Photo-response of the \(N=Z\) nucleus 28Si below 13.0 MeV

The electric E1 and magnetic M1 dipole responses of the \(N=Z\) nucleus ²⁸Si were investigated in a nuclear resonance fluorescence experiment at the ELBE accelerator of the Helmholtz Zentrum Dresden-Rossendorf. The investigated energy range extends to 13.0 MeV, which corresponds to the kinetic energy of the electrons that were used to produce the unpolarised bremsstrahlung in the entrance channel of the ²⁸Si(\(\gamma ,\gamma ^{\prime }\)) reaction. The bremsstrahlung photons excited three \(J^{\pi }=1^-\), seven \(J^{\pi }=1^+\), and several \(J^{\pi }=2^+\) states. De-excitation \(\gamma \) rays were detected using the four high-purity germanium detectors of the \(\gamma \)ELBE setup. The excellent background conditions allowed to identify nine previously unobserved \(\gamma \)-ray transitions. In the investigated energy region of up to 13.0 MeV a total \(\sum _i B(M1, 0^+ \rightarrow 1^+_i)= 5.0(3)~\mu _N^2\) strength is firmly observed with a possible addition of \(B(M1, 0^+ \rightarrow 1^+_i) \le 0.14~\mu _N^2\) from levels for which the data allows to establish only an upper limit. Furthermore, below 13 MeV this \(N=Z\) nucleus exhibits a marginal isoscalar E1 strength of \(\sum _i B(E1, 0^+ \rightarrow 1^-)= 2.6(3) \times 10^{-3}\) \(\hbox {e}^2 \hbox {fm}^2\), which exhausts only 0.026(2) % of the energy-weighted sum rule.