First evidence for spin-flip M1 strength in Ar-40

The $^{40}\mathrm{Ar}$($\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}},{\ensuremath{\gamma}}^{\ensuremath{'}}$) photon scattering reaction was used to search for spin-flip $M1$ strength in $^{40}\mathrm{Ar}$. The nearly monochromatic, linearly polarized photon beam of HI\ensuremath{\gamma}S, in an energy region from 7.7 to 11 MeV, was employed in this study. 28 dipole excitations were observed. The azimuthal intensity asymmetry indicated that all of these states were $E1$ except for the state at ${E}_{x}=9.757$ MeV. Shell-model calculations were used to interpret this state as one fragment of the spin-flip $M1$ strength in $^{40}\mathrm{Ar}$.