The existing experimental values of the ratios B 4/2=B(E2:41 + 21 ) / B(E2:21 + 01 ) and B6/4 = B(E2:61 + 41 ) / B(E2:41 + 21 ) in Ce are less than unity which is unexpected on traditional collective models and is highly anomalous . In non-magic nuclei there are only a handful of s uch cases, and Ce has more valence nucleons than most of them, so it would be among th e least likely to have B 4/2<1. Therefore, lifetime measurements on the yrast 2 , 4 and 6 states in Ce using a plunger device and the Recoil Distance D opplerShift (RDDS) method in coincidence mode are propose d. Excited levels of Ce will be populated via a Sn(O, 4n) reaction. With the new lifetime, and hence B (E2) measurements, the anomaly will either be resolved or a significant structural challenge pres ented.

Excited low-spin, nonyrast states in {sup 170,172,174}Hf were populated in {beta}{sup +}/{epsilon} decay and studied through off-beam {gamma}-ray spectroscopy. New coincidence data allowed for a substantial revision of the level schemes of {sup 170,172}Hf and a confirmation of the level scheme of {sup 174}Hf. The Hf isotopes represent a unique situation in which a crossing of collective intrinsic excitations occurs, enhancing significantly the effects of mixing. Using branching ratios from excited 2{sup +} states, this mixing is followed and studied. The resulting mixing matrix elements are found to be {approx}30 keV - an order of magnitude larger than estimated previously for nearby nuclei. In the case of {sup 170}Hf, the 2{sub {beta}}{sup +} and 2{sub {gamma}}{sup +} level are shown to be completely mixed.

Identification of near-yrast states in the beta-stable nucleus Zr-91(51) has been carried out using the fusion evaporation reactions Se-82(C-13,4n)Zr-91 at an incident beam energy of 50 MeV. States above the reported inverted perpendicular = 6 mu s, I-pi = 21/2(+) isomeric level at E-x = 3167 keV have been identified for the first time in this work, a tentative decay scheme of near-yrast states to excitation energies in excess of 10 MeV has been constructed. These states are of relevance to shell model structures formed via limited valence-space configurations in this semi-magic N = 51 nucleus.

S.F. Ashley, A. Linnemann, J. Jolie, P.H. Regan K. Andgrena,c, A. Dewald, E.A. McCutchan, B. Melon O. Möller, N.V. Zamfird,e, L. Amond,f , N. Boelaertb,g R.B. Cakirlid,f , R.F. Casten, R.M. Clark, C. Fransen W. Gelletly, G. Gürdald,i, M. Heidemann, K.L. Keyes M.N-. Erduran , D.A. Meyer, A. Papenberg, C. Plettner G. Rainovski, R.V. Ribas, N.J. Thomasa,c, J. Vinson D.D. Warner, V. Werner, E. Williams, K.O. Zell

Two separate experiments using the Differential Decay Curve Method have been performed to extract mean lifetimes of excited states in 106 Cd. The inedium-spin states of interest were populated by the Mo-98(C-12, 4n) Cd-106 reaction performed at the Wright Nuclear Structure Lab., Yale University. From this experiment, two isomeric state mean lifetimes have been deduced. The low-lying states were populated by the Mo-96(C-13, 3n)Cd-106 reaction performed at the Institut fur Kernphysik, Universitat zu Koln. The mean lifetime of the I-pi = 2(1)(+) state was deduced, tentatively, as 16.4(9) ps. This value differs from the previously accepted literature value from Coulomb excitation of 10.43(9) ps.

A new family of IBM Hamiltonians, characterized by certain parameter values, was found about 15 years ago by Alhassid and Whelan to display almost regular dynamics, and yet these solutions to the IBM do not belong to any of the known dynamical symmetry limits (vibrational, rotational and gamma - unstable). Rather, they comprise an "Arc of Regularity" cutting through the interior of the symmetry triangle from U(5) to SU(3) where suddenly there is a decrease in chaoticity and a significant increase in regularity. A few years ago, the first set of nuclei lying along this arc was discovered. The purpose of the present work is to search more broadly in the nuclear chart at all nuclei from Z = 40 - 100 for other examples of such "regular" nuclei. Using a unique signature for such nuclei involving energy differences of certain excited states, we have identified an additional set of 12 nuclei lying near or along the arc. Some of these nuclei are known to have low-lying intruder states and therefore care must be taken, however, in judging their structure. The regularity exhibited by nuclei near the arc presumably reflects the validity or partial validity of some new, as yet unknown, quantum number describing these systems and giving the regularity found for them.

The g factor of the 2(1)(+) state of Hf-170 was measured by perturbed gamma-gamma angular correlation in a static external magnetic field. The result, g(2(1)(+))=0.28(5), extends the systematics of g factors of even-even Hf isotopes to N = 98 and enables a better test of theoretical models. The g(2(1)(+)) experimental values of these isotopes exhibit a remarkable constancy as a function of neutron number. This phenomenon, which was also observed for other isotopic chains in the Gd-W range, is explained in terms of a recently proposed empirical model.

Recently, the first nuclei to lie along an arc of regularity in the collective symmetry triangle were identified. These nuclei are predicted to display significantly more ordered spectra than their neighbors. The nuclei identified all lie in the rare earth region from A=156-180. Here we present the results of a search of all known nuclei from A=90-208 and have identified an additional set of such candidates for regular nuclei using a particular degeneracy condition as a signature.

