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

Subtopic:  Radioactivity (OLD NCERT) |
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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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Two radioactive substances A and B have decay constant 5λ and λ respectively. At t = 0 they have the same number of nuclei. The ratio of the number of nuclei of A to those of B will be (1/e)2 after a time interval of:

1. 14λ  

2. 4λ  

3. 2λ 

4.  12λ

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AIIMS - 2007
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The activity of the radioactive element decreases to one-third of the original activity A0 in a period of 7 years. After a further lapse of 7 years, its activity will be:

1.  A0  

2.  23A0  

3.  A06  

4.  A09

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A radioactive nucleus X decays to a stable nucleus 'y'. The graph of the rate of formation of 'y' against time 't' will be:

1. 2.
3. 4.
Subtopic:  Radioactivity (OLD NCERT) |
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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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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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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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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 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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