부산대학교 도서관

Background: The radioactive 220Th* compound nucleus (CN) is of interest since the evaporation residue (ER) cross sections are available for various entrance channels 16O+204Pb, 40Ar+180Hf, 48Ca+172Yb, and 82Se+138Ba at near barrier energies. Within the dynamical cluster-decay model (DCM), the radioactive CNs 215Fr*, 242Pu*, 246Bk*, and 254Fm* are studied where the main decay mode is fission, with very small predicted ER cross section. 220Th* provides a first case with experimentally observed ER cross section instead of fission. Purpose: To look for the optimum "hot-compact" target-projectile (t-p) combinations for the synthesis of "cold" 220Th* and then its decay. For best fitting of the measured ER cross sections, with quasifission (qf) content, if any, the fusion-fission (ff) component is predicted. The magic-shell structure and entrance channel mass-asymmetry effects are analyzed, and the behavior of CN formation and survival probabilities PCN and Psurv is studied. Methods: The quantum-mechanical fragmentation theory (QMFT) is used to predict the possible cold t-p combinations for synthesizing 220Th*, and the QMFT-based DCM is used to analyze its decay channels for the experimentally studied entrance channels. The only parameter of the model, the neck length ΔR, varies smoothly with the excitation energy E* of CN and is used to best fit the ER data and predict qf and ff cross sections. Results: The hot-compact and "cold-elongated" fragmentation paths show dissimilar results, whose comparisons with measured fission yields result in t-p combinations, the cold reaction valleys. For the decay process, the fixed ΔR fit the measured ER cross section nicely, but not the individual decay-channel cross sections, which require the presence of qf effects, less so for asymmetric t-p combinations, and large (predicted) ff cross section. Conclusions: The calculated yields for hot-compact fragmentation path compared favorably with the observed asymmetric fission-mass distribution, resulting in mostly the same t-p reactions as are used in experiments. The best fitted ΔR are found to be independent of the entrance channels, despite their very different cross sections, in conformity with our earlier works. The entrance-channel effects lead to the largest decay cross section for the most asymmetric and doubly magic t-p combination, the magicity taking over asymmetry for both the total ER and channel cross section. Furthermore, the variation of both PCN and Psurv with E* fit in with the known systematic of other radioactive CN studied so far, thereby giving credence to our DCM analysis of 220Th*. [ABSTRACT FROM AUTHOR]