Experimental nuclear moments of inertia at high spins along the yrast line have been determined systematically and found to differ from the rigid-body values. The difference is attributed to shell effects and these have been calculated microscopically. The data and quantal calculations are interpreted by means of the semiclassical Periodic Orbit Theory. From this new perspective, features in the moments of inertia as a function of neutron number and spin, as well as their relation to the shell energies can be understood. Gross shell effects persist up to the highest angular momenta observed.

We h a ve searched all even{even nuclei with Z 20, N 20 to nd examples which display c haracteristics of X(5) critical point b e h a vior. On the basis of the yrast state energies and yrast intraband transition strengths, the best candidates are 126 Ba, 130 Ce, and the previously suggested examples of the N=90 isotones of Nd, Sm, Gd, and Dy.PACS numbers: 21.60.-n 21.10.-k Typeset using REVT E X 1 Notable benchmarks of collective n uclear behavior are the harmonic vibrator 1], the symmetrically deformed rotor 2], and the triaxially soft rotor 3]. While nuclei may display behavior near these idealized limits, many lie in transitional regions between them. Recently, it has been suggested that a useful approach is to apply the ideas of a phase transition of the nuclear shape and to try to de ne critical points of the shape change as new benchmarks against which n uclear properties can be compared 4,5]. In particular, the transition from a spherical harmonic vibrator to an axially deformed rotor has been described analytically 5] by i n troducing a dynamic symmetry, denoted as X(5), which arises when the potential in the Bohr Hamiltonian 2] is decoupled into two components { an in nite square well potential for the quadrupole deformation parameter, , and a harmonic potential well for the triaxiality deformation parameter, .Several empirical examples of nuclei that may be close to an X(5) critical point h a ve been suggested. These include 150 Nd (Z=60, N=90) 6], 152 Sm (Z=62, N=90) 7], and 104 Mo (Z=42, N=62) 8]. For the N=90 isotones a recent paper 9] has shown that the properties of these nuclei, especially the intersequence transition strengths, are not reproduced by the X(5) description. An alternative picture involving simple couplings between rotational bands (and microscopically justi ed by the Pairing{Plus{Quadrupole model 10]) fares better. In the case of 104 Mo, the reduced transition strengths, derived from recent lifetime measurements of states in the yrast sequence 11,12], were used to demonstrate that this nucleus does not display X ( 5 ) b e h a vior 12].If the X(5) description is to be taken as a benchmark for describing shape transitional behavior, then it is important t o n d n uclei which follow the predicted behavior more closely than the examples discussed above. Motivated by such considerations we h a ve searched all known even{even nuclei with Z 20, N 20.The experimental signatures for X(5) behavior are: a) the ratios of the energies of the yrast states, E(I spacings than the yrast sequence e) the B(E2 I!I-2) values for intrasequence transitions should be lower for the non{yrast sequence relative to those of the yrast sequence (these latter two p o i n ts re ect the fact that the non{yrast states have a l o wer expectation value of deformation than the states in the yrast sequence) f) intersequence B(E2) values should show a c haracteristic pattern. We shall use all of the above points in our search for nuclei displaying behavior similar to the X(5) predictions.As a rst step we used...

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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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