Texas Industrial Radiography Gamma Practice Test

Using a Cobalt 60 source, if you determine the exposure time required to give the desired results for a certain set-up, in two years how much will your exposure time need to be increased to achieve the same results?

1. 100% longer

2. 60% longer

3. 40% longer

4. 15% longer

The correct answer is: 40% longer

When using a radioactive source like Cobalt 60, it is essential to understand how the activity of the source diminishes over time due to its half-life. Cobalt 60 has a half-life of approximately 5.27 years, meaning that after this period, the intensity of radiation emitted by the source will be reduced by half. In the scenario where you have determined an initial exposure time required to achieve the desired results, after two years, the amount of radiation emitted from the source will decrease, necessitating an adjustment in exposure time to achieve the same results. After two years, which is approximately 38% of the half-life of Cobalt 60, the exposure time would need to be increased. To calculate the increased exposure time, one can understand that approximately 65% of the original activity remains after two years (because after one half-life, only 50% remains, and you will be slightly above that after two years, due to the mathematical decline of exponential decay). This means that the exposure time will need to be adjusted to maintain the same level of exposure, and this is where the 40% increase comes into play. Over two years, the source has lost some of its effectiveness, which results in the need to