Isospin forbidden E1 transitions in 16O and 40Ca and the applicability of Siegert's theorem.

Abstract

We reexamine the well-known problem of isospin forbidden E1 decays in self-conjugate nuclei in the light of a recent claim suggesting that the E1 decay in $^{16}\mathrm{O}$ violates Siegert's theorem. We first discuss the importance of including, through isospin mixing, giant dipole and monopole components if agreement with observed lifetimes has to be achieved. We then show that the same microscopic calculation which yields broad agreement with (e,e') and lifetimes results in $^{16}\mathrm{O}$ and $^{40}\mathrm{Ca}$ also asserts the validity of Siegert's theorem. We conclude by suggesting that the claim regarding a violation of Siegert's theorem can only be definitely ascertained once additional data at low momentum transfer become available.