The half-life of a radioactive substance is 20 minutes. The time between 20% and 80% decay will be: 

1. 20 minutes

2. 40 minutes

3. 30 minutes

4. 25 minutes

After 280 days, the activity of a radioactive sample is 6000 dps. The activity reduces to 3000 dps after another 140 days. The initial activity of the sample in dps is

(1) 6000           

(2) 9000

(3) 3000           

(4) 24000

A radioactive sample is $\mathrm{\alpha }$-emitter with half life 138.6 days is observed by a student to have 2000 disintegration/sec. The number of radioactive nuclei for given activity are
(a) $3.45\times {10}^{10}$               (b) $1\times {10}^{10}$
(c) $3.45\times {10}^{15}$               (d) $2.75\times {10}^{11}$

The ratio of radii of nuclei ${}_{13}{}^{27}\mathrm{Al}$ and ${}_{52}{}^{\mathrm{A}}\mathrm{X}$ is 3 : 5. The number of neutrons in the nuclei of X will be
1. 52             

2. 73

3. 125             

4. 13

The dependence of binding energy per nucleon, ${\mathrm{B}}_{\mathrm{N}}$ on the mass number, A, is represented by

Binding energy per nucleon plot against the mass number for stable nuclei is shown in the figure. Which curve is correct ?

1. A
2. B
3. C
4. D

A radioactive nucleus undergoes $\mathrm{\alpha }$-emission to form a stable element. What will be the recoil velocity of the daughter nucleus if v is the velocity of $\mathrm{\alpha }$-particle and A is the atomic mass of radioactive nucleus ?

1. $\frac{4\mathrm{V}}{\mathrm{A}-4}$             2. $\frac{2\mathrm{V}}{\mathrm{A}-4}$
3. $\frac{4\mathrm{V}}{\mathrm{A}+4}$             4. $\frac{2\mathrm{V}}{\mathrm{A}+4}$

For a substance, the average life for $\mathrm{\alpha }$-emission is 1620 years and for $\mathrm{\beta }$ emission is 405 years. After how much time the 1/4 of the material remains after simultaneous emission:

1. 1500 years            2. 300 years
3. 449 years              4. 810 years

The half life period of a radioactive element X is same as the mean life time of another radioactive element Y. Initially both of them have the same number of atoms. Then

(1) X and Y have the same decay rate initially

(2) X and Y decay at the same rate always

(3) Y will decay at a faster rate than X

(4) X will decay at a faster rate than Y

A radioactive material decays by simultaneous emission of two particles with respective half lives 1620 and 810 years. The time (in years) after which one- fourth of the material remains is 
(a) 1080         (b) 2430
(c) 3240         (d) 4860