摘要:

This paper contains a review of the current status of the experimental study of heavy‐ion fusion at sub‐barrier energies. Emphasis is placed on the comparison of the experimentally observed quantities with theoretical expectations. Results of measurements of the spin distributions of the composite systems formed following fusion are critically examined with a view to understanding the large discrepancies between theory and experiment which exist for some systems.

ISSN:0094-243X    

DOI:10.1063/1.42027

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Pure elastic scattering cross sections for the systems58Ni+90,94Zrhave been measured at energies of 247.5 and 232.5 MeV, making use of the ENMA spectrograph which is particularly suited for heavy‐ion reactions. Also the differential cross sections for the excitation of the first 2+inelastic state of58Niin the58Ni+90Zrcombination were extracted. Simple coupled channel calculations successfully reproduce the data at 247.5 MeV while some improvement is needed for the lower energy. Calculations with the code CC‐FUS, based on the real optical model parameters extracted from the elastic scattering measurements, are compared with the previously measured fusion cross sections at energies near and below the Coulomb barrier.

ISSN:0094-243X    

DOI:10.1063/1.42010

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The large enhancement of the fusion cross section of heavy ion collisions at energies below the Coulomb barrier has stimulated many experimental as well as theoretical studies in the past decade. I overview the present status of the theoretical research of this subject and discuss some prospects for the future.

ISSN:0094-243X    

DOI:10.1063/1.42016

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

It has been suggested that damped reaction reaction products at backward angles from the28Si+12C,24Mg+12C and20Ne+12C reactions result from the formation of long‐lived orbiting complexes. This suggestion was originally made on the basis of the observation that the total integrated binary yields are much larger than the predictions of compound nucleus calculations although they follow a 1/sin &thgr;c.m.angular distribution near 180°. Subsequent studies have confirmed the noncompound origin of these yields through the observations of entrance channel dependence of back‐angle yields and strong selectivity of natural parity states. However, it has also been found that the formation of an orbiting complex and the associated back‐angle ‘‘anomalies’’ are not universal. It appears that the formation of orbiting complex dominates the back‐angle yields for systems having a small number of open channels. This is similar to the systematic behavior that was observed for the anomalous back‐angle elastic scattering and suggests that the same physical process might be responsible for back‐angle elastic scattering and suggests that the same physical process might be responsible for back‐angle anomalies in elastic and inelastic channels. Recently new insight has been obtained about orbiting reaction mechanism from the measurement of the complete gamma‐ray angular correlations for the28Si+12C orbiting system.It has been found that the measured density matrices for the12C(2+) and28Si states are almost diagonal with respect to the direction of motion of the fragments. The measured density matrices and spin alignments are consistent with the picture of formation of a long‐lived dinuclear complex undergoing orbiting, bending and wriggling motions, but not with those obtained from statistical compound nucleus or sticking model calculations.

ISSN:0094-243X    

DOI:10.1063/1.42008

出版商:AIP    

年代:1992

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.42009

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

To study the decay mechanisms of intermediate mass fragments and fission products from a hot compound nucleus, we made an experiment for84Kr+27Al at the energy of 10.6 MeV/u using Riken Ring Cyclotron Facility. The distributions of nuclear element Z and its isotope mass are extensively analyzed in terms of Extended Hauser‐Feshbach Method (EHFM) and it is concluded that the sequential decay plays an important role in the decay process from the highly excited compound nucleus. The dependence of element distributions on angular momentum is intensively discussed by comparing the two systems84Kr+27Al and3He+Ag, which form the same compound nucleus with different angular momentum window at the same excitation energy.

ISSN:0094-243X    

DOI:10.1063/1.42031

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

In order to study the mechanism of complex fragments production from highly excited light and medium compound nucleus induced by relatively low energy heavy ion reactions, the full statistical analysis have been performed in terms of Extended Hauser Feshbach (EHF) method. At first the charge‐ and kinetic energy‐distributions of35Cl+12Creaction at (ELab=180,200 MeV) and23Na+24Mgreaction at (ELab=89 MeV) which form the47Vcompound nucleus of almost same excitation energy are extensively investigated as a prototype of the light mass system. The variations in observed cross section from fragment to fragment are understood by the variations of binding energy of the lighter fragments of binary decay from the compound nucleus. The difference of the yield in the measured cross sections between the reactions is interpreted as the entrance channel effect that23Na+24Mgchannel has the larger critical angular momentum for fusion cross section than35Cl+12Cchannel. The measured kinetic energy distributions in the laboratory system of the complex fragments are shown to be well reproduced by the EHF‐method. Therefore the observed complex fragment production are understood as the statistical binary decay from the compound nucleus induced by heavy‐ion reaction. Next, the EHF‐method is extensively applied to the study of the complex production from the111Incompound nucleus which formed by the84Kr+27Alreaction at (ELab=890 MeV). Because the complex fragments (fissioning fragments) in the first step decay from this compound nucleus have large spins and highly excitation energies, the complex fragments decay sequentially by emitting the light particles. It is shown that the effect of multi‐step cascade decay of fissioning fragments is very important for reproducing the general trend of the observed quantities such as the observed isotope‐mass distributions.

ISSN:0094-243X    

DOI:10.1063/1.42032

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We investigate the dynamic polarization potential (DPP) due to the coupling to collective states, which is an important element in a unified approach to describe all the different types of nuclear reaction processes. The construction of the nonlocal andl‐dependent DPP is based on the projection operator formalism of Feshbach. Dispersion relations between real and imaginary parts are discussed. We present an approach that can calculate the elastic scattering, direct reaction, and fusion cross sections simultaneously with a single DPP. For this purpose, a rapid method of solving the nonlocal Schro¨dinger equation is first developed to obtain theexactsolution. The separation of the DPP into direct reaction and fusion parts is then formulated. We show that such a separation made phenomenologically in a local space nicely explains the observed data. Finally the localization of the nonlocal DPP is discussed in order to justify the phenomenological local optical model potential.

ISSN:0094-243X    

DOI:10.1063/1.42038

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

In this paper we discuss properties of transfer reactions which are important for understanding the dynamics of heavy‐ion grazing collisions. In particular we stress the evolution of the reaction mechanism as a function of the incident beam energy and discuss the various components of a continuum energy spectrum.

ISSN:0094-243X    

DOI:10.1063/1.42033

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The process of hadron‐nucleus interaction associated with the knockout of considerable number of nucleons from the nucleus is investigated in the framework of intra‐nuclei cascade model. It is shown that final states of target nucleus corresponding to the knockout of all possible numbers of nucleons form a continuous spectrum. Obtained results are discussed in connection with the problem of possible existence of multi‐nucleon clusters in the nuclei. It is shown that intra‐nuclear cascade model allows great occurrences of exited nuclear states corresponding to the knockout of &agr;‐clusters in hadron‐nucleus scattering process without the assumption of their existence in the nucleus.

ISSN:0094-243X    

DOI:10.1063/1.42034

出版商:AIP    

年代:1992

数据来源: AIP