Radioactive decay
Radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is considered radioactive. Three of the most common types of decay are alpha, beta, and gamma decay. The weak force is the mechanism that is responsible for beta decay, while the other two are governed by the electromagnetic and nuclear forces.
Radioactive decay is a random process at the level of single atoms. According to quantum theory, it is impossible to predict when a particular atom will decay, regardless of how long the atom has existed. However, for a significant number of identical atoms, the overall decay rate can be expressed as a decay constant or as a half-life. The half-lives of radioactive atoms have a huge range: from nearly instantaneous to far longer than the age of the universe.
The decaying nucleus is called the parent radionuclide, and the process produces at least one daughter nuclide. Except for gamma decay or internal conversion from a nuclear excited state, the decay is a nuclear transmutation resulting in a daughter containing a different number of protons or neutrons. When the number of protons changes, an atom of a different chemical element is created.
There are 28 naturally occurring chemical elements on Earth that are radioactive, consisting of 35 radionuclides that date before the time of formation of the Solar System. These 35 are known as primordial radionuclides. Well-known examples are uranium and thorium, but also included are naturally occurring long-lived radioisotopes, such as potassium-40. Each of the heavy primordial radionuclides participates in one of the four decay chains.
History of discovery
laid the seeds for the discovery of radioactivity through his interest in and studies of X-rays, which significantly influenced physicist Henri Becquerel. Radioactivity was discovered in 1896 by Becquerel and independently by Marie Curie, while working with phosphorescent materials. These materials glow in the dark after exposure to light, and Becquerel suspected that the glow produced in cathode-ray tubes by X-rays might be associated with phosphorescence. He wrapped a photographic plate in black paper and placed various phosphorescent salts on it. All results were negative until he used uranium salts. The uranium salts caused a blackening of the plate in spite of the plate being wrapped in black paper. Curie named the radiation rayons de Becquerel, "Becquerel Rays" and showed these rays were a property of atoms.While X-rays were produced using electrical energy, the source of energy for radiation was a mystery.
In 1899, Julius Elster and Hans Geitel performed key experiments to find the energy source for radioactivity, excluding extraction of energy from air by measurements in a vacuum and extraction of energy from outer space by measurements 300m down a mine in the Harz mountains. If the atoms themselves were the source of energy, this meant the seemingly immutable atoms must be altered when emitting the rays. In 1900 Curie summarized the puzzle of radioactivity as a choice between two equally unlikely possibilities: either energy was not conserved or chemical elements could be transmuted.
Rutherford was the first to realize that all such elements decay in accordance with the same mathematical exponential formula. Rutherford and his student Frederick Soddy were the first to realize that many decay processes resulted in the transmutation of one element to another. Subsequently, the radioactive displacement law of Fajans and Soddy was formulated to describe the products of alpha and beta decay.
The early researchers also discovered that many other chemical elements, besides uranium, have radioactive isotopes. A systematic search for the total radioactivity in uranium ores also guided Pierre and Marie Curie to isolate two new elements: polonium and radium. Except for the radioactivity of radium, the chemical similarity of radium to barium made these two elements difficult to distinguish.
Marie and Pierre Curie also coined the term "radioactivity" to define the emission of ionizing radiation by some heavy elements. Their research on the penetrating rays in uranium and the discovery of radium launched an era of using radium for the treatment of cancer. Their exploration of radium could be seen as the first peaceful use of nuclear energy and the start of modern nuclear medicine.
Early health dangers
The dangers of ionizing radiation due to radioactivity and X-rays were not immediately recognized.X-rays
The discovery of X‑rays by Wilhelm Röntgen in 1895 led to widespread experimentation by scientists, physicians, and inventors. Many people began recounting stories of burns, hair loss and worse in technical journals as early as 1896. In February of that year, Professor Daniel and Dr. Dudley of Vanderbilt University performed an experiment involving X-raying Dudley's head that resulted in his hair loss. A report by Dr. H.D. Hawks, of his suffering severe hand and chest burns in an X-ray demonstration, was the first of many other reports in Electrical Review.Other experimenters, including Elihu Thomson and Nikola Tesla, also reported burns. Thomson deliberately exposed a finger to an X-ray tube over a period of time and suffered pain, swelling, and blistering. Other effects, including ultraviolet rays and ozone, were sometimes blamed for the damage, and many physicians still claimed that there were no effects from X-ray exposure at all.
