Giant Quadrupole Resonances of 20Ne Based on Cluster Model

Abstract

571Giant quadrupole resonances (GQR's) of 2°Ne are investigated by the (ISO_a) +(lSO'(lp-lh)-a) cluster model which well explains many properties of low-lying states in 2°Ne. It is shown that highly excited states (Ex ~ 16 MeV) with a developed ISO' -a structure carry large fragment of the E2-transition strength to the ground state. The calculated E2-strength of the lSO*_a cluster states is about 14% of the energy weighted sum rule (760e 2 fm 4 MeV) and well describes that for the lower-energy (Ex~16 MeV) GQR component in the 2°Ne (e, e') experiment. § 1. Introduction Giant resonances are known as a typical collective vibrational mode of nuclei. In medium to heavy nuclei, isoscalar giant quadrupole resonances (GQR's) have been observed in a rather narrow excitation energy region at about Ex=63A-1/3 MeV and exhaust 50~100% of the energy-weighted sum rule (EWSR)_I) These observations are interpreted from the standard liquid drop picture of the nuclei. In light nuclei (A < 40), however, considerably broader GQR's have been observed 2 ) and in most of the light nuclei, 30~70% of the EWSR is yet undetected up to the GQR energy region. 3 ) Moreover, splitting of the GQR states (one near 63A-1/3 MeV and one at a lower energy)4),S),9) and large a-decay widths from the GQR states 5 )-7) are observed in some nuclei. These facts imply that the standard picture based on the vibration of a liquid drop is not so appropriate to describe GQR phenomena in light nuclei and that all degrees of freedom which carry E2-collectivity do not come out in the GQR energy region. Thus, it is necessary to try another approach which can naturally treat the a-decay width as well as E2-collectivity. Furthermore, since the E2-strengths of the low-lying states are fairly large, it should be necessary to investigate the GQR phenomena in light nuclei on the basis of appropriate description of the structure of low-lying states.Cluster models succeed in explaining many properties, for example, large a-decay widths and show the importance of the cluster correlation at least in the low excitation energy region (E x <10 MeV) of the light nuclei.IO),ll) The 2°Ne nucleus has been intensively studied by cluster models. 12 )-16) The standard 160_a cluster model l2 ) well reproduces the observed aldecay widths, but underestimates E2-transition strengths in the ground band. For the purpose of overcoming the defects, the coupling between 160_a clustering and quadrupole-type core polarization was discussed by Banda. 15 ) The core polarization which is induced by operating the quadrupole operator for each cluster in the 160_a model corresponds essentially to a modelled GQR state which is given by operating the quadrupole operator for the ground state of 2°N e.Recently, an (I 6 0-a ) + (I 6 0*-a) cluster modeF 6 ) was proposed for explaining low-lying states of 2°Ne. As the results, many structures are obtained as shown in the previous paper;16) 1) the well-developed 160_a cluster structure of the 0-(5.78 MeV) and 04+(8.3 MeV) bands, 2) the transient character of ...