放ほう射しゃ性せい核かく種しゅ

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Meanings

radionuclide; radioactive nuclide

Technical term in nuclear physics and radiochemistry. Refers to an atom with an unstable nucleus that undergoes radioactive decay.

放ほう射しゃ性せい同どう位い体たい
Often used interchangeably, but 放射性同位体 (radioisotope) emphasizes the isotopic nature, while 放射性核種 is a broader term for any radioactive nuclear species.
核かく種しゅ
A general term for a nuclide (a species of atom characterized by its nuclear composition). 放射性核種 specifies that the nuclide is radioactive.

Compound of 放射性 (radioactive) + 核種 (nuclide). The term is a direct calque from English 'radionuclide'.