In order to explain the drop off shown, we need to discuss the inverse square law. The inverse square law states the farther you get from a source, the less intense the effects of the source get by a 1 over distance square value. In this case, the intensity of the radiation and the radiation waves drop off by a 1/d^2 relationship where d is equal to distance. If the source is 2x as far away, it's 1/4 as much exposure. If it's 10x farther away, the radiation exposure is 100x less.