부산대학교 도서관

Background: The formation and decay of the 220Th* compound nucleus (CN) formed via some entrance channels (16O+204Pb,40Ar+180Hf,48Ca+172Yb,82Se+138Ba) at near barrier energies has been studied within the dynamical cluster-decay model (DCM) [Hemdeep et al. Phys. Rev. C 95, 014609 (2017)], for quadrupole deformations (β2i) and "optimum" orientations (θopt) of the two nuclei or decay fragments lying in the same plane (coplanar nuclei, Φ=0∘). Purpose: We aim to investigate the role of higher-multipole deformations, the octupole (β3i) and hexadecupole (β4i), and "compact" orientations (θci) together with the noncoplanarity degree of freedom (Φc) in the noncompound nucleus (nCN) cross section, already observed in the above mentioned study with quadrupole deformations (β2i) alone, the Φ=0∘ case. Methods: The dynamical cluster-decay model (DCM), based on the quantum mechanical fragmentation theory (QMFT), is used to analyze the decay channel cross sections σxn for various experimentally studied entrance channels. The parameter Ra (equivalently, the neck length ΔR in Ra=R1+R2+ΔR), which fixes both the preformation and penetration paths, is used to best fit both unobserved (1n,2n) and observed (3n-5n) decay channel cross sections, keeping the root-mean-square (r.m.s) deviation to the minimum, which allows us to predict the nCN effects, if any, and fusion-fission (ff) cross sections in various reactions at different CN excitation energies E*. Results: For the decay of CN 220Th∗, the mass fragmentation potential V(Ai) and preformation yields P0(Ai) show an asymmetric fission mass distribution, in agreement with one observed in experiments, independent of adding or not adding (β3i,β4i), and irrespective of large changes (by 36° and 34°), respectively, in "compact" orientations θci and noncoplanarity Φc, and also in the potential energy surface V(Ai) in light mass (1n-5n) decays. Whereas the 3n- and 5n-decay channels fit nearly exactly, i.e., they are always the pure CN decays, the 4n-decay channel shows the presence of large (∼95%) nCN content whose magnitude in every case remains the same within <1% and hence does not get modified, in contrast to our earlier studies of other CN. Also, the near constancy of best fitted Ra(≡ΔR) with E∗, and with an upper limiting value for reactions with magic nuclei as reaction partner(s), independent of the entrance channel nuclei, allows us to predict the decay channel cross sections σxn,x=3-5 for 16O+204Pb reaction, whose sum (=∑53σxn) fits the observed σER data nicely. Also, the variations of CN fusion/formation probability PCN and survival probability Psurv follow the required systematic behavior, giving credence to our DCM analysis. Conclusions: With the inclusion of higher-multipole deformations and "compact" noncoplanarity degree of freedom (Φc≠0), the results of our above-mentioned earlier study, using quadrupole deformation (β2i) alone for coplanar (Φc=0) nuclei, remain the same; i.e., of the measured 3n-5n decay channels of CN 220Th∗, the 3n and 5n decays are always pure CN decays and the 4n decay is mainly of nCN content σnCN, whose magnitude also remains constant (within <1%) under all approximations. Furthermore, the upper limiting value of the linear dependence of first turning point Ra on E∗ is shown to be a better choice for predicting the decay channel cross sections σxn for reactions like 16O+204Pb using magic nuclei, whose experimental determination will be a good test of our model. [ABSTRACT FROM AUTHOR]