. For atomic numbers larger than 121 cluster decay has a good chance to compete. While calculated α decay half-lives are in agreement with experimental data within one order of magnitude and cluster decay experiments are also very well accounted for, the discrepancy between theory and experiment can be as high as ten orders of magnitude for spontaneous fission. We analyze some ways of improving the accuracy: using a semiempirical formula for α decay and changing the parameters of analytical superasymmetric fission and of the universal curve for cluster decay. For spontaneous fission we act on nuclear dynamics based on potential barriers computed by the macroscopic–microscopic method and employing various nuclear inertia variation laws. Applications are illustrated for 284Cn and Z = 118–124 even–even parent nuclei. Communicated by Steffen Bass