Spectator isobars in deuteron collisions

Estimates are made for the production of spectator isobars in collisions with deuterons. The crucial question, if this is to be used to deduce the isobar content of the deuteron, is whether one can distinguish an isobar "created" in the collision from one which "preexisted" in the deuteron and was shaken loose as a spectator. We examine an important source of such created isobars and show how these are distributed differently from the spectator isobars. The argument is illustrated by considering the explicit example of ${\Delta }^{-}$ production in $d p$ collisions.