Systematics of the nuclear level densities

Neutron resonance data are applied to study properties of highly excited states in nuclei. A level density systematics of compound resonances for a large number of nuclei of different even-odd character is discussed. The interpretation of the rather complicated structure in the systematics is that there is a step-shaped base line for the level density parameter formed by nuclei with minimal effects from residual interaction and no shell effects. This base line is used to answer questions relevant not only to level densities but also to nuclear physics in general. The step-like behaviour of the base line for light and medium-light nuclei is interpreted as changes in the level density due to changes of the average number of particles and holes participating in the compound excitation. The proof that the compound states in light and neutron closed shell nuclei are doorway (2plh) states and therefore do not fulfil the Bohr assumption for a compound state may explain non-statistical effects observed in the capture process. Deviations of the level density parameter from the base line are interpreted as the effect of short range forces. A few examples are given and discussed for different mass regions. In the neighbourhood A approx. 75 the reduction of the pairing energy is assumed to be due to the blocking effect. In the mass range of the actinides the pairing energy has been readjusted at high excitation energy and at A approx. 105 it is shown that for neutron rich nuclei n-p interactions play the major role. Finally, the question is discussed whether or not there is a collective enhancement at higher excitation energy. 22 references