Influence of the shell effects on fission fragment angular anisotropies

The influence of the damping shell corrections with increasing excitation energy on the fission fragment angular anisotropies is considered. In the framework of the statistical approach to nuclear fission, experimental data on fission fragment angular anisotropies obtained in the ⁴He + ²³⁸U reaction is analyzed. Information about the energy dependence of the shell corrections is obtained from this analysis.     

A three-center shell model for ternary fission

We propose a generalization of the phenomenological shell model based on the harmonic oscillator potential with spin-orbit term andl ²-corrections to systems made up of three clusters. The centers of these may be in arbitrary geometrical configurations and the clusters may be of different masses. The method of determining the eigenstates of the single-particle Hamiltonian is sketched and results for the cluster structure of light nuclei and the ternary fission of a superheavy system are presented.     

Dynamical model of fission fragment angular distributions

A dynamical model of fission fragment angular distributions is developed. The experimental data on the angular anisotropy of fission fragments is analyzed for the ¹⁶O + ²⁰⁸Pb, ²³²Th, ²³⁸U, and ²⁴⁸Cm reactions at energies of the incident ¹⁶O ions ranging from 90 to 160 MeV. This analysis allows us to extract the relaxation time for the tilting mode. It was also demonstrated that the angular distributions are sensitive to the deformation dependence of the nuclear friction.

     

Comparison of cluster and shell model wave functions of deformed32S states

The 786 keV beta-286keV gamma angular correlation in the ground state decay of¹⁴⁹Pm is measured for the first time, using the conventional slow-fast scintillation assembly. Integral correlation results characterize the involved beta transition to be of first-forbidden non-unique type. The present results combined with the other experimental parameters, strongly support a 9/2^? spin-parity character to the 286 keV level in¹⁴⁹Sm. The energy independence of the reduced correlation coefficient as found in this work shows the validity ofξ-approximation for the 786 keV beta transition in¹⁴⁹Pm and is also consistent with the logft value and spectrum shape reported by earlier authors.     

Search for dynamical effects in the fission decay in the 240 MeV 32S + 100Mo reaction

Light particles in coincidence with evaporation residues and heavy fragments have been measured by a 4π charged-particle detector at the INFN Laboratori Nazionali di Legnaro (Padua) for the 240 MeV ³²S + ¹⁰⁰Mo reaction leading to the ¹³²Ce composite system at 152 MeV of excitation energy. Energy spectra of the alpha-particles in coincidence with fission fragments were extracted for many correlation angles both in plane and out of plane. A prominent out-of-plane emission was observed, consistent with the pattern for the near-scission emission. From the fit to the spectra, the pre-scission alpha-particle multiplicity of 0.040±0.006 was obtained. This value is reproduced by the code PACE2 without the inclusion of a delay time for fission. The presence of fast fission, which could be responsible for this result, is discussed. Received: 6 August 2002 / Accepted: 9 October 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: moro@na.infn.it Communicated by C. Signorini     

Lifetime estimates for asymmetric fission of Pu,including shell effects

The lifetime for spontaneous fission of ²⁴⁰Pu is evaluated in an adiabatic approach with the WKB-approximation, where left-right asymmetry is taken into account in both the potential (Strutinsky's method) and the kinetic energy (cranking model). The ratio of the lifetimes for symmetric and asymmetric fission agrees with the available data. The predicted peak-to-peak ratio in the fragment mass distribution is slightly larger (∼1.5) than the experimental one.     

Shell effects and pairing correlations in fission investigated with radioactive beams

The secondary-beam facility at GSI allows to produce a large variety of exotic nuclei at relativistic energies. This technique offers a unique oportunity to investigate systematically fission in inverse kinematics. In the present experiment, the fission properties of more than 70 different actinides and preactinides were investigated at low excitation energy. The elemental yields and kinetic energies of the fission residues present new signatures of shell structure and pairing correlations. Received: 1 May 2001 / Accepted: 4 December 2001     

Influence of shell effects on the angular distributions of fission fragments

A dynamical-statistical model is used to analyze the experimental angular distributions of fission fragments in the reactions α + ²³⁸U, ²³⁷Np at E _α = 20–100 MeV, as well as to determine the Am isotope fission probabilities and the shape isomer yields in the reactions d + ^242,240Pu at E _d = 20–30 MeV. Manifestations of shell effects are found in the fission barrier structure up to the excitation energies of 50–60 MeV.     

The shell structure and the asymmetry of nuclear fission for 140Pu in a statistical model

The dependence on excitation energy of the mass distribution of the fragments arising from the fission of ²⁴⁰Pu is studied in a statistical model. The level densities needed are calculated on the basis of the single-particle energies obtained from a deformed Woods-Saxon potential. First we investigate when and how the shell effects disappear in ²⁴⁰Pu with increasing excitation energy. Using the transition state method we then calculate some characteristic properties of the fragment mass distributions and compare the results with the experimental observations. Reasonable agreement is obtained.     

