Observation of Octupole Structures in Radon and Radium Isotopes and Their Contrasting Behavior at High Spin

Abstract

Multinucleon transfer reactions have been used, for the first time, to populate high-spin bands of alternating parity states in 218,220,222 Rn and 222,224,226 Ra. The behavior of the angular momentum alignment with rotational frequency for the Rn isotopes is very different when compared with Ra and Th isotopes with N ഠ 134, indicating a transition from octupole vibrational to stable octupole deformation. Throughout the measured spin range the values of jD 0 ͞Q 0 j remain constant for 222 Ra and 226 Ra and have a very small value for 224 Ra, suggesting that the charge and mass distributions are not affected appreciably by rotations. [S0031-9007(97)02928-1] PACS numbers: 21.10. Re, 23.20.Lv, 25.70.Gh, 27.90. + b Of all nuclear species, radium (Z 88) and thorium (Z 90) isotopes with N ഠ 134 show the best evidence for octupole instability in their ground state [1-3]. These nuclei have low-lying negative-parity states and relatively strong B͑E1͒ values for the transitions between the bands of opposite parity; for the single case of 226 Ra large B͑E3͒ values have been measured consistent with its interpretation as a rotating pear shape [4]. The inaccessibility of these nuclei has, however, meant that there are large gaps in our knowledge of octupole effects in heavy nuclei. Comprehensive measurements of the high-spin behavior of the yrast octupole band exist only for the isotopes of thorium. For the radium isotopes such measurements are available for the weakly quadrupole coupled 218,220 Ra and the strongly coupled 226 Ra. There is only a limited amount of data on 224 Ra and virtually no information exists for 222 Ra. The scarce data do, however, suggest cancellation effects for the electric dipole moment for 224 Ra [5] which do not occur in the thorium isotopes. This effect is not properly established as the spin-dependent behavior for 222 Ra has not yet been measured. There are almost no data on the octupole structures for the radon isotopes. Systematic measurement of the variation of angular momentum with rotational frequency of the octupole bands should provide an insight into the nature of the strength of the octupole interactions in these nuclei.In order to populate the nuclei of interest the properties of multinucleon transfer reactions have been exploited. Previously, yields have been mapped out following the bombardment of a thick 232 Th target with various projectiles [6]. As the reaction 136 Xe 1 232 Th offered the largest yield for radon and radium isotopes with N ഠ 134, this reaction was chosen in order to make spectroscopic measurements of the heavy products.High-spin states in 218,220,222 Rn and 222,224,226 Ra were simultaneously populated following multinucleon transfer between 136 Xe and 232 Th. The 136 Xe projectile was accelerated to an energy of 833 MeV by the 88 in. cyclotron at Lawrence Berkeley National Laboratory. This bombarded a 232 Th target of thickness 36 mg͞cm 2 . Deexcitation gamma rays emitted from reaction products were collected for 49 h with the Gammasphere spectrometer which cons...

Nuclear Schiff moment and soft vibrational modes

“…Here we assume that the collective modes with spin-parity characteristics 2 + and 3 − have low excitation energies ω 2 and ω 3 , well below the gap energy 2∆ that separates the ground state 0 + from the threshold of two-quasiparticle states. This situation, for example, can be found in the chain of even-even xenon isotopes [24] and, most clearly, in light isotopes of radon and radium [22], where we see soft quadrupole and octupole collective modes evidently correlated in their energetics. As the energy spectrum does not display a rotational pattern, we can assume that these nuclei are still spherical.…”

“…As mentioned earlier, the dipole transitions between the states of the quadrupole and octupole bands are empirically known to be enhanced in nuclei of our interest, such as light radium and radon isotopes [22]. The collective contribution to this operator can be written in terms of our phonon variables as…”

“Beat” patterns for the odd-even staggering in octupole bands from a quadrupole-octupole Hamiltonian

“…In particular it will be shown that the model formalism developed in the present work proposes a schematic explanation of the recently observed [5] "beat" patterns for the odd-even staggering in octupole bands of light actinides based on recent experimental data [6,7].…”

“…Furthermore, some "beat" patterns have been observed recently for the odd-even staggering (the relative displacement of the odd levels with respect to the positions at which they should have been located according to a fit of the even levels by the formula E(I) = AI(I + 1), where I denotes the angular momentum) in octupole bands of light actinides [5] based on recent experimental data [6,7], calling for a study of the interactions which could give rise to such shapes.…”

Studies of the electric dipole transition of deformed rare-earth nuclei

“…There have been intensive interests in the studies of octupole degree of freedom in nuclear structure recently both experimentally [1][2][3][4][5][6][7][8][9] and theoretically [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27].…”