Abstract. In the present note it is shown that distinguishing between pre-saddle and saddle-to-scission neutrons removes a discrepancy previously found [2][3][4] between data for the fission fragment anisotropy and an analysis in the framework of the transition state theory in which it has been assumed that all pre-scission neutrons influence the temperature at the saddle point. The analysis of the present paper requires a fairly weak (fl = 2 * 1021s -1) friction at compact shapes, in agreement with a previous [9,10] analysis of prescission neutron multiplicities and fission probabilities.Large efforts have been made in the last years to obtain information on the nuclear viscosity from studying heavy-ion induced fission, for a recent review see Hilscher and Rossner [1].In the present note we study the angular distribution of fission fragments in this connection. In refs.[2-4] data for fission fragment anisotropies have been analysed in the framework of transition state theory. In particular, the anisotropy for the emission of fission fragments can be approximately expressedwhere < 12 > is the mean square spin of the compound nucleus, and Ko 2 is the variance of the K-distribution at the transition state [5], where K is the projection of the compound nucleus spin I on the symmetry axis of the fissioning nucleus. The quantity Ko 2 is given by:here E*,a/,J~]/ and Ts are the excitation energy, tim level density parameter, the effective moment of inertia, and the temperature at the transition state which is usually assumed to be the saddle point. The average excitation energy at the saddle has been calculated according towhere Q is the formation Q-value, B/(< I S >) the fission barrier at < 12 >, Erot the corresponding rotational energy, and E~ is the energy removed by the pre-scission neutrons consisting of the binding energy B~ and the average kinetic energy < en >. In refs [2-4] it had been assumed that all neutrons are emitted prior to the saddle point. This improved the agreement between the theoretical description and the data at lowAlso Fachbereich Physik at the Freie Universit/it Berlin energies considerably as compared to the analysis with the conventional description in which the cooling by neutron emission is not taken into account. However, there remained a discrepancy with the data at high energies where the conventional theory worked well; for an example see Fig.1. The main purpose of the present note is to show that this discrepancy can be removed to a large extent by taking into account the fact that not all neutrons are emitted before the saddle but that with increasing energy more and more neutrons are emitted on the way from the saddle to the scission point. These latter neutrons do no longer influence the fission angular distribution since it is assumed that fission is decided at the saddle.In a series of papers [6-11] a dynamical theory for fission of hot nuclei has been developed on the basis of Langevin trajectories, along which the emission of light particles and 7-quanta is allowed by a Monte ...
