RADIOACTIVITY

Radioactivity is the emission of radiation by unstable nuclei. That radiation may exist in the form of subatomic particles (primarily alpha and beta particles) or in the form of energy (primarily gamma rays).

• Radioactivity was discovered accidentally in 1896 by French physicist Henri Becquerel (1852–1908).
• In the decades that followed Becquerel’s discovery, research on radioactivity produced revolutionary breakthroughs in our understanding of the nature of matter and led to a number of important practical applications.
• These applications include a host of new devices and techniques ranging from nuclear weapons and nuclear power plants to medical techniques that can be used for diagnosing and treating serious diseases.

Stable and unstable nuclei

• The nucleus of all atoms (with the exception of hydrogen) contains one or more protons and one or more neutrons. The nucleus of most carbon atoms, for instance, contains six protons and six neutrons.
• In most cases, the nuclei of atoms are stable; that is, they do not undergo changes on their own. A carbon nucleus will look exactly the same a hundred years from now (or a million years from now) as it does today.

But some nuclei are unstable. An unstable nucleus is one that undergoes some internal change spontaneously. In this change, the nucleus gives off a subatomic particle, or a burst of energy, or both.

As an example, an isotope of carbon, carbon-14, has a nucleus consisting of six protons and eight (rather than six) neutrons. A nucleus that gives off a particle or energy is said to undergo radioactive decay, or just decay.

 WORDS TO KNOW

• Alpha particle: The nucleus of a helium atom, consisting of two protons and two neutrons.
• Beta particle: An electron emitted by an atomic nucleus.
• Gamma ray: A high-energy form of electromagnetic radiation.
• Isotopes: Two or more forms of an element with the same number of protons but different numbers of neutrons in their atomic nuclei.
• Nucleus (atomic): The core of an atom, usually consisting of one or more protons and neutrons.
• Radioactive decay: The process by which an atomic nucleus gives off radiation and changes into a new nucleus.
• Radioactive family: A group of radioactive isotopes in which the decay of one isotope leads to the formation of another radioactive isotope.
• Stable nucleus: An atomic nucleus that does not undergo any changes spontaneously.
• Subatomic particle: Basic unit of matter and energy (proton, neutron, electron, neutrino, and positron) smaller than an atom.
• Unstable nucleus: An atomic nucleus that undergoes some internal change spontaneously.

Scientists are not entirely clear as to what makes a nucleus unstable. It seems that some nuclei contain an excess number of protons or neutrons or an excess amount of energy. These nuclei restore what must for them be a proper balance of protons, neutrons, and energy by giving off a subatomic particle or a burst of energy.

In this process, the nucleus changes its composition and may actually become a different nucleus entirely. For example, in its attempt to achieve stability, a carbon-14 nucleus gives off a beta particle.

After the carbon-14 nucleus has lost the beta particle, it consists of seven protons and seven neutrons. But a nucleus consisting of seven protons and seven neutrons is no longer a carbon nucleus. It is now the nucleus of a nitrogen atom. By giving off a beta particle, the carbon-14 atom has changed into a nitrogen atom.

Alpha (a) decay occurs when the neutron to proton ratio is too low. Alpha decay emits an alpha particle, which consists of two protons and two neutrons.

• This is the same as a helium nucleus and often uses the same chemical symbol ⁴He₂. Alpha particles are highly ionizing (e.g. deposits energy over a short distance).
• Since alpha particles lose energy over a short distance, they cannot travel far in most media. For example, the range of a 5 MeV alpha particle in air is only 3.5 cm.
• Consequently, alpha particles will not normally penetrate the outermost layer of the skin. Therefore, alpha particles pose little external radiation field hazard.
• Shielding of alpha particles is easily accomplished with minimal amounts of shielding. Examples of alpha particle emitting radio-nuclides include

 ²³⁸U, ²³⁹Pu, and ²⁴¹Am.

²³⁸U₉₂ à ²³⁴Th₉₀ + ⁴He₂.

²³⁹Pu₉₄ à ²³⁵U₉₂ + ⁴He₂.

²⁴¹Am₉₅ à  ²³⁷Np₉₃  + ⁴He₂.

After the emission of an a particle, the daughter product remaining, will be reduced by 4 in its mass number, and 2 in its atomic number, as could be verified in the examples above.

Beta (b^–) decay occurs when the neutron to proton ratio is too high. The radioactive nucleus emits a beta particle, which is essentially an electron, in order to bring this to a more favourable ratio.

• Beta particles are less ionizing than alpha particles. The range of beta particles depends on the energy, and some have enough to be of concern regarding external exposure.
• A 1 MeV beta particle can travel approximately 12 feet in air. Energetic beta particles can penetrate into the body and deposit dose to internal structures near the surface.
• Since beta particles are less ionizing than alpha particles, greater shielding is required. Low Z materials are selected as beta particle shields to take care of X-ray emissions associated with slowing down of beta particles while they travel in a medium.

In b emission, the neutron to proton ratio is reduced by converting a neutron into proton as:

¹n₀  à ¹p₁ + e-.  The electron ejected is the b particle that is released. Thus b emission results in the increase of the proton number, i.e. Z, by 1, but the mass number A is unaltered. Example of bdecay:  ⁴⁰K₁₉ à ⁴⁰Ca₂₀ + β^-1

Gamma (g) rays are not particulate radiation like the alpha and beta, but a form of high-energy electromagnetic wave.

• Gamma rays are the least ionizing of the three forms discussed. A 1 MeV gamma ray can travel an average of 130 meters in air.
• Since gamma radiation can travel far in air, it poses a significant external radiation hazard. Further, if ingested, it may pose an internal radiation hazard.
• Shielding of gamma rays is normally accomplished with high atomic number materials such as lead. [Gamma rays are electro-magnetic radiations with energies higher than X-rays.  X-rays are produced when electrons of an atom jump from one orbital location to another.
• The gamma rays are released when an atomic nucleus releases its excess energy. It is clear from this that nuclear transitions involve much larger energies than the atomic transitions. In other words, energies of nuclear origin are many (10³ – 10⁶) times greater than the energies of atomic origin].

Emission of γ-rays doesn’t change the Mass number nor Atomic number. If an atom is in exited state it comes to stable state by emitting a γ radiation.

Usually after α or β decay, the product nucleus is formed in an excited state and it reaches a stable state after γ emission.

LEARNING ALL ABOUT RADIOACTIVITY