Can strong electric dipole transitions in deformed rare earth nuclei be explained by the macroscopic behavior of the octupole phonon?

Abstract

Abstract. Strong E1 transitions of greater than 10 -3 Weisskopf units occur in many octupole states in the deformed rare earth region. It is shown using the droplet model that the electric dipole moment resulting from the macroscopic behaviour of the octupole phonon cannot by itself account for the observed E 1 strengths, and it is observed tha~this result is consistent with the proposal of Donner and Greiner and of Zilges yon Brentano and Richter that admixtures of the giant dipole resonance into the low energy octupole states are responsible for the fast E 1 transitions. It is also suggested that calculations similar to those performed by Egido and Robledo for N < 92 nuclei may be able to reproduce E 1 transitions in N = 94-104 nuclei. PACS: 21.60.Ev; 23.20.Lv During the past decade a significant amount of attention was focused on nuclei possessing stable reflection asymmetric shapes. One of the interesting aspects of such nuclei is the presence of strong electric dipole transitions, occassionally as large as 10 .2 Weisskopf units (W.u.). However, it has been demonstrated that strong E1 transitions can occur in nuclei which do not have stable reflection asymmetric shapes. For example, strong E1 transitions associated with octupole vibrations occur in the spherical nuclei 96Zr [1] and 1448m [2]. Strong E1 transitions (> 10-3W.u.) associated with octupole vibrations have also been observed in even-even rare earth nuclei in Coulomb excitation experiments (for example see [3]) or by using the technique of nuclear resonance fluorescence (see [4] and references therein).The discussions of the E1 strengths associated with octupole vibrations in the rare earth nuclei have centered on two possible origins. The first possibility is that Supported by the National Science Foundation and the State of Florida the strong E1 transitions arise from the macroscopic behavior of the ocutpole phonon. This mechanism was first suggested more than thirty years ago [5,6] and has more recently been discussed by Iachello [7]. The second hypothesis is that small admixtures of the giant dipole resonance (GDR) are responsible for the strong E1 transitions. That is, the J= 1 state of a K--0 or 1 octupole band would have a wavefunction of the formwhere fl ~ e. States of higher angular momentum would be obtained by applying the appropriate operators to (1). The mixing of the giant dipole resonance into low energy octupole states was a feature of the model of Donner and Greiner [8]. More recently, Zilges et al. [9] have used this argument to explain the results of their measurements of 1-states in even-even rare earth nuclei. The measurements reported in [9] imply that f12, as defined in (1), is near 5 • 10 -4 for the deformed rare earth nuclei. In this article, the hypothesis that the macroscopic behavior of the octupole phonon is responsible for the strong E1 transitions is tested by using the droplet model prescription of Dorso et al. [10] to make realistic estimates of the electric dipole moment caused by an axial octupole shape, and...

Origin of Low-Lying EnhancedE1Strength in Rare-Earth Nuclei

“…Indeed, several enhanced E1 transitions were observed in 142−150 Nd [36][37][38], 148−154 Sm [39], 156−160 Gd [40,41], 162 Dy [37], and other rare-earth nuclei [42,43] tempts to qualitatively describe the experimentally observed peculiar increase of the E1 strength connected with the proposed quadrupole-octupole coupled (QOC) 1 − 1 state [4] towards shell closures [43] in a microscopic manner were presented in Refs. [44][45][46][47][48][49][50]. There, it was proposed that one-particle-one-hole (1p−1h) admixtures, originating from the tail of the electric giant dipole resonance (GDR), into the QOC state could explain the enhanced E1 strength and that destructive interference of both components causes the decrease when moving away from the shell closure.…”

“…1 state [4] towards shell closures [43] in a microscopic manner were presented in Refs. [44][45][46][47][48][49][50]. There, it was proposed that one-particle-one-hole (1p−1h) admixtures, originating from the tail of the electric giant dipole resonance (GDR), into the QOC state could explain the enhanced E1 strength and that destructive interference of both components causes the decrease when moving away from the shell closure.…”