A radioactive substance has a half life of 60 minutes. After 3 hours, the fraction of atom that have decayed would be

(1) 12.5%               

(2) 87.5%

(3) 8.5%                 

(4) 25.1%

An element used for radioactive carbon dating for more than 5600 years is 

(1) C-14         

(2) U-234

(3) U-238       

(4) Po-94

N atoms of a radioactive element emit n alpha particles per second. The half life of the element is

(1) $\frac{\mathrm{n}}{\mathrm{N}}$ sec                   

(2) $\frac{\mathrm{N}}{\mathrm{n}}$ sec

(3) $\frac{0.693 \mathrm{N}}{\mathrm{n}}$ sec           

(4)  $\frac{0.693 \mathrm{n}}{\mathrm{N}}$ sec

The radioactive nucleus ${}_7{}^{13}\mathrm{N}$ decays to ${}_6{}^{13}\mathrm{C}$ through the emission of 

(1) Neutron             

(2) Proton

(3) Electron             

(4) Positron

The radioactive decay of uranium into thorium is expressed by the equation ${}_{92}{}^{238}\mathrm{U}\to {}_{90}{}^{234}\mathrm{Th}+\mathrm{X}$ where 'X' is

(1) An electron               

(2) A proton

(3) A deuteron                 

(4) An alpha particle

In a radioactive decay, neither the atomic number nor the mass number changes. Which of the following would be emitted in the decay process 

(1) Proton             

(2) Neutron

(3) Electron           

(4) Photon

The half life period of a radioactive substance is 5 min. The amount of substance decayed in 20 min will be

(1) 93.75%                 

(2) 75%

(3) 25%                       

(4) 6.25%

In the uranium radioactive series, the initial nucleus is ${}_{92}{}^{238}\mathrm{U}$ and the final nucleus is ${}_{82}{}^{206}\mathrm{Pb}$. When the uranium nucleus decays to lead, the number of  $\mathrm{\alpha }$-particles emitted will be 

(1) 1               

(2) 2

(3) 4               

(4) 8

Which of the following is a correct statement

(1) Beta rays are same as cathode rays

(2) Gamma rays are high energy neutrons

(3) Alpha particles are singly ionized helium atoms

(4) Protons and neutrons have exactly the same mass

The half life of ${}_{131}\mathrm{I}$ is 8 days. Given a sample of ${}_{131}\mathrm{I}$ at time t = 0, we can assert that 
(1) No nucleus will decay before t = 4 days

(2) No nucleus will decay before t = 8 days

(3) All nuclei will decay before t = 16 days

(4) A given nucleus may decay at any time after t = 0