Tuesday, October 13, 2009

The discovery of the electron towards the end of the nineteenth century was the starting point of new avenues of research in science, which were to give physicists an insight into the structure and nature of the atoms of matter.

In 1896, Henri Becquerel discovered that when a photographic plate wrapped in black paper was placed near a salt of uranium, it got affected. The same observation was repeated with other salts of uranium. This led him to conclude that the uranium salt emitted some deep penetrating radiation. Further experiments showed that the intensity of the emitted radiation depended directly on the concentration of uranium in its various salts. The emission of radiation was completely unaffected by any change in the physical and chemical conditions of the system. Becquerel concluded that the origin of the radiation was somehow rooted in the nucleus of the uranium atom. The radiation discovered by Becquerel was initially named Becquerel rays.

Later on, Madam Curie and Pierre Curie discovered other substances i.e., polonium, radium, etc., which were more active than uranium.

These substances, which have the property of spontaneous emission of radiation are called radioactive substances and the process of spontaneous emission of radiation is called Radioactivity.

It was later found that all elements, which have an atomic number greater than 82 are naturally radioactive.

Radioactivity is a nuclear phenomenon. It is the spontaneous emission of radiation from the nucleus.

Alpha, beta and gamma radiations

In 1899, the study of radioactivity was taken up by Ernest Rutherford. He placed a little radium at the bottom of a small lead box and subjected the rays that emerged from it to the action of a very strong magnetic field at right angles to their direction. He found that the rays separated into three distinct constituents as shown in the figure below.

Alpha beta gamma radiations
Lead box containing radium

For convenience, Rutherford called the three types of radiation alpha (a), beta (b) and gamma (g) rays. The a-rays were deflected in a direction opposite to that of b-rays. This showed that the a-rays carried a positive charge, b-rays carried a negative charge and those which passed undeviated were neutral or uncharged were g-rays.

Similarly, if the radiations given out by a radioactive substance are subjected to an electric field perpendicular to their path, they separate into three constituents. Those which turn towards the negative plate are the positively charged alpha particles. Those, which turn towards the positive plate, are the negatively charged beta particles. Those, which pass undeviated, are the uncharged gamma radiations.

Further investigation has shown that an alpha ray is a stream of helium nuclei, a beta ray is a stream of electrons and a gamma ray is an electromagnetic radiation whose frequency is higher than that of X-rays.

Properties of alpha particles

  • They are deflected by electric and magnetic fields. The deflection is less compared to that of bparticles, because a particles are heavy.
  • They affect a photographic plate and cause fluorescence on striking a fluorescent material.
  • They ionise the gas through which they pass.
  • The mass of an alpha particle is 6.643 x 10-27 kg, which is roughly four times the mass of a proton. Its charge is +3.2 x 10-19 C, which is two times the charge of a proton. Thus its specific charges i.e., q/m value is 4.826 x 107 C/kg.
  • An alpha particle consists of two protons and two neutrons.
  • The speed of a-particle is of the order of 107 m/s.
  • Its penetrating power is very small.
  • They have large kinetic energy.
  • They destroy living cells and cause biological damage.
  • They get scattered while passing through thin mica or gold foils.

Properties of Beta particles

  • They get deflected by electric and magnetic fields. The deflection is large since a beta particle is lighter than an a-particle.
  • They affect photographic plates.
  • They ionise the gas through which they pass.
  • The mass is 9.1 x 10-31 kg and its charge is 1.6x10-19 C. Thus its specific charge q/m value is 1.76 x 1011 C/kg.
  • They are fast moving electrons emitted from the nucleus of the atom.
  • Its speed is of the order of 108 m/s.
  • The penetrating power of b particles is more than that of particles.
  • They cause fluorescence on striking a fluorescent material.
  • They produce X-rays when they are stopped by metals of high atomic number and high melting point such as tungsten.
  • They cause greater radiation damage as they can easily pass through the skin of the body.

Properties of gamma radiation

  • They are not deflected by electric and magnetic fields.
  • They affect photographic plates.
  • The ionising power is very low compared to alpha-particles and beta-particles.
  • They are electromagnetic waves like X-rays and light rays. The wavelength of these rays are shorter than that of X-rays.
  • The speed is the same as the speed of light.
  • The penetrating power is high.
  • They cause fluorescence when they strike a fluorescent material.
  • They are diffracted by crystals.
  • Although X-rays and g-rays have similar properties, their origin is different. X-rays originate in the electron cloud outside the nucleus, where as gamma rays originate in the nucleus.
  • They can easily pass through the human body and cause immense biological damage.
  • They are very useful in the treatment of cancer.
  • Distinction between the properties of a, b and g radiation
Property α-particle β-particle γ-radiation
Nature Stream of positively charged particles i.e, helium nuclei, Stream of negatively charged particles i.e., energetic electrons. Electromagnetic waves.
Speed Nearly 107 ms-1 About 90% of the speed of light or 2.7 x 108 ms-1 3 x lO8 ms-1 (in vacuum)
Rest mass 4 times the mass of proton i.e., 6.64 X 10-27 kg Massof electron i.e,9.1X10-31kg No mass
Charge Positive charge +3.2x10-19C (+2e) Negative charge —1.6x10-19C(-e) No charge
Specific charge[q/m] 4.83X107C kg-1 1.76X1011C kg-1 -
Wavelength 10-13m or 10-3A0
Effect of electric and magnetic fields Less deflected More deflected in direction opposite to alpha particles. Unaffected
Ionising power Maximum (10,000 times of γ) less than alpha(100 times of γ) Minimum
Penetrating power Small (3-8 cm in air) Large (About 1 mm of lead or about 5 mm of Al) Very large (About 30 cm of iron)
Biological damage Cause some damage Cause more damage Cause immense damage

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