Despite this, there were some early systematic hazard investigations, and as early as 1902 William Herbert Rollins wrote almost despairingly that his warnings about the dangers involved in the careless use of X-rays were not being heeded, either by industry or by his colleagues. By this time, Rollins had proved that X-rays could kill experimental animals, could cause a pregnant guinea pig to abort, and that they could kill a foetus. He also stressed that "animals vary in susceptibility to the external action of X-light" and warned that these differences be considered when patients were treated by means of X-rays.
Radioactive substances
However, the biological effects of radiation due to radioactive substances were less easy to gauge. This gave the opportunity for many physicians and corporations to market radioactive substances as patent medicines. Examples were radium enema treatments, and radium-containing waters to be drunk as tonics. Marie Curie protested against this sort of treatment, warning that "radium is dangerous in untrained hands". Curie later died from aplastic anaemia, likely caused by exposure to ionizing radiation. By the 1930s, after a number of cases of bone necrosis and death of radium treatment enthusiasts, radium-containing medicinal products had been largely removed from the market.Radiation protection
Only a year after Röntgen's discovery of X-rays, the American engineer Wolfram Fuchs gave what is probably the first protection advice, but it was not until 1925 that the first International Congress of Radiology was held and considered establishing international protection standards. The effects of radiation on genes, including the effect of cancer risk, were recognized much later. In 1927, Hermann Joseph Muller published research showing genetic effects and, in 1946, was awarded the Nobel Prize in Physiology or Medicine for his findings.The second ICR was held in Stockholm in 1928 and proposed the adoption of the röntgen unit, and the International X-ray and Radium Protection Committee was formed. Rolf Sievert was named chairman, but a driving force was George Kaye of the British National Physical Laboratory. The committee met in 1931, 1934, and 1937.
After World War II, the increased range and quantity of radioactive substances being handled as a result of military and civil nuclear programs led to large groups of occupational workers and the public being potentially exposed to harmful levels of ionising radiation. This was considered at the first post-war ICR convened in London in 1950, when the present International Commission on Radiological Protection was born.
Since then the ICRP has developed the present international system of radiation protection, covering all aspects of radiation hazards.
In 2020, Hauptmann and another 15 international researchers from eight nations, International Agency for Research on Cancer studied definitively through meta-analysis the damage resulting from the "low doses" that have afflicted survivors of the atomic bombings of Hiroshima and Nagasaki and also in numerous accidents at nuclear plants that have occurred. These scientists reported, in JNCI Monographs: Epidemiological Studies of Low Dose Ionizing Radiation and Cancer Risk, that the new epidemiological studies directly support excess cancer risks from low-dose ionizing radiation. In 2021, Italian researcher Sebastiano Venturi reported the first correlations between radio-caesium and pancreatic cancer with the role of caesium in biology, in pancreatitis and in diabetes of pancreatic origin.
Units
The International System of Units unit of radioactive activity is the becquerel, named in honor of the scientist Henri Becquerel. One Bq is defined as one transformation per second.An older unit of radioactivity is the curie, Ci, which was originally defined as "the quantity or mass of radium emanation in equilibrium with one gram of radium ". Today, the curie is defined as disintegrations per second, so that 1 curie =.
For radiological protection purposes, although the United States Nuclear Regulatory Commission permits the use of the unit curie alongside SI units, the European Union European units of measurement directives required that its use for "public health... purposes" be phased out by 31 December 1985.
The effects of ionizing radiation are often measured in units of gray for mechanical or sievert for damage to tissue.