The statement that unstable nuclei form due to imbalances in nuclear forces (such as the ratio between the strong nuclear force and electromagnetic repulsion) is a foundational physical fact taught across standard physics curricula, requiring no citation.
The claim represents a fundamental textbook principle of nuclear physics: atomic nuclei are stable when the attractive strong nuclear force balances the repulsive electrostatic forces between protons, and imbalances lead to instability and radioactivity. None of the retrieved papers specifically test or dispute this standard principle in a primary research context; rather, the retrieved papers cover applied radiopharmaceuticals, soil radioactivity, and niche theoretical models. Therefore, the claim is classified as common knowledge.