In a radioactive substance at t = 0, the number of atoms is 8×104, its half-life period is 3 yr. The number of atoms equal to 1×104 will remain after an interval of:

1. 9 yr

2. 8 yr 

3. 6 yr 

4. 24 yr

Subtopic:  Radioactivity (OLD NCERT) |
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In a radioactive sample the fraction of initial number of radioactive nuclei, which remains undecayed after n mean lives is:

1. 1en 

2.  en   

3. 1-1en 

4. 1e-1n

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At time t = 0, N1 nuclei of decay constant λ1 and N2 nuclei of decay constant λ2 are mixed. The decay rate of the mixture is:

1. -N1N2e-λ1+λ2t 

2. -N1N2 e-λ1+λ2t 

3. -N1λ1e-λ1t + N2λ2e-λ2t 

4. -N1λ1N2λ2e-λ1+λ2t

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The half-life period of a radioactive substance is 6 h.  If after 24 h, activity is 0.01 μCi, what was the initial activity? 

1. 0.04 μCi 

2. 0.08 μCi

3. 0.24 μCi 

4. 0.16 μCi

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The rate of disintegration of a fixed quantity of a radioactive substance can be increased by:

1. increasing the temperature.

2. increasing the pressure.

3. chemical reaction.

4. it is not possible.

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The counting rate observed from a radioactive source at t = 0 second was 1600 counts per second and at t = 8 seconds it was 100 counts per second. The counting rate observed, as counts per second, at t = 6 seconds will be:

1. 400

2. 300

3. 200

4. 150

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Half-lives of two radioactive substances A and B respectively are 20 min and 40 min.  Initially, the samples of A and B have an equal number of nuclei. After 80 min the ratio of the remaining number of A and B nuclei is:

1. 1 : 16

2. 4 : 1

3. 1 : 4 

4. 1 : 1

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What fraction of a radioactive material will get disintegrated in a period of two half-lives?

1. whole

2. half

3. one-fourth

4. three-fourth

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In a radioactive material, the activity at time t1 is R1 and at a later time, t2 is R2. If the decay constant of the material is λ, then:

1. R1=R2e-λt1-t2   

2. R1=R2eλt1-t2

3. R1=R2t2t1 

4. R1=R2

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A and B are two radioactive substances whose half-lives are 1 and 2 years respectively. Initially 10 g of A and 1 g of B is taken. The time (approximate) after which they will have the same quantity remaining is:

1. 6.62 yr

2. 5 yr   

3. 3.2 yr 

4. 7 yr

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