Photo-response of the N=Z nucleus ^24 Mg

The electric E 1 and magnetic M 1 dipole responses of the N=Z nucleus ^24 Mg were investigated in an inelastic photon scattering experiment. The 13.0 MeV electrons, which were used to produce the unpolarised bremsstrahlung in the entrance channel of the ^24 Mg( γ ,γ ^' ) reaction, were delivered by the ELBE accelerator of the Helmholtz-Zentrum Dresden-Rossendorf. The collimated bremsstrahlung photons excited one J^π=1^- , four J^π=1^+ , and six J^π=2^+ states in ^24 Mg. De-excitation γ rays were detected using the four high-purity germanium detectors of the γ ELBE setup, which is dedicated to nuclear resonance fluorescence experiments. In the energy region up to 13.0 MeV a total B(M1)↑ = 2.7(3) μ _N^2 is observed, but this N=Z nucleus exhibits only marginal E 1 strength of less than ∑ B(E1)↑≤ 0.61 × 10^-3 e ^2 fm ^2 . The B( 1, 1^π_i → 2^+_1)/B( 1, 1^π_i → 0^+_gs) branching ratios in combination with the expected results from the Alaga rules demonstrate that K is a good approximative quantum number for ^24 Mg. The use of the known ρ ^2(E0, 0^+_2 → 0^+_gs) strength and the measured B(M1, 1^+ → 0^+_2)/B(M1, 1^+ → 0^+_gs) branching ratio of the 10.712 MeV 1^+ level allows, in a two-state mixing model, an extraction of the difference β _2^2 between the prolate ground-state structure and shape-coexisting superdeformed structure built upon the 6432-keV 0^+_2 level.