Thorium-232 (Th-232) is a naturally occurring radioactive isotope of thorium and the most stable and abundant isotope of this element. It has an extremely long half-life of approximately 14 billion years, making it nearly as old as the Earth itself. Th-232 undergoes alpha decay, eventually leading to a series of decay products that form the thorium decay series. During this decay process, gamma radiation is also emitted, including a prominent gamma-ray line at 186 keV, which is often used for detecting and identifying Th-232 in spectroscopic studies. This gamma emission originates as a secondary effect of the alpha decay, as the nucleus transitions from an excited to a lower energy state.

Th-232 is primarily used in the nuclear energy sector. It serves as a fertile material in nuclear reactors, where it can be transformed into uranium-233, a fissile material that can sustain a nuclear chain reaction. This property makes it a potential fuel for advanced thorium-based nuclear reactors, which are being researched for their safety, efficiency, and reduced long-lived radioactive waste compared to traditional uranium-based reactors. Th-232 is also used in high-temperature ceramics and magnesium-thorium alloys, particularly in aerospace and industrial applications.

Thorium-232 is naturally found in minerals such as monazite and thorite, which are rich in thorium. These minerals are widely distributed in the Earth's crust, especially in regions with heavy mineral sands. Th-232 contributes to natural background radiation and is often present in trace amounts in soils and rocks. Due to its natural abundance and potential applications, it is of significant interest in both industrial and scientific fields.