The ''island of inversion'' nucleus 32 Mg has been studied by a (t, p) two neutron transfer reaction in inverse kinematics at REX-ISOLDE. The shape coexistent excited 0 þ state in 32 Mg has been identified by the characteristic angular distribution of the protons of the ÁL ¼ 0 transfer. The excitation energy of 1058 keV is much lower than predicted by any theoretical model. The low-ray intensity observed for the decay of this 0 þ state indicates a lifetime of more than 10 ns. Deduced spectroscopic amplitudes are compared with occupation numbers from shell-model calculations. The evolution of shell structure in exotic nuclei as a function of the proton (Z) and neutron (N) number is currently at the center of many theoretical and experimental investigations [1,2]. It has been realized that the interaction of the last valence protons and neutrons, in particular, the monopole component of the residual interaction between those nucleons, can lead to significant shifts in the single-particle energies, leading to the disappearance of classic shell closures and the appearance of new shell gaps [3]. A prominent example is the collapse of the N ¼ 20 shell gap in the neutron-rich oxygen isotopes where instead a new magic shell gap appears for 24 O at N ¼ 16 [4,5]. Recent work showed that the disappearance of the N ¼ 20 shell can be attributed to the monopole effect of the tensor force [3,6,7]. The reduced strength of the attractive interaction between the proton d 5=2 and the neutron d 3=2 orbitals causes the d 3=2 orbital to rise in energy and come closer to the f 7=2 orbital. In regions without pronounced shell closures correlations between the valence nucleons may become as large as the spacing of the single-particle energies. This can thus lead to particle-hole excitations to higher-lying single-particle states enabling deformed configurations to be lowered in energy. This may result in low-lying collective excitations, the coexistence of different shapes at low energies or even the deformation of the ground state for nuclei with the conventional magic number N ¼ 20. Such an effect occurs in the ''island of inversion'', one of most studied regions of exotic nuclei in the nuclear chart. In this region of neutron-rich nuclei around the magic number N ¼ 20 strongly deformed ground states in Ne, Na, and Mg isotopes have been observed [8-11]. Because of the reduction of the N ¼ 20 shell gap, quadrupole correlations can enable low-lying deformed 2p-2h intruder states from the fp shell to compete with spherical normal neutron 0p-0h states of the sd shell. In this situation the promotion of a neutron pair across the N ¼ 20 gap can result in deformed intruder ground states. Consequentially, the competition of two configurations can lead to the coexistence of spherical and deformed 0 þ states in the neutron-rich 30;32 Mg nuclei [12]. Coulomb excitation experiments have shown that 30 Mg has a rather small BðE2Þ value for the 0 þ gs ! 2 þ 1 transition [13,14] placing this nucleus outside the island of inversion. The excited deform...
Rapid shape changes are observed for neutron-rich nuclei with A around 100. In particular, a sudden onset of ground-state deformation is observed in the Zr and Sr isotopic chains at N=60: low-lying states in N≤58 nuclei are nearly spherical, while those with N≥60 have a rotational character. Nuclear lifetimes as short as a few ps can be measured using fast-timing techniques with LaBr 3 (Ce)-scintillators, yielding a key ingredient in the systematic study of the shape evolution in this region. We used neutron-induced fission of 241 Pu and 235 U to study lifetimes of excited states in fission fragments in the A∼100 region with the EXILL-FATIMA array located at the PF1B cold neutron beam line at the Institut Laue-Langevin. In particular, we applied the generalized centroid difference method to deduce lifetimes of low-lying states for the nuclei 98 Zr (N=58), 100 Zr and 102 Zr (N≥60). The results are discussed in the context of the presumed phase transition in the Zr chain by comparing the experimental transition strengths with the theoretical calculations using the Interacting Boson Model and the Monte Carlo Shell Model.
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...
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