At any instant the ratio of the amount of radioactive substances is 2 : 1. If their half lives be respectively 12 and 16 hours, then after two days, what will be the ratio of the substances?

(1) 1 : 1                 

(2) 2 : 1

(3) 1 : 2                 

(4) 1 : 4

Which of the following isotopes is used for the treatment of cancer 

(1) ${\mathrm{K}}^{40}$        

(2) ${\mathrm{Co}}^{60}$

(3) ${\mathrm{Sr}}^{90}$       

(4) ${\mathrm{I}}^{131}$

If half-life of a radioactive atom is 2.3 days, then its decay constant would be 

(1) 0.1           

(2) 0.2

(3) 0.3           

(4) 2.3

A radio isotope has a half life of 75 years. The fraction of the atoms of this material that would decay in 150 years will be

(1) 66.6%               

(2) 85.5%

(3) 62.5%               

(4) 75%

An artificial radioactive decay series begins with unstable ${}_{94}{}^{241}\mathrm{Pu}$. The stable nuclide obtained after eight α decays and five $\mathrm{\beta }$-decays is 

(1) ${}_{83}{}^{209}\mathrm{Bi}$         

(2) ${}_{82}{}^{209}\mathrm{Pb}$

(3) ${}_{82}{}^{205}\mathrm{Ti}$         

(4) ${}_{82}{}^{201}\mathrm{Hg}$

The activity of a sample is $64\times {10}^{-5}$ Ci. Its half-life is 3 days. The activity will become $5\times {10}^{-6}$ Ci after 

(1) 12 days         

(2) 7 days

(3) 18 days         

(4) 21 days

The half-life of radon is 3.8 days. Three forth of a radon sample decay in

(1) 5.02 days         

(2) 15.2 days

(3) 7.6 days           

(4) 11.4 days

A nucleus of an element ${}_{84}{}^{202}\mathrm{X}$ emits an $\mathrm{\alpha }$-particle first, a $\mathrm{\beta }$-particle next and then a gamma photon. The final nucleus formed has an atomic number 

(1) 200           

(2) 199

(3) 83             

(4) 198

Plutonium decays with a half-life of 24000 years. If the plutonium is stored for 72000 years, then the fraction of plutonium that remains is 

(1) $\frac{1}{2}$          

(2) $\frac{1}{3}$
(3) $\frac{1}{4}$          

(4) $\frac{1}{8}$

A radioactive substance emits 

(1) $\mathrm{\alpha }$-rays               

(2) $\mathrm{\beta }$-rays

(3) $\mathrm{\gamma }$-rays                 

(4) All of these