L-dependent heavy ion fusion potentials

The energyE and angular momentuml dependence of optical potential for fusion of¹⁶O+²⁰⁸Pb system, observed by Christleyet al [5], is expressed as a function of radial kinetic energy (ɛ) instead of explicitE andl dependence. It is shown that the effects of different channel couplings, which result in different effective potentials, can also be parametrized as a function ofɛ. A correlation is obtained between the energy dependent part of this effective potential and the maximum of the spin enhancement around the Coulomb barrier and both these quantities depend on the details of the channel couplings.     

Pre-equilibrium effect in heavy ion induced fusion

The exciton model is applied to a multi-nucleon emission process after fusion caused by heavy-ion collisions. The master equations for the probability distribution are successively solved by assuming an initial distribution calculated by using a simple Fermi gas model. The equilibration process is discussed, and the results are compared with those of the equilibrium statistical theory and experimental data.     

Development of arc ion sources for heavy ion fusion

An overview of the development of arc ion sources for heavy ion fusion is presented. Two approaches to heavy ion fusion (HIF)-the RF linac-storage ring approach and the induction linac approach-are described. RF linac schemes require low emittance and moderate current levels, because the beam is accumulated in storage rings before being focused on target. The induction linac approach requires low emittance and high current, because this is a single-pass approach to HIF and one wishes to limit the number of beams in the machine. The RF scheme generally uses long pulse sources together with a buncher of RFQ. The induction linac approach requires sources in the microsecond pulse length range, with good optics being maintained during the pulse. Emphasis is on the induction linac approach pursued at the Lawrence Berkeley Laboratory     

Role of single particle transfer in heavy ion fusion

Fusion excitation functions in heavy ion collision have been calculated taking into account the effect of random transfer of single particles in a classical dynamical model. The model has been applied to fusion of ¹⁶O+²⁷Al and ⁴⁰Ar +¹⁰⁹Ag. The transfer does not affect the low energy fusion excitation function but the effect of transfer is appreciable for high energies.     

Dynamics of capture and fusion in heavy ion collisions

The effect of entrance channel on decrease of the complete fusion cross sections and on the yield of reaction products are associated with the quasifission which depends on the mass asymmetry and shell structure of colliding nuclei. In reactions of massive projectile and target nuclei, the competition between complete fusion and quasifission appears at the stage of compound nucleus formation, in addition to the increase of the fission probability. It is shown that the yield of quasifission products may be symmetric or asymmetric in dependence on peculiarities of shell structure of reaction fragments. Marima of mass or charge distributions are connected with the peculiarities of shell structure of reaction fragments.     

The role of rotational degrees of freedom in heavy ion collisions

The degrees of freedom affected by the angular momentum are identified. The relevance of the equilibrium fluctuations in a diffusive evolution of the system is discussed. The statistical limit is described and chosen as a reference for comparing with experiment. The rigid rotation regime is shown to be reached in a variety of reactions. The fragment spin alignment is measured from γ-ray multiplicities and anisotropies as well as from sequential fission angular distributions. Good agreement is obtained with the statistical model for the P_zz component of the polarization tensor. The P_xy component seems also to reach the statistical limit at large Q-value. The effect of shells on the angular momentum transferred to the fragments and on its misalignment is discussed theoretically and specific predictions are made.     

Analysis of the fusion hindrance in mass-symmetric heavy ion reactions

The fusion hindrance,which is also denominated by the term extra-push,is studied on mass-symmetric systems by the use of the liquid drop model with the two-center parameterization.Following the idea that the fusion hindrance exists only if the liquid drop barrier(saddle point) is located at the inner side of the contact point after overcoming the outer Coulomb barrier,the reactions in which two barriers are overlapped with each other are determined.It is shown that there are many systems where the fusion hi...     

Measurement and interpretation of heavy ion fusion excitation functions

Fusion excitation functions for ³²S induced reactions on ²⁴Mg, ²⁷Al, ⁴⁰Ca and ⁵⁸Ni are reported at incident ³²S ion energies of 65 to 132.5 MeV. Measurements were made using counter-telescopes with beams from Van de Graaff accelerators. From these data barrier heights and radii for fusion are extracted. These results are interpreted in terms of the nuclear diftuseness, and the nuclear attractive potentials at the fusion radii are deduced. Relative density overlaps at the fusion radius are estimated from electron scattering density distributions. Several parameterizations for the fusion radii and barrier heights are presented. Fusion cross sections are compared with reaction cross sections based on elastic scattering measurements coupled with optical model analysis. It is found that for the systems investigated, 90±10 % of the reaction cross section results in fusion.     

Classical dynamical theory of heavy ion fusion and scattering

A classical dynamical model which includes dissipative forces is suggested for heavy ion reactions high above the Coulomb barrier. Internal degrees of freedom corresponding to rotation of the ions are included. The reactions divide into three parts: (1) quasi-elastic scattering, with relatively small energy loss, associated with higher angular momenta, (2) deep inelastic scattering, with larger energy loss and considerable transfer of mass, associated with intermediate angular momenta, and (3) complete fusion where a highly excited compound state is formed, generally associated with the lowest angular momenta. One can predict a fusion cross section for two values of the friction coefficient, “weak” and “strong” friction cases. Reasonable values for fusion can be obtained in the weak friction case, but scattering angular distributions are not consistent with available experimental data. In the strong friction case it is more difficult to fit all the fusion cross sections with a single friction parameter. But the predicted angular distributions and energy losses are in better agreement with experiment than for the weak friction case.     

