GiantE1resonances inBe8from the reactionLi7(p,γ)Be8

The giant electric dipole resonances based on the ground state ($0^{+}$) and first excited state ($2^{+}$) of ${}_{}{}^8{}_{}{}^{}\mathrm{Be}$ have been studied with the reactions ${}_{}{}^7{}_{}{}^{}\mathrm{Li}(p,{\gamma }_0){}_{}{}^8{}_{}{}^{}\mathrm{Be}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}(p,{\gamma }_1){}_{}{}^8{}_{}{}^{}\mathrm{Be}$ over the range $E_p=0.8\mathrm{} \mathrm{to} 17.6$ MeV. Both resonances show a simple resonant shape with little fine structure. The ${\gamma }_1$ giant resonance is displaced upward from the ${\gamma }_0$ resonance by an energy of 2.2 MeV which is approximately equal to the excitation of the $2^{+}$ state. The reduced ($\gamma ,p_0$) strengths of the two resonances are approximately equal and each exhausts about 10% of the $E1\mathrm{}$ sum rule. The angular distributions for both ${\gamma }_0$ and ${\gamma }_1$ are quite constant over the giant resonance structure and are predominantly dipole in character. A simple model can be invoked to explain the dominant $E1\mathrm{}$ features. The presence of $P_1\left(cos\theta \right)$ and $P_3\left(cos\theta \right)$ terms in both angular distributions indicates $E2$ strength which probably increases above the $E1\mathrm{}$ resonances. Some parameters for the narrow levels at 18.15 and 19.06 MeV in ${}_{}{}^8{}_{}{}^{}\mathrm{Be}$ are given.