A radioactive sample S1 having an activity of 5μCi has twice the number of nuclei as another sample S2 which has an activity of 10μCi. The half lives of S1 and S2 can be
A
20 years and 5 years, respectively
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B
20 year and 10 years, respectively
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C
10 years each
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D
5 years each
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Solution
The correct option is A20 years and 5 years, respectively Given: R1=5μci,R2=10μci,N1=2N2
By, λN=R
0.693TN=R
T=0.693NR
For sample S1:
T1=0.693N15
T1=0.693×2N25 ......................eq1,
For sample S2:
T2=0.693N210 ....................eq2,
By dividing eq1 by eq2, we get:
T1T2=41
It is clear that the ratio of half lives of the two samples is 4:1, therefore half lives can be 20 (4k) years and 5 (1k) years respectively.