Symmetric Entrance Research Articles

Study of angular momentum variation due to entrance channel effect in heavy ion fusion reactions

A systematic investigation of the properties of hot nuclei may be studied by detecting the evaporated particles. These emissions reflect the behavior of the nucleus at various stages of the deexcitation cascade. When the nucleus is formed by the collision of a heavy nucleus with a light particle, the statistical model has done a good job of predicting the distribution of evaporated particles when reasonable choices were made for the level densities and yrast lines. Comparison to more specific measurements could, of course, provide a more severe test of the model and enable one to identify the deviations from the statistical model as the signature of other effects not included in the model. Some papers have claimed that experimental evaporation spectra from heavy-ion fusion reactions at higher excitation energies and angular momenta are no longer consistent with the predictions of the standard statistical model.In order to confirm this prediction we have employed two systems, a mass-symmetric (31P+45Sc) and a mass-asymmetric channel (12C+64Zn), leading to the same compound nucleus 76Kr* at the excitation energy of 75 MeV. Neutron energy spectra of the asymmetric system (12C+64Zn) at different angles are well described by the statistical model predictions using the normal value of the level density parameter a = A/8 MeV−1. However, in the case of the symmetric system (31P+45Sc), the statistical model interpretation of the data requires the change in the value of a = A/10 MeV−1. The delayed evolution of the compound system in case of the symmetric 31P+45Sc system may lead to the formation of a temperature equilibrated dinuclear complex, which may be responsible for the neutron emission at higher temperature, while the protons and alpha particles are evaporated after neutron emission when the system is sufficiently cooled down and the higher g-values do not contribute in the formation of the compound nucleus for the symmetric entrance channel in case of charged particle emission.

Alpha particle evaporation as a probe for dynamical deformations

Evaporation of alpha particles in heavy ion reactions followed by fusion has proved to be a powerful probe for the properties of emitting nuclei. Detailed experimental data and different model calculations allow us to probe whether the foundation of the statistical model holds for the compound nucleus populated in these reactions. It has been observed that in the case of composite nuclei at moderate energies and angular momenta, evaporation spectra are well explained in terms of the standard statistical model CASCADE code employing optical model transmission coefficients in the description of particle evaporation. However, it has been observed that experimental particle spectra from heavy ion induced fusion reactions are no longer consistent with the predictions of such models. It has been predicted by some authors that in these systems the emission barriers are lower than those expected from optical model transmission coefficients calculated for the respective inverse absorption channels. Some authors claim that these spectra may be well explained in terms of the statistical model incorporating only spin dependent level density and without lowering the emission barriers. The field is not yet free from the controversies. Furthermore, the assumption of the very short formation time in statistical model analysis is one extreme of the general evolution process which in fact is a continuous relaxation process, leading to the composite system from the entrance channel to the equilibrated configuration. Recent dynamical description of heavy ion collisions do not support this assumption in many cases. In symmetric entrance channels and for collisions where centre of mass energy is well above the Coulomb barrier, formation time can be even larger than decay time of the resulting composite system. In such cases realistic approach will be to couple the dynamical evolution of the intrinsic excitation of the composite system to a time dependent statistical model calculation. The above question has been addressed in the light of the alpha particle spectra taken in coincidence with the evaporation residues for the asymmetric 28Si+51V and the symmetric 28Si+27Al systems. The experimental data have been interpreted in the framework of dynamical trajectory model calculations.

Study of transfer channel coupling and entrance channel effects for the near and sub-barrier fusion of , and systems

