Time delay in nuclear reactions

The time delays involved under different circumstances in nuclear reactions, e.g., isolated or overlapping resonances, are analysed from the unified point of view of unitarity and of the statistical and analytical properties of theS-matrix. A general theorem is proved which says that the average over then open channels of the time delay of a wave packet covering many resonances (whose average separation isD) is given by?/(nD). The case of an incoherent superposition of monochromatic beams is also studied and the corresponding time delay is evaluated in the statistical model of Ericson.