Radioactivity

Radioactivity was discovered accidentally in 1896 by the French scientist Antoine Henry Becquerel. Studying a uranium mineral (pitchblende), Becquerel observed phosphorescence (a property of certain materials that allows them to absorb energy and then emit it in the form of radiation) without the mineral having been previously exposed to light. He found that the mineral emitted radiation capable of marking a photographic plate. Like X-rays, the rays from the pitchblende mineral were of high energy and it was impossible to deflect them with an external electromagnetic field. However, they differed from X-rays in that they were emitted spontaneously from the uranium material.

Marie Curie and her husband, Pierre, began to study the phenomenon and discovered that other minerals had the same property as pitchblende. They determined that the phenomenon was unique to the nuclei of atoms. Marie Curie (Becquerel’s disciple), proposed the term radioactivity for these spontaneous radiations of particles and energy. At the end of 1897, the Curies discovered two other radioactive elements, polonium (Po) and radium (Ra).

In 1902 Ernest Rutherford demonstrated that radioactivity generated spontaneous transformations and thus one element could be transformed into another. In 1903 Marie, Pierre and Becquerel received the Nobel Prize in Physics for the discovery of natural radioactivity. Finally, in 1911 Marie Curie isolated radium and obtained its atomic mass, the discovery earned her a second Nobel Prize.

Disintegration Processes

When an atom has an unstable nucleus, it can spontaneously emit radiation. These radiations occur with release of energy and often accompanied by particles with mass (neutrons for example). It is also possible (in nuclear reactors) to generate emissions from a non-radioactive atom. In both cases, the phenomenon is exothermic (energy is released) and the daughter nuclei generated are always more stable than their predecessor.

Considering the above reactions, they can be classified into two types. The first type of emission is called natural radioactivity while the emission from a stable, non-radioactive nucleus is known as artificial radioactivity.

In general, 5 types of natural radioactive decays are known:

  • Alpha particle emission (α)
  • Emission of beta particles (β)
  • Emission of gamma rays (γ)
  • Positron emission (β+)
  • Electronic capture (EC)
  • Alpha Emissions ()

Corresponds to particles with positive electric charge +2 and 4 atomic mass units. They are helium nuclei with low penetrating power and high ionizing capacity. They are preferentially emitted in high mass nuclei.

Beta Emissions ()

These are negatively charged particles (electrons) that travel at high speed. They are deflected by an electromagnetic field and are much more penetrating than alpha radiation. Beta emissions come from the nucleus resulting from the disintegration of a neutron. The atom remaining from the disintegration increases its atomic number by 1 unit, but maintains its mass number (we must remember that proton and neutron have similar masses).

Along with beta particles, antineutrinos (ν) are also emitted, which have no electric charge and have a mass less than 4-10-5 times the mass of the electron (energy). Whenever a radioactive atom disintegrates emitting a beta particle, the daughter nucleus will be isobarous of the atom that generated it.

Gamma emission (γ)

Corresponds to high-energy (electromagnetic) radiation without mass or electric charge. Some isotopes are known to emit gamma rays in pure form. Gamma emission can occur when a radioelement exists in two different forms (nuclear isomers), both with the same atomic number and mass number but with different energy. Gamma emission accompanies the transition from the higher energy isomer to the lower energy form. An example of this isomerism is the isotope protactinium 234 (Pa), which exists with 2 different energy states, and in which gamma-ray emission indicates the transition from one to the other.

In gamma ray emission there is no change in the number of protons and neutrons in the nucleus, therefore, there is no transmutation (change in the Z number of an element).

Positron emission ( or β+)

Positron emission occurs when a proton in the nucleus is transformed into a neutron by emitting a particle called a positron (β+).

Electronic Capture (EC)

Electronic capture occurs when an electron from the innermost layers of the atom falls into the nucleus, transforming a proton into a neutron. This causes a decrease in the atomic number, but the mass number remains constant.

Transmutation

This is the phenomenon where one atom is transformed into another by a change in the number of protons. This transformation can be natural when an atom emits radiation, positrons or electron capture, or it can also be produced by artificial means (bombardment of a stable nucleus with neutrons). In both cases the new element can also be radioactive and will continue to emit until it is transformed into another.

Radioactivity and its applications

Radioactive phenomena are used in many branches of science, with chemistry, physics and medicine having the greatest potential for application. Radioactive isotopes are used in nuclear medicine, mainly in medical imaging, to study the mode of action of drugs, to understand the functioning of the brain, to detect cardiac abnormalities, to discover cancer metastases, among others.

In industry: X-rays of alloys to detect faults, production control by measuring thickness, control of wear of materials, studies of detergents, detection of filtrations or leaks, generation of electric current, food preservation, sterilization of surgical instruments.

In chemistry: Use of tracers in reactions to be studied, analysis by neutron activation to determine traces of impurities (the latter is widely used in space science, geology, ecology, etc.).

In agriculture: in tracers to study how plants absorb fertilizers, insecticides and other products, to increase food preservation, to obtain, by mutations, more resistant and productive cereals, to better study animal feeding, to increase milk and egg production, etc.

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