I have read that the mechanism behind internal conversion, in which a nuclear transition leads to the ejection of an electron in one of the lower atomic orbitals, is related to the fact that the wavefunctions of some of these electrons penetrate the nucleus. Because of this, there is a finite probability that the electron will be in the nucleus, and, when this occurs, the electron may couple to the
excited state of the nucleus nuclear transition when it happens.
Is it conjecture that the electron must penetrate the nucleus in order for internal conversion to occur, or is this a straightforward result? I would have figured that since internal conversion is mediated by the electromagnetic force (which has long reach), for any electron within range of a virtual photon there will be a nonzero cross section for this type of decay. What details am I missing in thinking this?