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PHYSICAL REVIEW LETTERS 28 MAY 1979 ed for in the thermal and cascade models.We would like to thank Y. Yariv and Z. Fraenkel for making the computer code ISABEL available to us, and L. Oliveirafor computer assistance. We thank J. O. Rasmussen, M. Gyulassy, J. I. Kapusta, and P. J. Siemens for helpful discussions.

We have identified yrast states of 152 Dy to spin ~37, with unusually strong population of high-spin states in the ( 32 S,4wy) reaction. Yrast isomers with spins ~17, 21, 27, and 31 were detected. For J ^ 14 the yrast states scatter about a straight line on an E vs IQ +1) plot, consistent with predictions for aligned-p article configurations. The slope of the line yields an effective moment of inertia 18% larger than the rigid-sphere value. This and other arguments suggest possible oblate deformation at high spin.Investigations of high-spin nuclear states, / > 3Qfr, have so far been limited to studies of continuum y radiation. 1 ' 2 Such experiments by their nature provide information only on gross properties of nuclear states over a wide range of excitation energy above the yrast line. Data on the yrast line itself to very high spin will open up a domain of nuclear structure which has hitherto not been explored. In particular one might learn whether there is a general relation between the energies and spins of yrast states, as has been predicted for an independent-particle system, 3 and how shell effects modify this relation; whether dramatic shape changes are induced in nuclei by large angular moments; and how spin is generated with the least expenditure of energy.

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