About 5 hours after removal the second term in the brackets becomes very small, and the activity after that time will decay nearly according to an exponential law with the time, falling to half value in 11 hours. For any time of exposure T, the activity at time t after the removal (see equation 11, section 199) is given by
where I₀ is the initial value of the activity, immediately after removal, and
By variation of T the curves of variation of activity for any time of exposure can be accurately deduced from the equation, when the values of the two constants λ1, λ2 are substituted. Miss Brooks11 has examined the decay curves of excited activity for thorium for different times of exposure and has observed a substantial agreement between experiment and theory.