L.G. Moretto

Systematics of complex fragment emission in niobium-induced reactions

Abstract Complex fragments of 3 Z ⪅35 have been detected in the reverse-kinematics reactions of 93 Nb plus 9 Be, 12 C and 27 Al at bombarding energies of E / A =11.4, 14.7 and 18.0 MeV. Velocity spectra and angular distributions show the presence of projectile and target-like components along with a component isotropic (in the reaction plane). This latter component aappears as a Coulomb ring in the invariant cross section plots indicating the presence of a binary decay which is confirmed by the coincidence data. Statistical model calculations indicate that for the Nb+Be and C reactions, the isotropic component is associated with the binary decay of compound nuclei formed in complete fusion reactions. The charge distributions for these two systems are consistent with the conditional barriers predicted with the rotating finite-range model. For the Nb+Al reactions, there is an additional isotropic component besides compound nucleus decay, which may arise from fast fission.

Statistical emission of large fragments: a general theoretical approach

Abstract A theory for the statistical emission of large fragments is developed. In analogy with the fission saddle point, a ridge line in the potential energy surface is defined which controls the decay width of the system into any two given fragments. The normal modes at the ridge are separated into three classes: decay modes, amplifying modes, and non-amplifying modes. Amplifying modes are those whose thermal fluctuations are amplified and lead to a broadening of the kinetic energy distribution. Analytical expressions for the kinetic energy distributions are developed for various combinations of amplifying and non-amplifying modes. The limit for large amplifications is a Gaussian kinetic energy distribution. The limit for no amplification is a Maxwellian-like distribution. Thus the formalism comprehends the fission decay on one hand and the neutron evaporation on the other. The angular distributions are evaluated in terms of the ridge line principal moments of inertia. A general analytical expression has been derived which predicts, correctly in both limits, the angular distributions of the evaporated neutrons and of the fission fragments.

STATISTICAL DESCRIPTION OF DEFORMATION IN EXCITED NUCLEI AND DISAPPEARANCE OF SHELL EFFECTS WITH EXCITATION ENERGY.

Abstract The deformation of excited nuclei is studied on the basis of the methods of statistical mechanics. The statistical coupling between a system with many internal degrees of freedom and a one-dimensional or multidimensional harmonic oscillator is considered. The relevance of the phase-space volume is discussed in the semiclassical limit. The conclusions are employed to develop a general theory of nuclear deformation for actual nuclei. Calculations for five nuclei are performed on the basis of the Nilsson diagram and of the BCS Hamiltonian. The potential energies of nuclei as a function of deformation are calculated by means of the Strutinsky procedure. The level densities are also calculated directly from single-particle levels and include pairing and its energy dependence. The deformation probability as a function of excitation energy is derived. The nuclear fluctuations in shape are considered and in particular the disappearance of the ground state deformations with increasing excitation energy is described. The calculations account in a natural way for the washing out of shell effects with excitation energy. Applications of the theory are discussed.

STATISTICAL DESCRIPTION OF A PAIRED NUCLEUS WITH THE INCLUSION OF ANGULAR MOMENTUM

Abstract The effect of angular momentum on an excited paired nucleus has been studied. The BCS Hamiltonian, modified to include the z-projection of angular momentum has been diagonalized and expressed in terms of the quasiparticle occupation numbers. The grand partition function and all the relevant thermodynamical functions as well as the level-density expression have been derived for the general case of an arbitrary set of single-particle levels. Furthermore, the formalism has been applied to the uniform model and, whenever possible, analytical expressions have been derived. In particular the zero-temperature angular momentum dependence of the gap parameter, the critical angular momentum as well as the yrast line have been calculated. The critical temperature as a function of angular momentum, which defines the phase transition between paired and unpaired systems, has been calculated. A new effect called the thermally assisted pairing correlation, involving an increase of pairing with increasing temperature has been predicted. The completeness of the formalism as applied to spherical or deformed nuclei has been discussed.

Pairing fluctuations in excited nuclei and the absence of a second order phase transition

Abstract The statistical fluctuations in the nuclear gap parameter are studied. The difference between the average and most probable gap parameter is discussed and the absence of sharp second order phase transition is shown.

Influence of shells and pairing on the fission probabilities of nuclei below radium

Abstract Realistic level densities have been employed in analysing experimental fission probabilities. The inclusion of shell and pairing effects allows one to obtain reliable information concerning the fission barriers as well as the saddle point pairing and level densities.

The role of the compound nucleus in complex fragment emission at low and intermediate energies

Abstract The origin of complex fragments produced at low and intermediate energies is discussed and the leading theories as well as the experimental evidence are reviewed. Particular attention is focused on the compound nucleus formed in either complete or incomplete fusion as a possible source of complex fragments. Theoretical aspects of compound nucleus emission are reviewed in detail. The experimental demonstration of the existence of compound nucleus emission at extremely low energy is given and the process is followed by means of a series of reactions up to 50 MeV/u. These results allow one to conclude that, in the general class of reactions under investigation, complex fragments come either from statistical compound nucleus emission, or they are target and/or projectile-like fragments formed in quasi and deep inelastic reactions, or in incomplete fusion as spectators. The multifragmentation process is discussed in terms of novel theories like liquid-vapor equilibrium, nuclear shattering, nuclear disassembly, etc. as well as in terms of established physics like the process of comminution resulting from the sequential emission of many fragments from an extremely excited compound nucleus. The origin of fragments at higher energies is also briefly considered.

Emission of complex fragments from highly excited systems produced in 93Nb+9Be and 27Al reactions at EA = 25.4 and 30.3 MeV

Abstract Complex fragments with atomic numbers intermediate between that of the target and projectile have been detected in the reverse-kinematics reactions of 93 Nb plus 9 Be and 27 Al at bombarding energies of E A = 25.4 and 30.3 MeV. Experimental results from inclusive and coincidence measurements are presented and are used to characterize these fragments as the statistical, binary-decay products of highly excited compound nuclei formed in fusion-like reactions.

COMPLEX FRAGMENT EMISSION AT 50 MeV/u: COMPOUND NUCLEI FOREVER?

Abstract Fragments with 12⩽ Z ⩽35 have been detected in the reaction of 50 MeV/u 139 La on 12 C and are shown to be produced solely by the binary decay of a compound nucleaus formed in an incomplete fusion reaction. The source velocity, CM velocities, cross sections, and angular distributions extracted from the inclusive data are entirely consistent with this mechanism. The binary coincidence data confirm this interpretation and can be quantitatively accounted for by the singles data.

Shell-model calculations of fission decay widths and probabilities in superheavy nuclei

Abstract The stability of excited superheavy nuclei with respect to fission is studied on the basis of realistic shell models and of the BCS Hamiltonian. A statistical approach is used to calculate the deformation probabilities of excited nuclei as well as the fission and neutron decay widths. The results of the calculations show a rapid washing out with energy of the shell effects responsible for the high fission barriers predicted in the superheavy region. The first chance fission probabilities are calculated for nuclei in the region between Z = 108 and Z = 126. Two single-particle models proposed by Nilsson and by Bolsterli et al. have been used in the calculations. The results are discussed in terms of the possible production of superheavy elements by means of standard nuclear reactions.