Matrix Elements for the Nuclear Photoeffect

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Abstract
A phenomenological formulation of the interaction of nuclear systems with the transverse electromagnetic field is developed, and expressions for the nuclear photoeffect matrix elements are obtained. The development is based on the differential charge conservation law and the hypothesis that the charge and current density operators for a nuclear system can be expressed in terms of nucleon variables only, even though virtual mesons may play a role in the interaction of the system with the electromagnetic field. The charge and current density operators are expressed as the sum of one-particle, two-particle, etc., terms in analogy with the phenomenological treatment of potential interactions between nucleons. The treatment is sufficiently general to include velocity dependence of interaction charges and currents, but is essentially nonrelativistic. Methods of explicitly constructing the most general forms of the charge and current density operators are given, taking into account general invariance and symmetry conditions to which they are subject. A new feature found in the present treatment is the possible existence of interaction effects in the charge density operator which imply that long-wavelength electric dipole matrix elements can be affected by interactions between nucleons (contrary to recent statements in the literature). The effects of the principles of charge symmetry and charge independence on nuclear-electromagnetic interactions are discussed. The form in which the photoeffect matrix elements are presented is such as to allow computation of all multipoles in an already summed form.