http://arxiv.org/abs/1710.06352
At the long-wavelength approximation, $E1$ transitions are forbidden between isospin-zero states. Hence $E1$ radiative capture is strongly hindered in reactions involving $N = Z$ nuclei but the $E1$ $S$ factor may remain comparable to, or larger than, the $E2$ one. Theoretical expressions of the isoscalar and isovector contributions to $E1$ capture are analyzed in microscopic and three-body approaches in the context of the $\alpha(d,\gamma)^6$Li reaction. The lowest non-vanishing terms of the operators are derived and the dominant contributions to matrix elements are discussed. Some of these contributions computed in a three-body model are compatible with an interpretation of the low-energy experimental data in terms of dominant isovector transitions involving small isospin-one admixtures in the wave functions. This suggests that the exact-masses prescription which is often used to avoid the disappearance of the $E1$ matrix element in potential models is not founded at the microscopic level. The importance of capture components from an initial $S$ scattering wave is also discussed.
D. Baye and E. Tursunov
Wed, 18 Oct 2017
34/62
Comments: 23 pages, 1 table, 2 figures