Which of the following is NOT a long-lived constancy source?

The correct answer identifies 133Xe as the option that is not a long-lived constancy source. In the context of radiation, a long-lived constancy source is typically characterized by its stability and longevity, allowing it to provide a reliable reference over an extended period.

133Xe, or xenon-133, has a half-life of about 5.2 days, which is considerably shorter than that of the other isotopes listed. This relatively short half-life means that 133Xe cannot maintain a stable, constant source for long-term calibration or quality assurance in PET scans.

On the other hand, the other isotopes—57Co, 135Ba, and 137Cs—have much longer half-lives and are commonly used as calibration sources in nuclear medicine and radiologic imaging because they remain stable over prolonged periods, making them suitable for long-term use in equipment calibration. 57Co has a half-life of about 271.5 days, 135Ba has a half-life of around 15.7 hours (longer than Xe but still not suitable for long-term referencing in most contexts), and 137Cs boasts a half-life of about 30.2 years, solidifying its role as a long-term reference.

Thus