||Probing cluster structure of sd-shell nuclei using isoscalar monopole and dipole transitions
Background Cluster structure is well established in p-shell nuclei but clustering in midsd-shell nuclei and that of heavier nuclei is still not established due to experi-mental and theoretical difficulties. Recently, isoscalar monopole transition was proposed as a probe for Jπ= 0+ cluster states and experimental difficulties are being overcame. Furthermore, the observed isoscalar dipole transition distri-butions of self-conjugate 4n nuclei show that many 1−states exist near the 0+ states observed in isoscalar monopole transition distributions but their nature has been unclear. Purpose I prove that the cluster states are strongly excited by isoscalar monopole and dipole transitions and provide reliable predictions of isoscalar monopole and dipole transitions of 24Mg and 28Si to clarify the nature of the 0+ and 1−states observed in the isoscalar monopole and dipole transition distributions. Method I derive analytical expression of isoscalar dipole transition matrix and estimate the magnitudes of isoscalar monopole and dipole transition metrics for 20Ne using simple cluster model. I perform realistic calculations by anti- symmetrized molecular dynamics for 24Mg and 28Si and numerically evaluate isoscalar monopole and dipole strengths and analyze the cluster structure of the excited states. Results It is shown that the isoscalar dipole transition matrix between the ground state and the excited 1−cluster states is comparable to the single particle strength and that its magnitude is naturally enhanced for parity-asymmetric cluster configuration and vanished for symmetric cluster configuration. Realistic calculations for 24Mg and 28Si show isoscalar monopole and dipole strengths to the cluster states are considerably large and comparable to the single particle strengths. Conclusion I conclude that the cluster states with asymmetric configuration is strongly excited by isoscalar dipole transitions. Combined with isoscalar monopole tran- sitions, the isoscalar monopole and dipole transitions are promising probes for asymmetric clusters.
Hokkaido University（北海道大学）. 博士(理学)