What is the radiation intensity at 30 feet from an unshielded 60 Ci Iridium-192 source?

To determine the radiation intensity at a specific distance from an unshielded radioactive source, it's essential to recognize how the intensity of radiation diminishes with distance. This follows the inverse square law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source.

For Iridium-192, which is commonly used in brachytherapy and industrial applications, the initial intensity can be estimated based on its curie activity. In this case, you have a source of 60 curies. At 1 meter (approximately 3.28 feet) from such a source, the intensity can typically be calculated, and from there, you can adjust for the distance of 30 feet.

Given that 30 feet roughly translates to about 9.14 meters, applying the inverse square law would result in a significant decrease in the intensity due to the increased distance from the source. Specifically, as you move further away, the radiation intensity diminishes in such a way that every doubling of distance results in a fourfold decrease in intensity.

In this context, with the calculations backed by the physics of radiation emission and the characteristics of Iridium-192, the most reasonable conclusion is that at 30 feet, the radiation intensity is