Quantum Tunneling in Nuclear Fusion

Recent theoretical advances in the study of heavy ion fusion reactions below the Coulomb barrier are reviewed. Particular emphasis is given to new ways of analyzing data, such as studying barrier distributions; new approaches to channel coupling, such as the path integral and Green function formalisms; and alternative methods to describe nuclear structure effects, such as those using the Interacting Boson Model. The roles of nucleon transfer, asymmetry effects, higher-order couplings, and shape-phase transitions are elucidated. The current status of the fusion of unstable nuclei and very massive systems are briefly discussed.Comment: To appear in the January 1998 issue of Reviews of Modern Physics. 13 Figures (postscript file for Figure 6 is not available; a hard copy can be requested from the authors). Full text and figures are also available at http://nucth.physics.wisc.edu/preprints

Recent experimental results in sub- and near-barrier heavy ion fusion reactions

Recent advances obtained in the field of near and sub-barrier heavy-ion fusion reactions are reviewed. Emphasis is given to the results obtained in the last decade, and focus will be mainly on the experimental work performed concerning the influence of transfer channels on fusion cross sections and the hindrance phenomenon far below the barrier. Indeed, early data of sub-barrier fusion taught us that cross sections may strongly depend on the low-energy collective modes of the colliding nuclei, and, possibly, on couplings to transfer channels. The coupled-channels (CC) model has been quite successful in the interpretation of the experimental evidences. Fusion barrier distributions often yield the fingerprint of the relevant coupled channels. Recent results obtained by using radioactive beams are reported. At deep sub-barrier energies, the slope of the excitation function in a semi-logarithmic plot keeps increasing in many cases and standard CC calculations over-predict the cross sections. This was named a hindrance phenomenon, and its physical origin is still a matter of debate. Recent theoretical developments suggest that this effect, at least partially, may be a consequence of the Pauli exclusion principle. The hindrance may have far-reaching consequences in astrophysics where fusion of light systems determines stellar evolution during the carbon and oxygen burning stages, and yields important information for exotic reactions that take place in the inner crust of accreting neutron stars.Comment: 40 pages, 63 figures, review paper accepted for EPJ

Étude de la désintégration 194Hg → 194Au

The disintegration of Hg194 has been studied by β and γ spectroscopy. The half life was found to be 700 ± 100 days. No γ nor β rays have been detected between 2 et 80 keV, the upper limit of the transition energy being 83 keV (XK). The branching ratio through a possible isomeric state of Au194 is less than 1 %.La désintégration de 194Hg a été étudiée par des méthodes de spectroscopie β et γ. On a déterminé une période de 700 ± 100 jours. Aucun rayonnement γ ou β d'énergie comprise entre 2 et 80 keV n'a été détecté, la limite supérieure de l'énergie de la transition étant de 83 keV (XK). Le rapport d'embranchement vers un éventuel niveau isomérique de 194Au est inférieure à 1 %

Fonctions d'excitation des réactions (p, xn) sur l'or entre 40 et 155 Mev

Mercury isotopes of mass between 197 and 190 have been separated by electromagnetic deflection. Results have been normalized with regard to 195Hg appearing at all energies, the excitation function of which has been determined independently.Les isotopes du mercure de masse comprise entre 197 et 190 ont été obtenus par séparation électromagnétique après irradiation. Les résultats ont été normalisés par rapport au 195Hg visible à toutes les énergies et dont la fonction d'excitation a été déterminée indépendamment