The thermal neutron cross section for the reaction n${+}^3$He${\to }^4$He+γ is calculated with the Monte Carlo method from realistic variational wave functions obtained for the Argonne ${\mathit{v}}_{14}$ two-nucleon and Urbana VII three-nucleon interaction models. The calculated cross section is found to be almost entirely due to exchange currents. With the exchange current operator constructed so as to satisfy the continuity equation with the Argonne ${\mathit{v}}_{14}$ interaction, the calculated cross section is 71 μb, which is close to the empirical value 54±6 μb. The less certainly known contributions from model-dependent exchange current mechanisms increase the predicted value to about 112 μb. The calculated cross section depends strongly on the n${+}^3$He scattering length, varying between 140 and 71 μb as the scattering length is changed from 3.25 to 3.75 fm.

• Received 2 April 1990

DOI:https://doi.org/10.1103/PhysRevC.42.830