Radioactivity: The Start of Modern Physics

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Nineteenth century was the century of science evolution. Many revolutionary ideas were presented during this century. In biology, Charles Darwin introduced the idea of evolution and natural selection. The germ theory of diseases was proved. In physics, the experiments, theories and discoveries of the likes of Michael Faraday, Andre-Marie Ampere, James Clerk Maxwell, and others led to the creation of electromagnetism as a new branch of science. Thermodynamics led to an understanding of heat and the notion of energy was defined. However, one of the most influential discoveries of all times in physics was the discovery of the phenomenon of radioactivity.

Radioactivity,or radioactive decay, is a spontaneous process in which an unstable atomic nucleus, the positively charged core of an atom, loses energy by emitting radiations like alpha particles, beta particles, and gamma radiation. The materials that undergo radioactive decay are called radioactive elements. On the periodic table, the elements with atomic numbers greater than 82 are all radioactive. duration in which half of the atomic nuclei undergo radioactive decay and turn into nuclei of another element is called half-life. After the first half-life, half of nuclei change into nuclei of another element. After the second half-life, the half of remaining atomic nuclei undergo radioactive decay. Each element has a specific half-life.

Pictorial representation of radioactivity phenomenon

The discovery of radioactivity was accidental. In 1896, a French physicist, Henri Becquerel, was using naturally fluorescent minerals to study the properties of X-rays. He exposed potassium uranyl sulfate (K2UO2(SO4)2) to sunlight and then placed it on photographic plates wrapped in black paper, believing that the uranium absorbed the sun’s energy and then emitted it as x-rays. To his dismay, this hypothesis was disproved. For some reason, Becquerel decided to develop his photographic plates anyway. To his surprise, the images were strong and clear, proving that the uranium emitted radiation without an external source of energy such as the sun. Hence, Becquerel had discovered radioactivity.

Henry Becquerel

In 1897, Marie Curie and her husband Pierre Curie started investigating natural ores that exhibited radioactivity. During their experiments, they discovered two new elements: polonium and radium. They went on to share the Nobel Prize in Physics with Henri Becquerel for their work. In 1899, Rutherford, discovered, through a series of experiments, that there were two types of radiations emitted from uranium. He named them alpha and beta radiations. In 1900, P. Villard had also found an additional penetrating radiation from uranium. This radiation was neither alpha nor beta, but more penetrating than the beta particles; it was given the name gamma.

The most prominent processes in radioactive decay are alpha decay, beta decay and gamma decay. Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms or ‘decays’ into a different atomic nucleus, with its mass number reduced by four and its atomic number reduced by two. Beta decay occurs when, in a nucleus with too many protons or too many neutrons, one of the protons or neutrons is transformed into the other. Gamma decay is a type of radioactivity in which some unstable atomic nuclei dissipate excess energy by a spontaneous electromagnetic process. Together, these three processes were the first phenomena to be observed in experiments on radioactive elements.

Fission process occurs in which a heavy nucleus splits into two different elements with lighter nuclei, accompanied by the emission of a very large amount of energy. When fission reaction is left uncontrolled, it releases very huge amount of kinetic energy and very high temperature which is almost equal to surface temperature of sun (6000ºC). This uncontrolled fission process is what fuels the atomic bomb.

Fission Reaction

Fusion reaction is the nuclear process in which two lighter elements fuse to form a heavier element. The repulsive force among the nuclei is very strong. In order to overcome this force, very high energy is required to bring the nuclei very close to fuse them together. This massive amount of energy is provided by a fission reaction before fusion. A typical example of fusion reaction is the fusion of hydrogen isotopes to form a helium nucleus. Hydrogen bomb is based on this principle.

Fusion Reaction

The applications of radioactive materials are almost uncountable. Cobalt-60 is a radioactive isotope that has been used to cure cancer. It is applied to the body via a medical treatment, where it destroys cancerous cells by radioactive exposure. Gamma radiation is used for sterilization process. Through carbon dating (C-14 isotope), samples of wood, charcoal, bone, and shell have been identified as having lived from 1 000 to 25 000 years ago. This knowledge has helped researchers reconstruct the history of living organism—including human—during that time span.

Radiation is not without its severe hazards and dangers. Radiations are highly penetrating, and they can alter chemical nature of molecules in our body. Exposure to very high levels of radiation can cause skin burns, cancer and cardiovascular disease. Low levels of radiation encountered in the environment does not cause immediate health effects. When radiations penetrate the body, they can damage the genetic information and can thus cause uncontrolled mutation of cells, which ultimately results in cancer. Even if the person does not show symptoms of cancer, the damaged genes can transfer to next generations which will cause cancer and other dangerous diseases in next generations – as evident in Japan where people were exposed to high levels of radiations during the atomic blast.

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