Modifiers matching decay:

• transient,
• internal,
• active,
• radioactive,
• urban,
• exotic,

Simple categories matching radioactive:

• substance,
• poison,
• mineral,
• material,
• contaminant,
• gas,
• surface,
• waste,
• substrate,
• log,
• isotope,
• constituent,
• source,
• ore,
• metal,
• particle,
• element,
• halogen,
• atom,
• solid,
• liquid,
• volatile,
• tracer,
• fluid,
• article,
• component,
• decay,
• drug,