Which Cardinal rule of radiation protection is mathematically defined by the inverse square law?

• A. Distance
• B. Time
• C. Shielding
• D. Dose rate

Answer: (A)

Distance is the key factor because the inverse square law shows that the dose rate from a point source falls off in proportion to 1 over the distance squared. As radiation energy radiates in all directions, moving farther from the source spreads that energy over a larger spherical surface area. Doubling the distance reduces the dose rate to one quarter, tripling it to one ninth, and so on. This is why increasing distance is such a powerful protective measure: small increases in distance yield large reductions in exposure. Time affects total dose accumulated but not the instantaneous rate described by the inverse square law, and shielding reduces exposure through absorption rather than changing the geometric spreading of radiation. The law defines how dose rate changes with distance, which is why distance is the correct choice.