Question

The activity of a freshly prepared  radioactive sample is 10^10 disintegrations per second, whose mean life is 10^9 sec.

The mass of an atom of this radioisotope is 10 ^ -25 kg.  The mass (in mg) of the radioactive sample is

 

Joshi sir comment

dN/dt = -λN

1010 = -[1/109] N                                               λ = 1/mean life

so N = 1019 disintegration

so mass of radioisotope = 1019 *10-25 = 10-6 Kg = 1 mg

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