R-matrix analysis of theβ∓-delayed alpha spectra from the decay ofLi8andB8

The general question of the analysis of beta-delayed particle emission is discussed. It is emphasized that neither an integrated Fermi function nor a beta branching ratio is, in general, rigorously defined for beta decay to a specific level (resonance). As an example, spectra of breakup α particles observed following ${}_{}{}^8{}_{}{}^{}\mathrm{Li}$(${\beta }^{\mathrm{-}}$ ${)}^8$Be and ${}_{}{}^8{}_{}{}^{}\mathrm{B}$(${\beta }^{+}$ ${)}^8$Be are analyzed by the many-level one-channel approximation of R-matrix theory. In addition, the L=2 α-α elastic scattering phase shifts are analyzed up to $E_{\alpha }$=34 MeV. It is found that satisfactory fits are obtained without introducing intruder states below 26-MeV excitation. Gamow-Teller matrix elements are extracted for decay to the 3.0-MeV first-excited state of ${}_{}{}^8{}_{}{}^{}\mathrm{Be}$ and for the doublet near 16 MeV. The width and location of the ${}_{}{}^8{}_{}{}^{}\mathrm{Be}$ 3.0-MeV state are also obtained. The results are compared to shell-model predictions.