Introduction: Spallation Reactions and their Occurrences

Abstract

This volume of collected articles deals with the type of nuclear reactions commonly called “spallation reactions” and with their occurrence in astrophysics, geophysics, radiotherapy, and radiobiology. There is no generally accepted definition of the term “spallation reaction” usage differs somewhat from discipline to discipline. Astrophysicists, who have little interest in the mechanism of the reaction but great interest in its outcome, tend to use the term with more abandon than the nuclear chemists, who over the years have done most of the systematic research on the process at high-energy accelerators. Cosmicray physicists, who have dealt with spallation reactions for decades, continue to refer to such reactions suffered by cosmic rays as “fragmentations”, to the dismay of nuclear chemists who reserve that term for a very specific and poorly understood aspect of spallation. But this lack of uniformity in terminology need not worry us here. For all practical purposes, spallation reactions are inelastic nuclear reactions in which at least one of the two collision partners is a complex nucleus and in which the energy available well exceeds the interaction energy between nucleons in the nucleus. Thus, a nucleon-nucleus or pionnucleus or nucleus-nucleus reaction in which the incident energy exceeds something like 50 or 100 MeV per a.m.u. is generally referred to as a spallation reaction. The term comes from the verb “to spall”, meaning to chip with a hammer, but actual usage has never required that there be wholesale chipping away of nucleons before a reaction can be called a spallation.