In a radioactive substance at t = 0, the number of atoms is 8$\times {10}^4$, its half-life period is 3 yr. The number of atoms equal to $1\times {10}^4$ will remain after an interval of:

1. 9 yr

2. 8 yr 

3. 6 yr 

4. 24 yr

The half-life of radium is 1622 years. How long will it take for seven-eighth of a given amount of radium to decay

1. 3244 years

2. 6488 years

3. 4866 years

4. 811 years

The power obtained in a reactor using \(\mathrm{U}^{235}\) disintegration is \(1000\) kW. The mass decay of \(\mathrm{U}^{235}\) per hour is:
1. \(1\) microgram
2. \(10\) microgram
3. \(20\) microgram
4. \(40\) microgram

The numbers of nuclei of a radioactive substance at time t = 0 are 1000 and 900 at time t = 2 sec. Then the number of nuclei at time t = 4 sec will be:

1. 800

2. 810 

3. 790 

4. 700

After two hours, one-sixteenth of the starting amount of a certain radioactive isotope remained undecayed. The half life of the isotope is

1. 15 minutes 

2. 30 minutes

3. 45 minutes

4. 1 hour

In the nuclear reaction: \(\mathrm{X}\left(n,\alpha\right){}_{3}^{7}\mathrm{Li}\) the term \({X}\) will be:

10 g of radioactive material of half-life 15 year is kept in store for 20 years. The disintegrated material mass is 

1. 12.5 g

2. 10.5 g

3. 6.03 g

4. 4.03 g

A sample of radioactive element has a mass of 10 gm at an instant t = 0. The approximate mass of this element in the sample after two means lives in 

1. 3.70 gm

2. 6.30 gm

3. 1.35 gm 

4. 2.50 gm

When two deuterium nuclei fuse together to form a tritium nuclei, we get a

1. Neutron

2. Deutron

3. $\alpha$-particle

4. Proton