CASCADE NUCLEOSYNTHESIS IN THE EARLY UNIVERSE

We describe a calculation of electromagnetic cascading in radiation an d matter in the early universe initiated by the decay of massive parti cles or by some other process. We have used a combination of Monte Car lo and numerical techniques which enables us to use exact cross sectio ns, where known, for all the relevant processes. In cascades initiated after the epoch of big bang nucleosynthesis gamma-rays in the cascade s will photodisintegrate He-4, producing He-3 and deuterium. Using the observed He-3 and deuterium abundances we are able to place constrain ts on the cascade energy deposition as a function of cosmic time. In t he case of the decay of massive primordial particles, we place limits on their density as a function of their mean decay time.