Angular distributions in reactions with fast fission

In the nuclear reactions 50Ti, 56Fe + 208Pb the angular distributions of reaction products were measured. The detection pursued via the K X-ray quanta of beta-unstable product nuclei emitted after electron capture and internal conversion. The differential distributions change their shape in dependence on the nuclear charge number Z. They are generally not symmetric around THETA=900; an exception are only the fragments with charges Zapprox.=Zsub(sym)=Zsub(total)/2. Thus the distributions are not compatible with the production of the fragments by fission of a long-lived compound nucleus equilibrated in all degrees of freedom; rather they are the result of a new reaction type ('fact fission'), which equalizes the mass asymmetry of projectile and target in the time framework of the rotational period. A method is described which permits to determine the variance of the tilting angular momentum K02 for the first time also from asymmetric angular distributions. K02 is then no longer the variance of a distribution in the statistical equilibrium but characterizes the excitation of the tilting mode dynamically excited during the reaction. The results suggest that the tilting mode in the fast fission is only partly excited. The results are compared with the sum of the variances of all statistical rotational modes of a double-nucleus which are accessible via the measurement of γ multiplicities. Furthermore by means of a folding method from the angular distributions for each Z the mean rotational angle was determined by which the reaction complex has been rotated between formation and decay. From these rotational angles a time scale for the dynamical development of K02 can be derived. The times lie in the range from 6x10-21 s to 13x10-21 s and are therefore of the same order of magnitude as the relaxation time for the mass drift. (orig./HSI)