Ep=1.118MeV resonance in theAl27(p,γ)Si28reaction

Abstract

The 12.664-MeV level in ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ has been populated by resonant proton capture at 1.118 MeV in the ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}(p,\gamma ){}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ reaction. $\gamma$-ray angular distributions have been measured with a 80-${\mathrm{cm}}^3$ Ge(Li) detector. The spin of the 11.432-MeV level is most probably 4 with $T=0\mathrm{}$; for the 8.413-MeV level, the assignment $J^{\pi }=4^{-}$ is in agreement with the present data. Mixing ratios for $\gamma$-ray transitions have been extracted from the angular distributions. Lifetime and branching-ratio studies have been made at $E_p=1.118,1.724, \mathrm{and} 2.876$ MeV. Emphasis was given to measurements on levels at 8.413, 9.702, and 11.577 MeV, as possible candidates for the $4^{-}$, $5^{-}$, and $6^{-}$ states of a $K^{\pi }=3^{-}$ rotational band based on the 6.879-MeV level. The observed $E2\mathrm{}$ transition strengths are not in agreement with the above assumption. A strength of 0.3 Weisskopf units is found for the 12.664→8.413-MeV $M1\mathrm{}$ transition, which is in conformity with a $J^{\pi }\to J^{\pi }$, $\Delta T=1\mathrm{}$ transition in a self-conjugate nucleus; this result may suggest the possibility of another interpretation of these negative-parity states, relied on one-particle - one-hole configurations.