175Lu

Lutetium-175, written as 175Lu, is the stable isotope of the element lutetium (Z = 71) with a mass number of 175. It is the only stable isotope of lutetium, and it makes up the majority of natural lutetium.

In natural lutetium, about 97.4% is 175Lu, while the remaining approximately 2.6% is 176Lu. The stability of 175Lu means it does not undergo radioactive decay under normal conditions. By contrast, 176Lu is a long-lived radioactive isotope that decays to hafnium-176 (176Hf) with a half-life of about 3.78 × 10^10 years, contributing to the natural isotopic composition over geological timescales.

Because 175Lu is stable, it serves as the dominant isotope in most measurements involving lutetium and is not used as a radiometric dating nuclide. The presence of 176Lu, however, underpins Lu–Hf geochronology, a dating and tracing system in Earth sciences that relies on the decay of 176Lu to 176Hf to monitor time-dependent changes in hafnium isotopic ratios.

Applications of lutetium in industry and research are driven by the element as a whole rather than by 175Lu specifically; lutetium compounds are used in catalysts, ceramics, and specialized scintillators. The isotopic composition, dominated by 175Lu, is a characteristic feature of natural lutetium and informs both fundamental studies and practical measurements involving the element.