60 



APPLIED RADIOACTIVITY 



Half-Life 



The number of seconds required for the radioactivity of a substance to 

 fall to half its original value is called its half-life or half-value period. 

 This value of t may be obtained by setting 



N 2 



Then 



log e 2 = X* 



or 



logio 2 



Xt 



2.3026 



To illustrate what is meant by half-life of a radioactive substance 

 one may use that therapeutically valuable radioactive gas radon. It 

 disintegrates in a typical way by the emission of an alpha particle. Its 

 decay constant is 2.097 X 10 -6 per sec. Figure II-2 shows graphically 



100 

 90 

 80 



t>0 



.= 70 



C 



§ 60 



■2 50 



= 40 

 o 



o 



2 30 



20 

 10 



6 8 



Time in days 



10 



12 



14 



Fig. II-2. The percentage of radon remaining after any time recorded in days. 

 A'o is taken as an arbitrary activity of 100 units. 



how radon disintegrates in the course of 14 days. Note that half of 

 the original number N remains after 3.825 days, and one-fourth after 

 7.65 days. It is simpler to calculate the number of days it takes for 

 the original amount to reduce to half-value, by using the above decay 



