2. Discovery of radioactivity
• Henery Bequrel during his studies of phosphorence (1896),found a
mineral Uranium salts produced invisible rays or emit radiation when
penetrate through small thickness of matter and effect photographic plate.
• The phenomenon of radioactivity is shown by nucleus of high atomic mass.
• In case of nucleus of high atomic mass the force of repulsion between
charged proton increase and hence the binding energy decreases. The
nucleus then becomes unstable
3. Radioactivity
• The phenomenon of spontaneous emission of
radiation by disintegration of nucleus is called
radioactivity and such substance which emit these
rays are called radioactive substance.
4. Type of emitting radiation
• Classified into three distinct type according to their penetrating power.
• 1-Alpha particle – positively charged ;can barely penetrate a piece of paper.
• 2-Beta particles – negatively charged ; pass through as much as 3 mm of
aluminum .
• 3-gamma rays- neutral ; extremely penetrating
5. Alpha-particle emission
• Emission of an alpha particle is associated with breakdown of heavy
elements such as uranium and radium
• Alpha particle is combination of four fundamental particle (two protons and
two neutrons)
• An alpha particle is identical in structure of the nucleus of helium atom.
6.
7. Properties of alpha particle
• Positive electric charge.
• They cause intense ionization in matter through which they pass.
• They are the heaviest of the particle emission .
• These rays are influenced by both electric and magnetic field, that’s why they
would be attracted towards a negatively charged electrode.
• Alpha particle have a very short range.
• The effect of alpha particles on living tissue is intense, but very localized .
8. Beta particle emission
• The emission of a negatively charged beta particle is associated with the breakdown
of nuclides which have to many neutrons.
• In this nuclides whose neutrons : protons ratio too high.
• Negative beta particle is identical to an electron having the same mass and carrying
the same charge.
• Beta particle may carry all of this energy , or it may carry only a part of the energy
released .
• In the latter case , the missing energy is carried by another particle known as an
Antineutrino( ν)
9.
10. Properties of beta particle
• Negative beta particle cause ionization in any medium they travels.
• They attracted by the positive charged.
• Beta particle have grater range than alpha particle and would not be much
attenuated by sheet of paper.
• These particle would be stopped by a layer of aluminium a few mm thick.
• Beta particle also have a localised damaging effect on body tissues.
11. Gamma ray emission
• Nucleus sheds it surplus energy by emitting one or more photons of high
energy electromagnetic radiation called gamma radiation.
• In case of prompt gamma decay , there is no measurable delay between the
emission of the beta particle and the emission of a gamma- ray photon.
• Temporary excited condition of the radionuclide is known as metastable
state and the subsequent transformation from metastable to stable state is
called an isomeric transition
• contd…..
12. Contd….
• Isotopes of barium, which was produced as a daughter product in our
example of beta negative emission , behave in this way:
13. Properties of gamma rays
• Gamma rays photons have no mass and carry no electric charge being
electromagnetic radiation rather than matter.
• These rays are not influenced by electric or magnetic fields , but they do have an
ionizing effect on matter, interacting with it in the same way as x-rays .
• Gamma rays can be far more penetrating than alpha or beta particles.
• Gamma rays able to penetrate into body tissues and are therefore used in the
radiotherapy of deep lying lesion.
• Gamma ray emitters are also used extensively for diagnostic purposes in the field of
nuclear medicine .
14.
15. Radioactive half life
• The time taken for the activity of any radionuclide to decrease by 50% is
called as radioactive or physical half life (t1⁄2 )
16. Contd…..
• The value of half life for different radio nuclides varies tremendously small
fraction of a sec (10-9s ) to over a billion years (1010 years).
• Decay constant and half life are related by the equation :
17. Half-lives use in medicine for several
reason
• 1-they are cheaper to manufacture .
• 2-the patient and staff receive less radiation dose because the activity reduce
more quickly .
• 3-when the treatment or investigation is over , the residual activity is low.
