ASNT Industrial Radiography Radiation Safety Practice Test

If you have a source of lr-192 with 50 Ci, what will the surface reading be after 150 days?

1. 50 mR/h

2. 25 mR/h

3. 12.5 mR/h

4. 0 mR/h

The correct answer is: 12.5 mR/h

To determine the surface reading after 150 days for a source of Iridium-192 (Ir-192) with an initial activity of 50 Curie (Ci), it's important to understand the concept of radioactive decay. Ir-192 has a half-life of approximately 74 days. This means that every 74 days, the radioactivity will decrease to half of its previous value. Since 150 days is more than two half-lives (approximately 148 days), we can calculate how much the activity decreases over this period. 1. After the first half-life (74 days), the activity decreases from 50 Ci to 25 Ci. 2. After the second half-life (another 74 days, totaling 148 days), the activity decreases from 25 Ci to 12.5 Ci. After 150 days, since we've passed two complete half-lives but not yet reached the next half-life, the activity will still be just under 12.5 Ci. However, in terms of the surface reading measured in millirads per hour (mR/h), we round down to 12.5 mR/h to reflect the approximate conditions. This understanding of half-lives and decay rates helps establish the decrease in radioactivity over