The microscopic origin of the $\gamma$ softness (fluctuations in the triaxiality parameter $\gamma$ of the nuclear shape) observed in atomic nuclei is studied in the framework of the triaxial projected shell model approach, which is based on a multiquasiparticle configuration space generated from a deformed mean field. It is demonstrated that the coupling to quasiparticle excitations drives the system from a $\gamma$-rigid to a $\gamma$-soft pattern. As an illustrative example for a $\gamma$-soft nucleus, a detailed study has been performed for the ${}^{104}\mathrm{Ru}$ nucleus. The experimental energies and a large sample of measured $E2$ matrix elements available for this nucleus are reproduced quite accurately. The shape invariant analysis of the calculated $E2$ matrix elements elucidates the $\gamma$-soft nature of ${}^{104}\mathrm{Ru}$.

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• Collective levels
• Electromagnetic transitions
• Nuclear shapes and moments
• Nuclear structure & decays
• Nucleon distribution
• 90 ≤ A ≤ 149
• Shell model