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Radioactivity





1) Introduction

  • Phenomenon of radioactivity was first discovered by A.H.Bacquerel in 1896 while studying fluorescence and phosphorence of compounds irradiated by visible light
  • these phosphorescent materials glow in dark after being exposed to visible light
  • while conducting experiment on uranium salts, he found that uranium salts has a capability to blacken the photographic plate kept in a dark place wrapped through a paper
  • Subsequent experiments showed that radioactivity is a nuclear phenomenon in which an unstable nucleus under goes a decay process referred as radioactive decay
  • There are three types of radioactivity decays that occur in nature .These are α decay ,β decay and γ decay.
  • We now define radioactive decay as the process by which unstable atomic nucleus looses energy by emitting ionizing particles or radiations ( α,β and γ rays)
  • Radioactive decay of an atomic nucleus is a spontaneous process and can occur without any interaction of other particles outside the atom
  • This process of radioactive decay is random and we can not predict whether a given radioactive atom will emit radiations at a particular instant of time or not
  • Phenomenon of radioactivity is observed in heavy elements like uranium and unstable isotopes like carbon 14



2) Properties of radioactive decay

  • Radioactive rays ionize the surrounding air and affect photographic plate
  • Radioactive rays acts differently on different biological cells and tissues
  • A beam of radioactive rays from a radium sample into three components in presence of strong magnetic or electric fields
I.The alpha rays(particles)
  • The alpha particles are nuclei of helium atoms
  • Alpha particles was first identified by Rutherford and Royds in 1909 by spectroscopic method where they found traces of helium in an originally pure sample of Radon gas which is an α emitter.
  • Examples of α decay are

    (A)     222Rn864He2+218Po84

    (B) 238U924He2+234Th90

  • α rays can be stopped by thin sheet of paper.
  • α rays can cause intense ionization in air.
  • Any group of α particles emitted from same type of nuclei always have definite energy and definite velocity.
  • Most α particles are emitted with velocities between ˜ 1.5x107 and ˜ 2.2x107.
  • The α particles cover a definite distance in a material without any loss of intensity and suddenly in a small distance they are absorbed completely.
  • The distance α rays travel within a given material is called their range in that material.
II.The beta rays(particles)
  • β particles are identical with electrons.
  • They have mass 1 ⁄ 1836 of mass of proton.
  • examples of β decay are
    (A)     234Th90234Pa91+e-

    (B) 210Bi83210Po84+e-

    (C) 14C614N7+e-

  • Mass number and charge are conserved and the daughter product moves one place up in the periodic table, as loss of negative charge by nucleus implies gain of positive charge.
  • β rays cause much less ionization in air , but are ˜ 100 times more penetrating then α rays.
  • β rays can penetrate a aluminum sheet of few mm thickness.
  • A particular β active element emits β particles with energies varying between zero and a certain maximum.
  • This maximum energy is called end point energy.
III. The Gamma rays:-
  • They are part of EM spectrum λγ < λX-rays
  • γ ray photons are more energetic and more penetrating then X-rays photons
  • λγ rages between 1.7X10-8 cm and 4.0X10-6 cm
  • Ionization due to Gamma rays is a photoelectric effect
  • Owing to their large energies ,the Gamma rays photons can dislodge electrons not only from outer orbits( valence orbits on conduction band) of atoms but also from the inner orbits
  • Besides photoelectric effect ,gamma rays loose energy by
    i) Compton effect ,in which the gamma photon collides with an electron and gets scattered with a shift in wavelength

    ii) Pair production ,in which a gamma photon is converted into a pair consisting of an electron and a positron( particle having mass and charge equal to electron but carrying positive change)



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