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The superdeformed band in 149 Gd has been populated with various input angular momenta and excitation energies. These, together with all other available data, indicate that the necessary criterion for populating superdeformed bands is to form cold residual nuclei at spins higher than those where the superdeformed states become yrast. In addition, the decrease in the superdeformed band intensity as it is gated by higher-energy y rays does not support the predictions based on a recently proposed feeding mechanism of these states.PACS numbers: 21.10. Re, 25.70.Gh, 27.60,+j The recent discovery 1 ' 2 of superdeformed (SD) shapes at very high spin offers exciting possibilities for the study of nuclear structure at the extreme limits of rotational stress and large shape changes, and in the absence of pairing correlations. A major challenge to our understanding of nuclear behavior at very high spin is to comprehend the conditions which would favor the population of SD states. The first known SD band, in 152 Dy, was populated much more weakly in ( 40 Ar,4«) than in ( 48 Ca,4/z) reactions. 3 Although this result could have been interpreted to indicate a dependence on the input angular momentum and the temperature of the residual nucleus, it could not preclude an explanation based on the use of different projectiles. The fact that the SD band in 149 Gd was formed 2 in the reaction ( 30 Si,5rt) refuted the possibility that such bands could be populated only by reactions of neutron-rich and moderately heavy projectiles. In the present work, we have tested the spin and temperature dependence directly by measuring the excitation function of the SD states in 149 Gd and by comparing reactions with very similar target-projectile combinations.A related issue is the feeding mechanism of these SD bands. In both 152 Dy and 149 Gd, the highest discrete spin state (/--60/i) is populated rather strongly (~~0.2% of the residual nucleus cross section). Such a high intensity exceeds what would be expected from a simple extrapolation in "normally" deformed nuclei by an order of magnitude. Herskind et al. 4 have proposed an ingenious mechanism whereby the large splitting of the giant dipole resonance (GDR) built on excited states in the SD minimum, together with the low level density expected in this minimum, will give rise to statistical E 1 transitions that would cool the SD state an order of magnitude faster than the normal nuclei. A consequence of this mechanism is the prediction 4 that the E 1 continuum spectra associated with the normal and SD states are very different. Such spectra have been measured for the first time in our work and are reported in this Letter together with the excitation function.The y-ray yields of the SD band in 149 Gd as well as of the 4AZ, 5/I, and 6n channels were measured with the 8/r spectrometer 5 for the reaction 124 Sn+ 30 Si at bombarding energies of 140, 145, 150, 155, and 160 MeV. The target consisted of two stacked 0.4-mg-cm -2 Sn foils (enriched to 96.4% 124 Sn) and the beam was provided by the M...

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