The mean-lifetimes, tau, of various medium-spin excited states in Pd-103 and Cd-106,Cd-107 have been deduced using the Recoil Distance Doppler Shift technique and the Differential Decay Curve Method. In Cd-106, the mean-lifetimes of the I-pi=12(+) state at E-x=5418 keV and the I-pi=11(-) state at E-x=4324 keV have been deduced as 11.4(17)ps and 8.2(7)ps, respectively. The associated beta(2) deformation within the axially-symmetric deformed rotor model for these states are 0.14(1) and 0.14(1), respectively. The beta(2) deformation of 0.14(1) for the I-pi=12(+) state in Cd-106 compares with a predicted beta(2) value from total Routhian surface (TRS) calculations of 0.17. In addition, the mean-lifetimes of the yrast I-pi = 15(-)/2 states in Pd-103 (at E-x=1262 keV) and Cd-107 (at E-x=1360 keV) have been deduced to be 31.2(44)ps and 31.4(17)ps, respectively, corresponding to beta(2) values of 0.16(1) and 0.12(1) assuming axial symmetry. Agreement with TRS calculations are good for Pd-103 but deviate for that predicted for Cd-107.

Preliminary lifetime values have been measured for a number of near-yrast states in the odd-A transitional nuclei Cd-107 and Pd-103. The reaction used to populate the nuclei of interest was Mo-98(C-12, 3nx alpha)Cd-107, Pd-103, with the beam delivered by the tandem accelerator of the Wright Nuclear Structure Laboratory at an incident beam energy of 60 MeV. Our experiment was aimed at the investigation of collective excitations built on the unnatural parity, v h(11/2) orbital, specifically by measuring the B(E2) values of decays from the excited levels built on this intrinsic structure, using the Doppler Recoil Distance Method. We report lifetimes and associated transition probabilities for decays from the 15/2(-) and the 19/2(-) states in Cd-107 and the first measurement of the 15/2- state in Pd-103. These results suggest that neither a simple rotational or vibrational interpretation is sufficient to explain the observed structures.

Near-yrast states have been identified in the stable Zr-91,Zr-92 isotopes using the fusion evaporation reaction Se-82(C-13,xn)Zr95-x at an incident beam energy of 50 MeV. Gamma-ray spectroscopy of states above the reported tau = 6 mu s, I-pi=(21)/(+)(2) isomer in Zr-91 are reported for the first time with consequences for stimulating new shell model calculations which incorporate either the breaking of the N=50 shell closure and/or the inclusion of the negative parity h(11/2) neutron intruder orbital.

Lifetimes of excited states in 128 Ce were measured using the recoil distance Doppler-shift (RDDS) and the Doppler-shift attenuation (DSAM) methods. The experiments were performed at the Wright Nuclear Structure Laboratory of Yale University. Excited states of 128 Ce were populated in the 100 Mo ( 32 Si ,4 n ) reaction at 120 MeV and the nuclear γ decay was measured with an array of eight Clover detectors positioned at forward and backward angles. The deduced yrast transition strengths together with the energies of the levels within the ground-state (gs) band of 128 Ce are in agreement with the predicted values for the X (5) critical point symmetry. Thus, we suggest 128 Ce as a benchmark X (5) nucleus in the mass A ≈ 130 region.

1 Department of Physics, Royal Institute of Technology, S-104 05 Stockholm, Sweden 2 Department of Physics, University of Surrey, Guildford GU2 7XH, UK 3 WNSL, Yale University, New Haven, CT 06520, USA 4 Department of Physics, Istanbul University, Istanbul, Turkey 5 Lawrence Berkeley National Lab, Berkeley, CA 94720, USA 6 Clark University, Worcester, MA, 01610-1477, USA 7 Institute of Physical Research, University of Paisley, Paisley PA1 2BE, UK 8 Department of Physics and Astronomy, SUNY, Stony Brook, NY 11794, USA 9 Instituto de Fı́sica, Uni. de Sao Paulo, SP, Caixa Postal 05315-970, Brazil 10 CCLRC, Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK

Lifetimes for decays linking near-yrast states in 107Cd have been measured using the recoil distance method (RDM). The nucleus of interest was populated via the 98Mo(12C,3n)107Cd fusion–evaporation reaction at an incident beam energy of 60 MeV. From the measured lifetimes, transition probabilities have been deduced and compared with the theoretical B(E2) values for limiting cases of harmonic vibrational and axially deformed rotational systems. Our initial results suggest a rotor-like behaviour for the structure based on the unnatural-parity, h11/2 orbital in 107Cd, providing further evidence for the role of this 'shape-polarizing' orbital in stabilizing the nuclear deformation in the A ~ 100 transitional region.

The aim was to search for reflection asymmetric high-K states in Pu-240 by measuring coincidences across the T-1/2 = 165 ns, K-pi = 5(-) two-quasiproton isomer. The measurements were undertaken with the AFRODITE gamma-ray spectrometer when a pulsed beam of 30MeV alpha particles was directed onto a U-238 target. No evidence was found for the known gamma rays that decay from the isomer into the ground state band.