Radioactivity in Physics

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Radioactivity in Physics

  • Hendry Becquerel ( 1896 ) observed that a photographic plate blackened, when placed near double sulphate of potassium and uranium.
  • He further observed that uranium emitted special kind of Rays. They were called Becquerel Rays.
  • Pierre and Marrie Curie observed that the radiation from pitchblende was four times stronger than uranium.
  • In 1898, they finally discovered two new substances – Polonium 84Po210 ) and Radium ( 88Ra226 )
  • These newly discovered substances were called radioactive substances and this property of these substances was named radioactivity.
  • Many more radioactive substances were discovered later on.

Schmidt and Curie ( 1898 ) discovered Thorium and Debierne ( 1899 ) discovered Actinium

  • Radioactivity is exhibited by elements of high atomic weight ( Z > 82 ) where nuclei are unstable and break up of their own accord into simpler elements giving out radiation.
  • The process is spontaneous i.e., it can neither be started, stopped, accelerated nor retarded under any physical circumstances.

Properties of Radioactive Substances :

  • They affect a photographic plate like light or X-Rays.
  • They penetrate through matter, the thickness depending upon the nature of radiations and source.
  • They ionize the gas through which they pass.

Nature of Radioactive Rays :

  • Rutherford ( 1902 ) studied the effects of electric and magnetic fields on the radioactive Rays emitted by different radioactive substances.
  • He kept a radioactive substance in a thick walled lead box and subjected an electrostatic field to the radioactive Rays emerging from a narrow opening in the box.
  • He also studied the behaviour of these radiations in magnetic field.
  • He observed that radioactive Rays or particles are three types.

(1) Alpha Rays ( α – Rays ) : Rays which are deflected towards the negative plate are called alpha Rays.

Properties of α – Rays

  • α – particles carry double the positive charge of proton and are four times as heavy.
  • α – particles produce fluorescence in substance like zinc sulphide and barium platinocyanide.
  • α – particles are deflected by electric and magnetic fields.
  • α – particles get scattered while passing through metal foils.
  • α – particles affect a photographic plate.
  • α – particles can produce artificial radioactivity in certain elements and can produce nuclear reaction.
  • α – particles may cause burns on human body.
  • On being stopped α – particles produce heating effect.

(2) Beta Rays ( β – Rays ) : Rays which are deflected towards the positive plate are called beta Rays.

Properties of β – Rays

  • β – particles can easily through a few mm thick aluminium foil.
  • The range of β – particles in air is several meters
  • β – particles can also produce fluorescence in materials like zinc sulphide and barium platinocyanide.
  • β – particles are deflected by electric and magnetic fields.
  • β – particles are affect a photographic plate.
  • They can produce artificial radioactivity.

(3) Gamma Rays ( γ – Rays ) : Rays which go undeflected by the electric field are called Gamma Rays.

Properties of γ – Rays

  • They are electromagnetic waves and move with velocity of light.
  • They are highly penetrating.
  • They can produce fluorescence in a substance like willemite.
  • They can affect photographic plate
  • They are not deflected by electric and magnetic field.
  • They knock out electrons from the surface on which they fall.
  • Hard γ – Rays are used in radiotherapy.

No radioactive substance emits both α and β – particles simultaneously.

Some substances emits α – particles while some other emits β – particles. γ – Rays are emitted along with both α – and β – particles.

Sody – Fajan’s Displacement Law :

  • When a radioactive atom emits an α – particle ( mass 4 and charge 2e ), it is converted into another element of atomic number two less and atomic weight four less than that of the parent element and thus the place of the new atom is two groups lower in the periodic table.

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  • When a radioactive atom emits a β – particle ( mass – 0, charge – e ) it is converted into a new element of atomic number one greater than the parent element but of the same atomic weight as the parent substance and thus its place is shifted one group higher in the periodic table. These are isotopes.
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Carbon Dating :

  • It is used to find the age of earth and old civilizations in archaeology. This method is developed by Libby.
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  • The half – life of C14 is 5730 yr and it emits a β – particle of 0.155 MeV transforming itself back in N14.

The age is calculated by the formula :

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