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Radioactive Decay as First Order Process
Appears in
Concepts tested here
- Amount after n half lives
- Initial activity from half lives
- Number of half lives
All Questions
2003 AIPMT 1 question
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The radioisotope, tritium ($^{3}_{1}H$) has a half-life of 12.3 years. If the initial amount of tritium is 32 mg, how many milligrams of it would remain after 49.2 years :Number of half lives: $n = \dfrac{49.2}{12.3} = 4$
Amount remaining $= N_0\left(\dfrac{1}{2}\right)^n = 32 \times \left(\dfrac{1}{2}\right)^4 = \dfrac{32}{16}$
$= 2$ mg
(32 mg $\rightarrow$ 16 $\rightarrow$ 8 $\rightarrow$ 4 $\rightarrow$ 2 mg.)
2001 AIPMT 1 question
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A human body requires 0.01 Curie activity of radioactive substance after 24 hours. Half life of radioactive substance is 6 hours. Then max. activity of radioactive substance that can be injected will be : -The activity at the end of 24 hours must be 0.01 Ci.
Number of half lives in 24 hours: $n = \dfrac{24}{6} = 4$
Activity after n half lives $= \dfrac{A_0}{2^n}$
$\dfrac{A_0}{2^4} = 0.01 \Rightarrow A_0 = 0.01 \times 16 = 0.16$ Ci
So 0.16 Ci must be injected.
2000 AIPMT 1 question
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A 300 gram radioactive sample has half life of 3 hours. After 18 hours remaining quantity will be :Number of half lives: $n = \dfrac{18}{3} = 6$
Remaining amount $= \dfrac{N_0}{2^n} = \dfrac{300}{2^6} = \dfrac{300}{64}$
$= 4.6875 \approx 4.68$ g