(i) calculate the activity of two mg sample `""_(28Sr^(90)` whose half - life period is 28 yr.
(ii) An experiment isperformed to determine the half -life of a radioactive substance which emits one beta particle for each decay process . Observations show that an averaghe of 8.4 `beta` -particles are emittied each sc0ond by 2.5 Mg of the substance ,the atomic weight of the substance is 230 . calculate the half -life ofhte substance .

Explanation

Given that activity = `8.4 sec^(-1)`
According to Avogadro's hypothesis, the number of atoms in 2.5 mg
`N=(6.02xx10^(23))/(230)xx2.5xx10^(16)`
Now `lambdaN=8.4sec^(-1)`
`therefore` `lambda=(8.4)/(N)= (8.4)/( 6.54xx10^(18))=1.28xx10^(-18)sec^(-1)` `therefore` `T=(0.6931)/(lambda)=(0.6931)/(1.28xx10^(-18))sec=1.71xx10^(10)`years