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We discuss a possible mechanism, namely the maximization of the overlap of nucleonic wave functions by alignment, for the formation of isomers in the yrast band in the rare nuclei at very high spin states 0 Our results indicate that the possibility of existence of such yrast traps is restricted to a few nuclei with special irregularities in the singleparticle level s e With the advent of heavy-ion accelerators it has been made possible to transfer in a fusion experiment angular momenta as large 1 as ~ 60/z into compound nuclei. This makes feasible the experimental studies of very high-spin states of the nuclei. The information from these experiments, 1,2 however, is restricted to measurements of the continuum y rays 1 and y-ray multiplicities, 2 ' 3 since it is impossible to resolve single y lines above spin ~ 22/L The experimental situation would, therefore, be greatly improved, if an isomeric state at a very high angular momentum (yrast trap) could be found as discussed first by Bohr and Mottelson. 4 The purpose of this note is to suggest more in detail a mechanism for the formation of such yrast traps.The calculations of the deformation energy surfaces performed in the framework of the liquiddrop model 5 using a rigid-body moment of inertia in the rotational energy indicate that up to spin ~80# (rare-earth region) the nucleus is oblate (y = -60°) with the symmetry axis coinciding with the rotation axis. Calculations performed with the inclusion of the shell correction 6,7 and microscopic calculations, 8 however, do not yield such shapes for many nuclei. Even so in this note we want to restrict the quantitative considerations to rotation around the symmetry axis preferred by the liquid-drop model. In such a situation a collective rotation is forbidden out of general quantum-mechanical arguments. Any further increase of the total spin can be obtained only from the individual nucleonic excitations. Once such a stage is reached any irregularities on the yrast line may lead to isomeric states/ A possible mechanism for producing isomeric states is the maximization of overlap of nucleonic wave functions by alignment (MONA) of the single-particle angular momenta outside closed shells. This effect is clearly demonstrated for the 18 + state 9,10 of 212 Po 128 at 2.93 MeV with a half-life of 45 sec. This state is formed by aligning the (7r/z 9^) 2 (vz n/^) 2 configuration outside the doubly closed 208 Pb. Because of the short-range nature of the nucleon-nucleon interaction we have the largest overlap of the nucleonic wave functions for angular momentum J = 0. For intermediate angular momenta the overlap is reduced and increased again for the fully aligned configuration. Moreover, Schiffer 11 and Molinari et al. 12 found that a suitable normalized effective residual interaction depends only on the angle between the angular momenta: The residual interaction is almost symmetric around 90° and is strongly attractive for 0° and 180°. This property of the effective interaction lowers the J= 18 + state in 212 Po below the 16 ...

A time-dependent mean-field theory is developed for fast nucleon emission in nuclear heavy-ion reactions. The essence of the model is to treat the relative motion of the ions classically while treating the internal excitations quantum mechanically. General properties of the model are calculated by assuming a phenornenological external field. The model is applied to study fast neutron emission from the reaction ' 0+ 'Nb.

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