The half-life of a radioactive substance against $\mathrm{\alpha }$-decay is $1.2\times {10}^7$ s. What is the decay rate for $4\times {10}^{15}$ atoms of the substance

(1) $4.6\times {10}^{12} \mathrm{atoms}/\mathrm{s}$         

(2) $2.3\times {10}^{11} \mathrm{atoms}/\mathrm{s}$

(3) $4.6\times {10}^{10} \mathrm{atoms}/\mathrm{s}$          

(4) $2.3\times {10}^8 \mathrm{atoms}/\mathrm{s}$

In a sample of radioactive material, what percentage of the initial number of active nuclei will decay during one mean life 

(1) 69.3%           

(2) 63%

(3) 50%               

(4) 37%

The S.I. unit of radioactivity is 

(1) Roentgen         

(2) Rutherford

(3) Curie               

(4) Becqueral

A radioactive material has an initial amount 16 gm. After 120 days it reduces to 1 gm, then the half-life of radioactive material is 

(1) 60 days             

(2) 30 days

(3) 40 days             

(4) 240 days

The activity of a sample of a radioactive material is ${\mathrm{A}}_1$, at time ${\mathrm{t}}_1$ and ${\mathrm{A}}_2$ at time ${\mathrm{t}}_2$ (${\mathrm{t}}_2$>${\mathrm{t}}_1$). If its mean life T, then 

(1) ${\mathrm{A}}_1{\mathrm{t}}_1={\mathrm{A}}_2{\mathrm{t}}_2$           

(2) ${\mathrm{A}}_1-{\mathrm{A}}_2={\mathrm{t}}_2-{\mathrm{t}}_1$
(3) ${\mathrm{A}}_2={\mathrm{A}}_1{\mathrm{e}}^{\left({\mathrm{t}}_1-{\mathrm{t}}_2\right)/\mathrm{T}}$   

(4) ${\mathrm{A}}_2={\mathrm{A}}_1{\mathrm{e}}^{\left({\mathrm{t}}_1/{\mathrm{t}}_2\right)\mathrm{T}}$

Nucleus produced due to $\mathrm{\alpha }$-decay of the nucleus ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}$ is

(1) ${}_{\mathrm{Z}+2}{}^{\mathrm{A}+4}\mathrm{Y}$                     

(2) ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{Y}$

(3) ${}_{\mathrm{Z}-2}{}^{\mathrm{A}-4}\mathrm{Y}$                    

(4) ${}_{\mathrm{Z}-4}{}^{\mathrm{A}-2}\mathrm{Y}$

Which of the following processes represents a gamma-decay

(1) ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}+\mathrm{\gamma }\to {}_{\mathrm{Z}-1}{}^{\mathrm{A}}\mathrm{X}+\mathrm{a}+\mathrm{b}$ 

(2) ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}+{}_0{}^1\mathrm{n}\to {}_{\mathrm{Z}-2}{}^{\mathrm{A}-3}\mathrm{X}+\mathrm{c}$

(3) ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}\to {}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}+\mathrm{f}$

(4) ${}_{\mathrm{Z}}{}^{\mathrm{A}}\mathrm{X}+{\mathrm{e}}^{-1 }\to {}_{\mathrm{Z}-1}{}^{\mathrm{A}}\mathrm{X}+\mathrm{g}$

The half-life of ${}_{215}\mathrm{At}$ is 100 $\mathrm{\mu s}$. The time taken for the radioactivity of a sample of ${}_{215}\mathrm{At}$ to decay to 1/16th of its initial value is 

(1) 400 $\mathrm{\mu s}$               

(2) 6.3 $\mathrm{\mu s}$ 

(3) 40 $\mathrm{\mu s}$                 

(4) 300 $\mathrm{\mu s}$

The half-life of a radioactive substance is 48 hours. How much time will it take to disintegrate to its $\frac{1}{16}$th part

(1) 12 h             

(2) 16 h

(3) 48 h             

(4) 192 h

A radioactive substance has an average life of 5 hours. In a time of 5 hours 

(1) Half of the active nuclei decay

(2) Less than half of the active nuclei decay

(3) More than half of the active nuclei decay

(4) All active nuclei decay