After two half-lives, what fraction of the initial activity of a radioisotope remains?

Prepare for the Medical Dosimetry Certification Test with comprehensive flashcards and multiple choice questions, complete with hints and explanations. Ensure your success by studying key topics covered in the exam!

After two half-lives of a radioisotope, the fraction of the initial activity that remains can be determined by understanding what a half-life represents. A half-life is the time it takes for half of the radioactive substance to decay. Therefore, after one half-life, 50% of the original activity remains.

When you reach the second half-life, you take this remaining amount (50%) and again reduce it by half. This results in 25% of the original activity still being present. In fractional terms, this is expressed as 1/4 of the initial activity.

The process can be illustrated as follows:

  • Initially, you start with a fraction of 1 (100%).

  • After the first half-life, the fraction remaining is 1/2.

  • After the second half-life, the remaining fraction from the first half-life (1/2) is halved again, leading to 1/4.

This clearly shows that after two half-lives, 1/4 of the initial activity remains. Thus, the correct fraction of the initial activity of a radioisotope that remains after two half-lives is accurately represented by option A.

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