Ix] EXCITED RADIO-ACTIVITY 263 
curve is unaltered for all times of exposure to the emanation greater 
than 24 hours. 
After an interval of 2°5 hours, the logarithmic decay curve for 
long times of exposure is a straight line, z.e. the activity falls off 
im an exponential law with the time, falling to half value in 
28 minutes. P. Curie and Danne found that for any time ¢ 
3 
Decay |of Excited Activity 
from|Radium for varying 
times of exposure 


Log. of Intensity of Radiation 






Time in Hours 
Fig. 52. 
after removal the intensity J; was given by the difference of two 
exponentials, viz. 
f 
L =ae-’—(a—1)e™, 
where Ay = yqyq and Ay = 7—45q With the second as the unit of time. 
The numerical constant a = 420. The explanation of this law of 
decay is given in section 177. 
The decay curve varies greatly with the time of exposure. For 
example, in an exposure of 5 minutes, the activity at first decreases 
