WIDTH AND STRENGTH OF THE HOT GIANT-DIPOLE RESONANCE - THE ROLE OF THE LIFE TIME OF THE COMPOUND NUCLEUS AND THE TRANSITION FROM ORDER TO CHAOS

In this article we investigate the role of the particle evaporation on the properties of the Giant Dipole Resonance built on excited states. We discuss two phenomena. The first one is based on the idea that the finite life time of the compound nucleus gives a finite width for any of the compound nucleus levels. Consequently, the gamma-ray spectrum coming from the transitions between such two levels must exhibit a broadening characterized by a width equal to twice the compound nucleus width. This leads to the prediction of a fast increase of the width of the observed GDR transitions which is compatible with the data and may partly explain the observed saturation of the photon yield. The second new physical effect is based on the fact that each individual particle emission induces a strong fluctuation of the dipole moment of the nucleus. Therefore, the description of the dipole collective variable is akin to the problem of a brownian motion in a harmonic potential. One can conclude that when the time between two particle emissions becomes comparable to the period of the harmonic oscillation, the transition between order and chaos is reached and the collective vibration is suppressed. From this simple argument a quenching factor can be deduced and compared with existing suppression factors.