Understanding the anomalous fragment anisotropies in heavyion induced fission in the dynamical trajectory model

Dynamical trajectory model has been used to calculate the fusion and non-fusion trajectories in ¹⁰ B, ¹² C, ¹⁶ O, ¹⁹ F +²³² Th, ²³⁷ Np reactions. It is seen that in some of the above systems, there exists an l-window (above fusion l-value) for which the trajectories are characterised by large mass exchange and energy relaxation (fission-like) before the system undergoes symmetric split, without formation of a shape equilibrated compound nucleus. These events would correspond to a small value of the variance of the K-distribution, thereby leading to large fragment angular anisotropies. The fission fragment angular distributions, calculated as an admixture of these two classes of events (fusion and non-fusion), are able to explain the anomalous angular anisotropies observed experimentally for some systems at the above barrier energies.     

P-odd and P-even correlations for third particles in ternary fission

Within quantum-mechanical fission theory, P-odd and P-even correlations in angular distributions of products of the ternary fission of nuclei that is induced by polarized cold and thermal neutrons are investigated on the basis of a nonevaporative mechanism of third-particle emission and under the assumption that a two-humped fission barrier exists. It is shown that these correlations for third particles are induced by the analogous correlations for ternary-fission fragments, the latter being transferred to the third particle because of the kinematical conditions of third-particle emission that are associated with the charge and mass asymmetry of fragments. Optimum methods for observing the above correlations for third particles are discussed. The possibility of discovering the emission of prescission neutrons in the fission process against the background of evaporated neutrons by means of studying P-odd and P-even correlations is explored.     

Critical analysis of the Colle-Salvetti model for electron correlation in closed shell systems: pair correlations

The Colle-Salvetti functional for electron correlation in closed shell systems is applied to a simple two-electron problem and compared with a variational wavefunction. The correlation hole is found to be too short range. As a result, the model is biased towards regions of large electron density and practically neglects pair correlations that are long range. In addition, the correlation energy per electron is found to be singular at the nucleus. The error from neglecting the single-particle operators is found to be of the order of magnitude of the correlation energy itself. It is concluded that the Colle-Salvetti model predicts inaccurate pair correlations and should be used with great care.     

p t -Multiplicity correlations in a multi-pomeron-exchange model with string collective effects

The 〈p _t 〉N _ch - N _ch correlations experimentally observed in the central rapidity region in pp and p collisions, in a wide energy range from the ISR to Tevatron, are described in the framework of a multi-Pomeron exchange model in which string collectivity has been included in an effective way. Three parameters are obtained from the fit to data: the string tension, the average number of particles per string, and a parameter which effectively introduces string collective effects. The model successfully reproduces the rise of 〈p _t 〉 of charged particles, the flattening with growing rapidity density of charged particles and with the collision energy, and the negative p _t -N _ch correlation at low energies. The string tension and the average number of particles per string are energy independent, while the parameter that includes effectively string collective effects shows a smooth increasing behavior with energy. The text was submitted by the authors in English.     

Effect of entrance channel parameters on fission fragment angular momentum in medium energy fission

Independent isomeric yield ratios of¹³²I were radiochemically determined in alpha particle induced fission of²³⁸U in the energy range 25–44 MeV. Fission fragment angular momenta were deduced from the measured isomeric yield ratios using spin dependent statistical model analysis. It was seen that angular momentum of¹³²I increases with increase of excitation energy and angular momentum of the fissioning nucleus. Comparison of the present data on¹³²I in²³⁸U(α,f) with the literature data for the same product in²³⁸U(p, f) and²³⁸U(γ, f) at various excitation energies show that fragment angular momentum strongly depends on the input angular momentum in the range of excitation energy considered. Experimental fragment angular momentum at all excitation energies were seen to be in agreement with the theoretical values calculated based on thermal equilibration of the various collective rotational degrees after considering the occurence of multichance fission. Thus, strong effect of input angular momentum as well as the statistical equilibration among the various collective rotational degrees of freedom in medium energy fission is corroborated.     

Influence of fission fragment nuclear structure on scission configuration in252Cf (S.F.)

With the aim of investigating the influence of fragment nuclear structure e.g. shell closure configuration and odd-even nature on the scission configuration, fragment angular momenta have been deduced from the radiochemically determined independent isomeric yield ratios of the fission products¹¹¹Pd,¹³¹Te,¹³³Te,¹³⁴I and¹³⁸Cs in²⁵²Cf spontaneous fission. The fragments' angular momenta deduced are 8.8± 1.5, 5.8 ±1.0, 6.1±1.1, 11.5±1.1 and 9.8±1.2 ? respectively. The angular momenta data from this work and literature have been analysed to arrive at fragments' deformations at the scission bringing out the influence of the spherical 82n and deformed 66n and 88n shells. The fragment angular momenta were also seen to be inversely correlated to their elemental yields exhibiting odd-even fluctuations. This has been explained in terms of equilibration of the collective degrees of freedom in low energy fission.     

Prompt radiation as a probe for fission dynamics

It is shown that the Strutinsky-Denisov induced polarization mechanism leads to the appearance of the prompt electric dipole radiation from fission fragments of ²³⁵Uby thermal neutrons in the domain of around 5 MeV. The probability of the radiation is at the level of 0.001 per fission, which is in agreement with experiment. The angular distribution exhibits left-right asymmetry with respect to the direction of the neutron polarization axis. That means that the emission of gamma quanta at the given angle depends on the neutron polarization. The asymmetry is at the level of 10⁻³. The study of this effect will give a direct information about the scission configuration, nuclear viscosity, and dissipation properties of the collective energy of the surface vibration in fragments with large amplitude. This will give a complete picture of the process of snapping back the nuclear surface.