NON-RADIATIVE MUON TRANSITIONS IN HEAVY ELEMENTS.

We have recently investigated a new type of process which takes place in heavy muonic atoms where it competes with the ordinary electromagnetic (radiative and Auger) transitions which make up most of the mu-atomic cascade process. The new process consists in the radiationless de-excitation of a muonic atom accompanied by an excitation of the nucleus. In many of the cases observed, but not always, the excitation energy is high enough so that neutron emission can take place from the excited nucleus. We still lack a complete understanding of this process, and I will, therefore, concern myself primarily with a presentation of the experimental facts. Before starting, I should mention, however, that one particular type of radiationless transition, namely that leading to fission, was predicted by Wheelerl) as early as 1949. This process was later studied experimentally as well as theoretically mainly by Russian groups, and is rather well understood today2).