ASNT Industrial Radiography Radiation Safety Practice Test

What is the current activity of a cobalt source that has decayed from 88 Ci (3256 GBq) after 3 half-lives?

1. 22 Ci (814 GBq)

2. 44 Ci (1628 GBq)

3. 11 Ci (407 GBq)

4. 55 Ci (2035 GBq)

The correct answer is: 11 Ci (407 GBq)

To determine the current activity of the cobalt source after 3 half-lives, it is essential to understand the concept of radioactive decay and how it relates to half-lives. A half-life is the time required for half of a radioactive substance to decay. Starting with an initial activity of 88 Ci, each half-life will reduce the activity by half. After the first half-life, the activity would be: 1st half-life: 88 Ci / 2 = 44 Ci After the second half-life, it would be: 2nd half-life: 44 Ci / 2 = 22 Ci After the third half-life, the activity reduces again: 3rd half-life: 22 Ci / 2 = 11 Ci Therefore, after 3 half-lives, the activity of the cobalt source is 11 Ci. This aligns perfectly with the calculation of activity reduction through successive half-lives, demonstrating that the selected answer accurately represents the ongoing decay of the radioactive source over the specified time period.