O A P Tavares

Systematics of alpha-decay half-life: new evaluations for alpha-emitter nuclides

A semiempirical model based on the quantum-mechanical tunnelling mechanism of alpha emission from nuclei has been used to systematize the alpha-decay half-lives of a set of 336 nuclides, comprising all the alpha-emitter nuclides whose T1/2α-data for ground-state to ground-state transitions of mutual angular momentum l = 0 are known. With a minimum of data rejection (only ~5% of cases), the procedure has been successful in reproducing quite satisfactorily (within a factor of ~2) most of the cases (~80%) investigated. A few significant discrepancies found between measured and calculated results are analysed and discussed. Also reported is the prediction from the model for possible new alpha-emitter nuclides, namely 180W, 184Os and 228Ra, for which cases the calculated partial alpha-decay half-lives fall within the range of half-lives measurable by the current techniques.

Alpha decay half-life of bismuth isotopes

The observed alpha decay half-life values of favoured alpha transitions of l = 5 in bismuth isotopes have been analysed in the framework of a model based on quantum mechanical tunnelling through a potential barrier where the centrifugal and overlapping effects are taken into account. In particular, the very recently measured alpha decay half-life value of (1.9 ± 0.2) × 1019 y for the unique naturally occurring 209Bi isotope has been reproduced by the present approach as (1.0 ± 0.3) × 1019 y. Also, the partial alpha decay half-lives for a number of unmeasured alpha transitions of l = 5 in bismuth isotopes are predicted by the model, thus making it possible to demonstrate the influence of the 126 neutron shell closure on the alpha decay half-life. The present approach is shown to be successfully applicable to other isotopic sequences of alpha-emitter nuclides.

The CRISP package for intermediate- and high-energy photonuclear reactions

The two-step process that characterizes the intermediate- and high-energy photonuclear reactions (between 40 MeV and 4 GeV) has been successfully described by Monte Carlo calculations. Recently, a new class of codes capable to perform those calculations according to a more realistic method has been developed, improving the possibilities for simulating the reactions in more details. In this work we present the CRISP package (standing for Rio–Sao Paulo Collaboration), which is a coupling of the multi collisional Monte Carlo (MCMC) and the Monte Carlo for evaporation–fission (MCEF) codes. The first one describes the intranuclear cascade process, while the second one is dedicated to the evaporation/fission competition step. Both codes have already shown to be useful for calculating several features of different nuclear reactions. The CRISP code, coupling these two software, represents a good tool to describe the complex characteristics of the nuclear reactions, and opens the opportunity for applications in quite different fields, such as studies of hadron physics inside the nucleus, specific nuclear reactions, spallation and/or fission processes initiated by different probes and many others.

The MCEF code for nuclear evaporation and fission calculations

We present an object oriented algorithm, written in the Java programming language, which performs a Monte Carlo calculation of the evaporation-fission process taking place inside an excited nucleus. We show that this nuclear physics problem is very suited for the object oriented programming by constructing two simple objects: one that handles all nuclear properties and another that takes care of the nuclear reaction. The MCEF code was used to calculate important results for nuclear reactions, and here we show examples of possible uses for this code.

Alpha decay and nuclear deformation: the case for favoured alpha transitions of even-even emitters ∗

Alpha decay half-life for ground-state to ground-state transitions of 174 even-even alpha emitters has been calculated from a simple, Gamow-like model in which the quadrupole deformation of the product nucleus (assumed to have an ellipsoidal shape) is taken into account. The assumption made is that before tunnelling through a purely Coulomb potential barrier the two-body system oscillates isotropically, thus giving rise to an equivalent, average polar direction θ0 (referred to the symmetry axis of the ellipsoid) for alpha emission. It is shown that the experimental half-life data are much better reproduced by the present description than in the spherical shaped approximation for the daughter nucleus.

Nuclear photoabsorption by quasi-deuterons and an updated evaluation of Levinger's constant

Levingers (1951) quasi-deuteron model of nuclear photoabsorption has been used together with modern root-mean-square radius data to obtain Levingers constant L of nuclei throughout the Periodic Table. It is found that L=6.8-11.2A-23/+5.7A-43/, which gives L values in good agreement with those obtained from measured total nuclear photoabsorption cross sections.

A Monte Carlo method for nuclear evaporation and fission at intermediate energies

Abstract We describe a Monte Carlo method to calculate the characteristics of the competition between particle evaporation and nuclear fission processes taking place in the compound nucleus formed after the intranuclear cascade following the absorption of intermediate energy photons by the nucleus. In this version we include not only neutrons, but also protons and alphas as possible evaporating particles. The present method allows the easy inclusion of other evaporating particles, such as deuteron or heavier clusters. Some fissility results are discussed for the target nuclei 237 Np, 238 U and 232 Th.

Effective liquid drop description for alpha decay of atomic nuclei

Alpha-decay half-lives are presented in the framework of an effective liquid drop model for different combinations of mass transfer descriptions and inertia coefficients. Calculated half-life values for ground-state-ground-state-favoured alpha transitions are compared with available, updated experimental data. Results have shown that the present model is very suitable to treat the alpha-decay process on an equal footing as cluster radioactivity and cold fission processes. Better agreement with the data is found when the subset of even-even alpha emitters is considered in the calculation.

Fragment mass distributions in the fission of heavy nuclei by intermediate- and high-energy probes

Recent experiments have shown that the multimode approach for describing the fission process is compatible with the observed results. A systematic analysis of the parameters obtained by fitting the fission-fragment mass distribution to the spontaneous and low-energy data has shown that the values for those parameters present a smooth dependence upon the nuclear mass number. In this work, a new methodology is introduced for studying fragment mass distributions through the multimode approach. It is shown that for fission induced by energetic probes ( E> 30 MeV) the mass distribution of the fissioning nuclei produced during the intranuclear cascade and evaporation processes must be considered in order to have a realistic description of the fission process. The method is applied to study 208 Pb, 238 U, 239 Np and 241 Am fission induced by protons or photons.

Photofission cross section and fissility of pre-actinide and intermediate-mass nuclei by 120 and 145 MeV Compton backscattered photons

Cross section measurements for photofission induced in , , , and by 120 and 145 MeV quasi-monochromatic photon beams have been performed at the ROKK-1M facility (BINP, Novosibirsk). The fission yields have been obtained using Makrofol sheets as solid-state fission track detectors. Nuclear fissility values have been deduced on the basis of Levingers modified quasi-deuteron model of photonuclear interaction, and compared with available literature data. The trend of fissility in the 60-145 MeV energy range has been analysed for various target nuclei as a function of energy and of the parameter .