The fusion excitation functions for the 46 Ti+ 64 Ni , 50 Ti+ 60 Ni and 19 F+ 93 Nb systems have been measured from ∼ 10% below to 15% above the nominal Coulomb barrier, utilizing the recoil mass separator, HIRA at the Nuclear Science Centre, New Delhi. Mean spins have been deduced from the fusion cross sections by two different methods, one using the statistical model and the second by a fit to the fusion cross sections. Good agreement is found between them. It is seen that the simplified coupled-channel calculations, with couplings to the lowest surface inelastic excitations alone, do not explain both the observed fusion cross sections and mean spins in the low energy domain for all the three systems. A systematic comparison is made between the systems 46 Ti+ 64 Ni and 50 Ti+ 60 Ni which have quite different ground-state transfer Q -values ( Q gg ) for the transfer channels. The 46 Ti+ 64 Ni system shows a significant enhancement of the sub-barrier fusion cross section and mean spin as compared with the 50 Ti+ 60 Ni system, indicating the importance of the transfer channel coupling. Fusion cross section and mean spin have been studied for the near and sub-barrier fusion of the highly mass asymmetric system 19 F+ 93 Nb and compared with those for the 48 Ti+ 64 Ni system leading to the same compound nucleus via the nearly symmetric entrance channel. Since the 19 F+ 93 Nb system has a large positive Q gg for one-proton pick-up, the fusion data have been complemented by one-nucleon transfer measurements at energies around the Coulomb barrier. A simultaneous analysis of the fusion excitation functions and mean spin data indicates no significant effects that may be related to the entrance channel mass asymmetry.

Dynamics of heavy-ion fusion probed by d/p double ratios from a cross bombardment.

The particle decay ensuing from the reactions 86.0 MeV $^{16}\mathrm{O}$ + $^{148}\mathrm{Sm}$ and 239.1 MeV $^{64}\mathrm{Ni}$ + $^{100}\mathrm{Mo}$ was studied. These reactions each form $^{164}\mathrm{Yb}$ compound nuclei excited to \ensuremath{\approxeq} 54 MeV. Particle decay from compound nucleus producing reactions was selected by gating on the gamma-ray fold and the angular region of the particle emission. While there are no discernable differences in the dominant decay channels between the two reactions, there are fewer deuterons from the more symmetric system. This difference can be interpreted two ways: as a suppression of the emission of energetically expensive clusters during the time required for shape equilibration (which is predicted to be longer for the more symmetric entrance channel), or as an enhancement of the emission of energetically expensive clusters from the more asymmetric system at the very early stage of the collision when the initial energy deposited is only available to a reduced number of nucleons. The first explanation is identical to that used in recent high energy photon work while the second could be identified as the result of the emission of clusters on the multistep compound branch leading to the fusion of the low energy heavy ions. If the first explanation is adopted, the observed suppression is larger than predicted by a standard statistical decay model coupled to a dynamical fusion model, but consistent with work using high energy photons as a probe of fusion dynamics.

Transitions in nuclear disassembly as viewed by fragment charge correlations.

Central heavy-ion collisions in the (nearly) symmetric entrance channels {sup 20}Ne+{sup 27}Al, {sup 40}Ar+{sup 45}Sc, {sup 87}Kr+{sup 93}Nb, and {sup 129}Xe+{sup 139}La have been systematically studied at many intermediate beam energies. The relative sizes of the three largest fragments in the central events are studied using new variables defined from charge Dalitz plots. Autocorrelations between these variables and the method used to select the central events are investigated. Also described are the hidden constraints on these variables that are imposed by definition. For increasing beam energies, the average values of these variables imply transitions from sequential binary disassembly to multifragmentation at beam energies of {similar_to}50 and {similar_to}30 MeV/nucleon in the {sup 40}Ar+{sup 45}Sc and {sup 129}Xe+{sup 139}La entrance channels, respectively. Similar trends are observed for the central {sup 20}Ne+{sup 27}Al and {sup 87}Kr+{sup 93}Nb reactions.

Entrance-channel dependence of the fission dynamics in 19F+209Bi vs 64Ni+165Ho.

