23-25. Distance provides very effective protection against penetrating gamma radiation because radiation intensity decreases by the square of the distance from the source. For example, if exposed to 1,000 cGy of radiation standing 30 centimeters (12 inches) from the source, at 60 centimeters (24 inches), you would only receive 250 cGy. Thus, when you double the distance, radiation decreases to (0.5)2 or 0.25 the amount. While this formula is valid for concentrated sources of radiation in small areas, it becomes more complicated for large areas of radiation such as fallout areas.