Light particles emission and incomplete fusion in heavy ion collisions

Light particles emission in heavy-ion collisions is considered in terms of direct-reaction mechanisms. The extreme high-energy part of the inclusive spectrum of light particles is shown to arise essentially from the stripping mechanism while the usually observed maximum of the spectrum is formed by a superposition of several mechanisms. The significance is emphasized of the heavy-fragment stripping in the synthesis of the exotic superheavy nuclei.     

Fast neutron emission from partial fusion heavy ion reactions

The Boltzmann master equation theory which succesfully reproduces the spectra of fast nucleons emitted in fusion and quasi-fusion heavy ion reactions is used to predict the spectra of fast nucleons in coincidence with a projectile-like fragment in the interaction of 35 MeV/ nucleon¹⁴N with¹⁶⁵Ho. In particular it is shown that the spectra of neutrons in coincidence with a high energy projectile-like fragment emitted at a very forward angle with energy corresponding to the beam velocity may be reproduced satisfactorily assuming an elastic breakup of the projectile followed by fusion of one of the fragments with the target nucleus.     

A new approach for heavy ion fusion spin distribution

The method of optical model analysis of generalized elastic scattering angular distributions (GESA) has been applied to heavy ion scattering to derive fusion spin distributions. This method is used to reproduce the coupled channel fusion spin distributions. When applied to experimental data, particularly to the fissile systems like¹⁶O +²³²Th, the method gives large mean square spin values in agreement with “anomalous” values derived from experimental fission fragment anisotropies.     

Liquid-drop model description of heavy ion fusion at sub-barrier energies

The enhancement of the heavy ion fusion cross section at sub-barrier energies is studied in the liquid-drop model approach. The shape of the system is described by two spheres smoothly connected by a neck, and the kinetic and potential energies are calculated within this parametrization. Underbarrier fusion cross sections for symmetric projectile-target combinations are calculated in the WKB approximation and a comparison with the available data is made. The agreement is quite satisfactory, except for those systems in which the reaction is strongly affected by the details of the nuclear structure of the collision partners.     

A new polarization potential for heavy ion fusion spin distribution

The real part of the polarization potential which depends on both energy and angular momentum is calculated in a simple way using dispersion relation. A barrier penetration model (BPM) has been used to explain the fusion cross-section and compound nucleus spin distribution for³²S+⁶⁴Ni system in the energy range 50–75 MeV. It is also shown that the polarization potential which only depends on energy, is not adequate to give rise to correct spin distribution even after including any radial dependence. The proposed polarization potential with implicitE andl dependences is able to explain both fusion cross-section and average spin values.     

SubthresholdK − production in heavy ion collisions: No evidence of collective phenomena as yet

Recent systematic experimental studies of subthreshold antikaon production are explained reasonably well by a multiple collision approach based on binary baryon-baryon encounters.     

Scaling behaviour and correlations in the string fusion model for heavy ion collisions

The KNO scaling, the scaling of multiplicities, the behaviour of fluctuations in the transverse momentum as a function of the multiplicity and the long range correlations in nucleus-nucleus collisions are studied by a Monte Carlo code based on the string fusion model. It is shown that the fusion of strings produces a strong reduction of the long range correlations at high multiplicities, which could be detected experimentally. On the contrary the KNO scaling, the scaling of multiplicities and the behaviour of fluctuations in the transverse momentum are not modified when string fusion is included.On leave of absence from the Department of High Energy Physics, University of St. Petersburg, 198904 St. Petersburg, Russia     

Generators of collective vibrations in heavy nuclei

Solving harmonic equations of motion for a realistic single-particle potential, the generators of various isoscalar collective vibrations up to a multipolarity of 4 are extracted. Transition strengths, transition currents, and energy systematics are studied in comparison with full RPA calculations, fluid dynamical calculations, and experimental data. It is shown that all these states can be well described by simple generators and that the influence of the detailed structure of the residual two-body force is small compared with that of the single-particle level scheme and of geometric constraints.     

Multifragmentation and incomplete fusion in central heavy ion collisions within a schematic phenomenological model

The projectile-energy thresholds for nuclear multifragmentation in the collisions of ¹²C, ²⁰Ne and ⁴⁰Ar with ¹⁹⁷Au have been calculated using a schematic model applicable to central collisions of intermediate energy heavy ions. The properties of the composite nucleus which remains following the incomplete fusion of the projectile and target nuclei are calculated using the master equation approach of Harp and Miller to determine the number of preequilibrium nucleons emitted and the energy and linear momentum removal by these nucleons. This nucleus is followed through an isentropic expansion phase and the probability distribution of clusters is determined using a percolation approach based on a 3-dimensional cubic lattice. The input parameters for the percolation calculation are derived from the results of the preequilibrium calculation. While the calculated thresholds in projectile energy per nucleon decrease rapidly as the projectile mass increases, the excitation energy at which multifragmentation is first predicted to occur is approximately the same for each system, i.e., 4.5 to 5 MeV/u. This result is consistent with the apparent disappearance of residual heavy products at such bombarding energies.     

The TMX heavy ion beam probe

A heavy ion beam probe has been used to measure the radial space potential distribution in the central cell of TMX. This was the first beam probe system to utilize computer control, CAMAC instrumentation, and fast time response for broadband fluctuation capabilities. The fast time response was obtained using off-line processing of the energy analyzer detector signals and wideband transimpedance amplifiers. The on-axis space potential was found to be 300-400 V, with φ_e/T_ec~8. The radial potential profile is parabolic when gas box fueling is used. The frequency of observed fluctuations was found to agree with the E×B plasma rotation frequency during the discharge. The measured Tl⁺⁺ secondary ion current level is consistent with calculations, given reasonable assumptions for beam attenuation