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

Abstract

The electric E1 and magnetic M1 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(gamma ,gamma ^{prime }) reaction, were delivered by the ELBE accelerator of the Helmholtz-Zentrum Dresden-Rossendorf. The collimated bremsstrahlung photons excited one J^{pi }=1^-, four J^{pi }=1^+, and six J^{pi }=2^+ states in ^{24}Mg. De-excitation gamma rays were detected using the four high-purity germanium detectors of the gamma ELBE setup, which is dedicated to nuclear resonance fluorescence experiments. In the energy region up to 13.0 MeV a total B(M1)uparrow = 2.7(3)~mu _N^2 is observed, but this N=Z nucleus exhibits only marginal E1 strength of less than sum B(E1)uparrow le 0.61 times 10^{-3} e^2 , fm^2. The B(varPi 1, 1^{pi }_i rightarrow 2^+_1)/B(varPi 1, 1^{pi }_i rightarrow 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 rho ^2(E0, 0^+_2 rightarrow 0^+_{gs}) strength and the measured B(M1, 1^+ rightarrow 0^+_2)/B(M1, 1^+ rightarrow 0^+_{gs}) branching ratio of the 10.712 MeV 1^+ level allows, in a two-state mixing model, an extraction of the difference varDelta beta _2^2 between the prolate ground-state structure and shape-coexisting superdeformed structure built upon the 6432-keV 0^+_2 level.