Which of the following is NOT a very long-lived, radioactive isotope?

• A. Cobalt-60 (Co-60)
• B. Uranium-238 (U-238)
• C. Potassium-40 (K-40)
• D. Thorium-232 (Th-232)

Answer: (A)

Verified step by step guidance

1. Understand the concept of a "very long-lived, radioactive isotope." These isotopes have half-lives on the order of millions or billions of years, meaning they decay very slowly and remain present in nature for a long time.
2. Review the half-lives of each isotope given in the problem: Uranium-238, Potassium-40, Thorium-232, and Cobalt-60. This information is key to determining which isotope is NOT very long-lived.
3. Recall that Uranium-238 has a half-life of about 4.5 billion years, Potassium-40 about 1.25 billion years, and Thorium-232 about 14 billion years. These are all extremely long half-lives, classifying them as very long-lived isotopes.
4. Compare these to Cobalt-60, which has a much shorter half-life of about 5.27 years. This is significantly shorter than the others, meaning it is not considered very long-lived.
5. Conclude that Cobalt-60 is the isotope that is NOT very long-lived, radioactive isotope, based on its much shorter half-life compared to the others.