ABSTRACT
We arrive now at a new turning point in our account of statistical decay. We shall henceforth be dealing with decay mechanisms which are more complex than the particle evaporation mechanism considered in the preceding chapter. The evolution of the decay mechanism is associated with increasing compound nucleus excitation energy and angular momentum. These ‘modern’ studies have been spurred on by the increasing availability of higher and higher energy heavy ion beams. Going beyond typical beam energies for single stage cyclotrons and linear accelerators (10-20 MeV/nucleon) has been possible using two coupled cyclotrons or single superconducting devices (typically 15 times lighter than equivalent conventional magnets). The development of high performance ion sources, notably the electron cyclotron resonance (ECR) source by Geller and collaborators [1], has also been a major contributing factor. The principal advantage of this device is that the energy input to the electron-ion plasma is supplied in the form of microwaves. The Lamour precession frequency of the electrons is in resonance with the microwaves at certain (magnetic) regions inside the source. The source gas is thus ionized by energetic electrons leading to formation of a magnetically confined plasma. Fragile and rapidly burned cathodes and heating filaments characteristic of older designs are thus absent resulting in increased longevity and robustness. The ECR ion source facilitates the attainment of high
energies because it produces highly charged ions which may be extracted and injected into an accelerator. Current beam energies used in studies of the decay of ‘hot’ nuclei range up to hundreds of MeV per nucleon. However, the range of energy with which we will be concerned is limited to below 500 MeV/nucleon and, for most purposes, below 100 MeV/nucleon.
