Radioactivity: Alpha Decay


What is radioactivity?

Due to nuclear instability, an atom’s nucleus exhibits the phenomenon of Radioactivity. Energy is lost due to radiation that is emitted out of the unstable nucleus of an atom. Two forces namely the force of repulsion that is electrostatic and the powerful forces of attraction of the nucleus keep the nucleus together. These two forces are considered extremely strong in the natural environment. The chance of encountering instability increases as the size of the nucleus increases because the mass of the nucleus becomes a lot when concentrated. That’s the reason why atoms of Plutonium, Uranium are extremely unstable and undergo the phenomenon of radioactivity.

Henry Becquerel discovered radioactivity by accident. A Uranium compound was placed in a drawer containing photographic plates, wrapped in a black paper. When the plates were examined later it was found that they were exposed! This exposure gave rise to the concept of Radioactive decay. Radioactivity can be seen in such forms

  • Gamma Decay (Photons having high energy are emitted)
  • Beta Decay (Emission consists of Electrons)
  • Alpha Decay (Emission consists of Helium nucleus)

Alpha Decay

Alpha decay is a type of radioactive decay where the unstable atomic nuclei emit a helium nucleus (alpha particle) and in the process transforms into another more stable element. The particle ejected out, the alpha particle, consists of 4 nucleons; two neutrons and two protons. Alpha radiation reduces the ratio of protons to neutrons in the parent nucleus, bringing it to a more stable configuration. An alpha particle is identical to the nucleus of a helium atom. The first observations and investigation into alpha decay were made by Ernest Rutherford who used alpha particles in his gold foil scattering experiment. As the alpha particle; made of two protons and neutron; exits the nucleus, the atomic number of the radioactive sample changes. The element left behind after alpha decay is 2 atomic numbers lesser and four mass number lower. For example, Uranium 238 92 decays to form Thorium23490.

\(_{92}^{238}\textrm{U} \rightarrow _{90}^{234}\textrm{Th} + _{2}^{4}\textrm{He}\)

Thus the transformation of a nucleus in an alpha decay can be written as;

\(_{Z}^{A}\textrm{X} \rightarrow _{Z-2}^{A-4}\textrm{Y} + _{2}^{4}\textrm{He}\)

Alpha decay occurs only in the heaviest of the elements. The nucleus of the element should be large or unstable enough to undergo spontaneous fission-type changes. It is the most common form of decay in such elements. The alpha particles emitted out of the nucleus usually have an energy level of around 5 MeV and have a speed of around 5% of light. It is important to note that alpha particles possess a charge of +2 due to the absence of electrons. Due to this charge and owing to its heavy mass, an alpha particle reacts with the surroundings vigorously to lose all of its energy almost immediately. Their forward motion can be stopped by a few centimetres of air.

Owing to their heaviness and their charge, this kind of radioactive decay reacts most violently with the human body. They have a high ionizing power due to which they can wreak havoc with tissue. An overdose of alpha radiation results in the formation of blisters and burns on the victim’s bodies.

Uses of Radioactivity

Americium-241 is an alpha emitter and is used for domestic smoke detectors in the United States. The alpha particles given out by the Americium sample ionizes the air in the chamber of the smoke detector leading to a small current in the chamber. When smoke enters the chamber, it causes a drop in current causing the alarm to go off. Although Alpha particles have a very short range, they are devastating when in close contact. Alpha emitters, when swallowed, come in close contact with tissue and are deadly in such circumstances and is therefore used in assassination attempts by radiation poisoning.

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Practise This Question

The relation between the gas pressure P and average kinetic energy per unit volume E is