Binary fragmentations of $^{19}\mathrm{F}$${+}^{209}$Bi (${\mathit{E}}_{\mathit{p}}$=491 MeV) and $^{64}\mathrm{Ni}$${+}^{165}$Ho (${\mathit{E}}_{\mathit{p}}$=778 MeV) leading to compound nuclei with ${\mathit{E}}^{\mathrm{*}}$\ensuremath{\approxeq}400 MeV and similar masses were investigated by measuring fission coincident neutrons and \ensuremath{\alpha} particles. The neutron field was decomposed into contributions from preequilibrium (PE), prescission, and the two fragment sources with a moving source analysis. It was performed for separate excitation energy classes deduced with the folding angle technique from the linear momentum transfer. The angular distribution of the PE neutrons from the $^{19}\mathrm{F}$${+}^{209}$Bi reaction indicates a pronounced out-of-plane anisotropy that would result in an overestimation of the PE multiplicity by 37% for a measurement only in plane. The PE multiplicities and temperatures are interpreted by a delay in formation for the more symmetric entrance channel $^{64}\mathrm{Ni}$${+}^{165}$Ho, which is typical for quasifission and in line with the enhancement of asymmetric mass splits. Prescission times derived from the neutron clock method extend from 30\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}22}$ s for $^{64}\mathrm{Ni}$${+}^{165}$Ho to 160\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}22}$ s for $^{19}\mathrm{F}$${+}^{209}$Bi. Here, a constant level density parameter of A/10 ${\mathrm{MeV}}^{\mathrm{\ensuremath{-}}1}$ was used which is an outcome of the simultaneous measurement of \ensuremath{\alpha} emission temperatures. The prescission times are qualitatively discussed in terms of the reaction dynamics.

Autocorrelations and intermediate-mass-fragment multiplicities in central heavy-ion collisions.

The average multiplicities of intermediate mass fragments (IMFs) for central heavy-ion collisions in the (nearly) symmetric entrance channels $^{20}\mathrm{Ne}$${+}^{27}$Al, $^{40}\mathrm{Ar}$${+}^{45}$Sc, $^{84}\mathrm{Kr}$${+}^{93}$Nb, and $^{129}\mathrm{Xe}$${+}^{139}$La, are systematically studied over a wide range of intermediate beam energies. Cuts on experimental variables commonly assumed to be correlated with the impact parameter are used to select the most central collisions. The results for six different centrality variables are compared, and the extent to which measurements of the multiplicities of IMFs in small impact parameter collisions are affected by the variable used to select the central events is discussed. General methods for locating such ``autocorrelations'' are described. The two centrality observables that are the least autocorrelated with the number of intermediate mass fragments are identified, and these variables are used to select the most central collisions. The entrance channel mass and beam energy dependence of the experimental IMF multiplicities are presented and compared to a variety of model predictions. The models picturing the disassembly as a sequential binary process always underpredict the experimental IMF multiplicities. A generally more accurate reproduction of these multiplicities is provided by several similar chemical equilibrium models commonly assumed to be the theoretical description of multifragmentation.

How well do we understand the synthesis of heavy elements by heavy-ion induced fusion?

Experimental cross sections to synthesize heavy ion induced fusion reactions are confronted with calculations using the conventional picture of complete fusion after passing the barrier plus statistical deexcitation after complete equilibration. The sensitivity of the calculations to some of the parameters is demonstrated. Despite this sensitivity, a single parameter set is found that reproduces well the large body of data from experiments using actinide targets. By comparison, for the more symmetric entrance channels using targets around208Pb, we find a fusion hindrance that increases steeply with increasing compound nuclear charge. Predictions for a few reactions of future interest for the synthesis of heavy elements are also presented.

Ghost peaks in a sector-focusing magnetic β-ray spectrometer

Zero-, single- and double-loop trajectories in a magnetic sector field, H = H φ ∼ 1 r , have been investigated. The focusing of single- and double-loop trajectories has been calculated for a K = 0.82 β-ray spectrometer with a symmetric entrance and exit profile shape. The trajectories are represented graphically. The counting ratio N 1/ N 0 of single-loop electrons/zero-loop electrons has been measured depending on the spectrometer entrance angle ψ s and the detector width Δz. An important decrease of N 1/ N 0 was achieved by a radial baffle S r placed in the central plane z = 0 of the spectrometer gap. The maximum length Δr max ( Δz, ψ s) of S r was found from the condition that the transmission T 0 ( Δz, ψ s) of the zero-loop electrons is unaffected by S r. Minimum obtainable values (N 1/N 0) min = ƒ min (Δz, ψ s ) are reported.