Meson Effects inn−pCapture

A contribution to the $n-p$ capture amplitude which results from the modification of the nucleon current operators by the nuclear interaction is calculated. The two-nucleon states are represented by Heitler-London states and the capture amplitude is related to single-nucleon matrix elements by means of an expansion corresponding to the exchange of mesons between the nucleons. These single-nucleon matrix elements are evaluated using the fixed-source theory. The one-meson excited Heitler-London state gives a contribution similar to the isobar model of Austern; the role of this state in the nuclear potential is also discussed. If a deuteron wave function with a relatively large $D$-state probability is used, the interaction effect is about 3% of the phenomenological cross section.