1. The problem statement, all variables and given/known data A point source of γ-radiation has a half-life of 30 minutes. The initial count rate, recorded by a Geiger counter placed 2.0 m from the source, is 360 s-1. The distance between the counter and the source is altered. After 1.5 hour the count rate recorded is 5 s-1. What is the new distance between the counter and the source? Answer: 6 m. 2. The attempt at a solution We have: T1 / 2 = 1800 s. d = 2 m R = 360 s-1 t = 5400 s d = ? R = 5 s-1 No idea what to do here. R = R0 e- λ t, but I don't see a way to find distance from this formula. There is also a formula from the experimental verification of the inverse square law for γ-rays: x = k R- 1 / 2 - c, where d = x + c (distance between actual position of source and effective point of detection), k = constant of proportionality. But I still see not much connection between these two formulas. Maybe I'm looking in the wrong direction?