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NUCLEAR CHEMISTRY (Nature of Radioactive Radiations)

Cnx chem 21 03 radiation

NATURE OF RADIOACTIVE RADIATIONS

 

In 1902,the Newzealand physicist Ernest Rutherford investigated the nature of the radiation emitted by radioactive substance, Pitch  blende (U3O8). Ernest Rutherford classified these rays in three types by passing through a strong  electric or magnetic field as shown in the figure:


              Figure: Emission of radioactive radiations

In the experiment , Pitch blende is taken in a lead box with small hoe at the top. when the radiation were passed through two oppositely charged plates( electric fields at right angle to the propagation),then the photographic plate is affected at three positions:

1.The radiations bending towards the negative plate (cathode) are positively charged rays, called alpha rays (α-rays). These are deflected to lesser extent from the original direction showing that they have large mass .

2.The radiations bending toward the positive plate (anode) are the negatively charged rays, called beta rays(β-rays). these rays are deflected to a greater extent showing that they are lighter than α-rays. 

3.The radiations which remained undeflected and passes straight forward are neutral in nature and are called gamma rays (γ- rays).

 

Properties ofα- rays

  1.      Nature: They are not rays, but are doubly ionized helium atom (He++), have 2 units of positive charge and 4 units of mass.
  2.      Velocity: They travel at the velocity between 1.5 to 2 *107m/s or nearly 1/10th of that of light.
  3.          Penetrating power:They an penetrate thin sheets (<0.1 mm thickness) of    aluminium and mica.
  4.      Ionizing power: They have high ionizing power and can ionize the gas through   which it pass due to the positive charge of  α- particle.
  5.    Luminescence: They cause luminescence on striking a zinc sulphide screen i.e. it     affects photographic plates.

Properties of β- rays

  1.       Nature: They are the stream of electrons having one negative charge and zero   mass (β, -1e0).
  2.      Velocity: They move at the velocity between 3*107 to 2.7*108 m/s i.e almost 30% to 90% of that of light. 
  3.      Penetrating power: They have high penetrating power than α- rays but can be stopped by about 1 cm thick aluminium sheet. 
  4.      Ionizing power: The ionizing power of the β- rays is 1/100th of the α-rays. 
  5.      Luminescence: They have less effect on the ZnS screen. This is due to the production of X-rays  on the interaction of β- rays with the matter.

Properties of γ- rays 

  1.      Nature:They are high energy electromagnetic radiations and are not deflected in the electric and magnetic field. This indicates that they do not carry any charge and no mass.
  2.      Velocity:They travel with the speed of light i.e. 3*108 m/s. As they don’t carry any charge and mass so they cannot be considered as particle.
  3.      Penetrating power:They have very high penetrating power than that of both α- rays and β- rays. They can penetrate upto a 100 cm thick aluminium sheet.
  4.      Ionizing power: They have very low ionization power since they do not possess mass.
  5.    Luminescence:They have no effect on the ZnS plate or the photographic plate.



COMPARISION AMONG THE PROPERTIES OF α, β & γ 


S.N.Characteristics

α-rays 

β-rays 

γ-rays 

1. NatureParticleParticle

Electromagnetic

radiation

2.Symbol

α, 2He4

β, -1e0

γ
3.Charge+2-1 0
4.MassApprox. 4 unit

About 1/1840 of H-atom

0
5.Ionizing powerMaximum

About 1/100 of that of α-particle

very low
6.Velocity

About 1/20 to 1/10 of that of light

About 30% to 90% of the light 

Same as the speed of light
7.Penetrating powerleast(2-7 cm in air)less  (several meters in air)very high
8.Kinetic EnergyMaximumLess         -----



Figure: Penetrating power of α, β γ  radiations