Isospin-forbidden electric dipole capture and the <i>α</i> ( <i>d</i> , <i>γ</i> ) <sup>6</sup> Li reaction
Abstract
Abstract At the long-wavelength approximation, E 1 transitions are forbidden between isospin-zero states. Hence E 1 radiative capture is strongly hindered in reactions involving N = Z nuclei but the E 1 astrophysical S factor may remain comparable to, or larger than, the E 2 one. Theoretical expressions of the isoscalar and isovector contributions to E 1 capture are analyzed in microscopic and three-body approaches in the context of the α ( d , γ ) 6 Li reaction. The lowest non-vanishing terms of the operators are derived and the dominant contributions to matrix elements are discussed. The astrophysical S factor computed with some of these contributions in a three-body α + n + p model is in agreement with the recent low-energy experimental data of the LUNA collaboration. This confirms that a correct treatment of the isovector E 1 transitions involving small isospin-one admixtures in the wave functions should be able to provide an explanation of the data without adjustable parameter. The exact-masses prescription which is often used to avoid the disappearance of the E 1 matrix element in potential models is not found at the microscopic level and should not be used for reactions of N = Z nuclei. The importance of capture components from an initial S scattering wave is also discussed. This is a board member communicated paper