Which formula is correct?

The inverse square law says intensity from a point source falls with the square of distance: I₂ = I₁ × (d₁² / d₂²). Doubling distance reduces intensity to one-quarter. In plain English Move away a little → exposure drops a lot. Distance is one of the fastest “controls” in radiation protection. Worked example 1 (doubling distance) Dose rate is 12 mR/hr at 1 meter. What at 2 meters?  I₂ = 12 × (1² / 2²) = 12 × (1/4) = 3 mR/hr Worked example 2 (solve for unknown distance) If dose rate is 20 µSv/hr at 1 m, at what distance is it 5 µSv/hr?  5 = 20 × (1² / d²)  5/20 = 1/d² → 1/4 = 1/d² →... Show more

The inverse square law says intensity from a point source falls with the square of distance:
I₂ = I₁ × (d₁² / d₂²).
Doubling distance reduces intensity to one-quarter.

In plain English
Move away a little → exposure drops a lot. Distance is one of the fastest “controls” in radiation protection.

Worked example 1 (doubling distance)
Dose rate is 12 mR/hr at 1 meter. What at 2 meters?
 I₂ = 12 × (1² / 2²) = 12 × (1/4) = 3 mR/hr

Worked example 2 (solve for unknown distance)
If dose rate is 20 µSv/hr at 1 m, at what distance is it 5 µSv/hr?
 5 = 20 × (1² / d²)
 5/20 = 1/d² → 1/4 = 1/d² → d² = 4 → d = 2 m

 

Also see this: Radiation Dose Units and Quantities: Gy Sv and REM

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