Beta Decay ofLi8

Abstract

The beta decay of ${\mathrm{Li}}^{8}$, formed in the ${\mathrm{Li}}^{7}(d, p){\mathrm{Li}}^{8}$ reaction, has been studied by measuring the energy distribution of the alpha particles that come from the subsequent breakup of ${\mathrm{Be}}^{8}$. The effects of the penetration into backing foils of the ${\mathrm{Li}}^{8}$ recoils have been corrected for by comparing, for several deuteron bombarding energies, the alpha-particle spectra seen using a very thin foil target and the same target backed by a thick foil. The resulting "correct" alpha-particle spectrum is adjusted for various small effects including that due to electron-neutrino recoil and then compared with a prediction based on the empirical alpha-alpha scattering phase shifts, themselves adjusted by the subtraction of a hard-sphere phase shift. It is shown that the prediction is rather insensitive to the choice of hard-sphere radius. The agreement between the beta-decay data and the alpha-alpha phase shifts in the peak position (the "2.9-MeV state" of ${\mathrm{Be}}^{8}$) is excellent as it is also in the shape of the transition probability distribution on the low- (alpha-particle) energy side of the peak where the falloff of intensity is here experimentally followed over two orders of magnitude. On the high-energy side of the peak, the familiar discrepancy is found in the sense that the transition probability is much too high to be explained by the first excited state alone. The present results, in addition to constituting an accurate comparison between ${\mathrm{Li}}^{8}$ beta decay and alpha-alpha scattering, strengthen the interpretation of the reaction ${\mathrm{Be}}^{9}(p, d){\mathrm{Be}}^{8}$ in terms of the "ghost" of the ground state of ${\mathrm{Be}}^{8}$ and provide necessary data for discussing ${\mathrm{Li}}^{8}$ and ${\mathrm{B}}^{8}$ decay to regions of higher excitation in ${\mathrm{Be}}^{8}$ where the effects of transitions to the tail of the first $T=1$ state of ${\mathrm{Be}}^{8}$ are probably important.