Related Clues:

• U-235 or C-14, e.g.
• Element form
• Heavy hydrogen, e.g.
• Carbon-14, e.g.
• Carbon 13 or cobalt 60
• Chemist's study
• Chemical relative
• Elemental form
• Physicist's atom
• Uranium 238, e.g.
• Particular form of an element
• Dating material
• Carbon 13, e.g.
• Uranium 235, for example
• Chemistry subject