Radioactivity

Radioactivity
Name	Radioactivity
Caption	The trefoil symbol is the international symbol for radioactivity.
Discovered	1896
Discoverer	Antoine Henri Becquerel

Radioactivity. It is the process by which an unstable atomic nucleus loses energy by emitting radiation. This spontaneous transformation, or decay, results in the nucleus changing into a different element or a lower-energy state. The phenomenon was first observed in uranium and is a fundamental property of certain elements and isotopes found throughout the universe.

Discovery and history

The discovery was made in 1896 by Antoine Henri Becquerel while investigating the connection between phosphorescence and newly discovered X-rays. He found that uranium salts could fog a photographic plate without an external energy source. This work was greatly expanded by Marie Curie and Pierre Curie, who isolated the new elements polonium and radium from pitchblende ore. Ernest Rutherford later identified and named alpha and beta radiation, with Paul Villard discovering gamma rays. Rutherford and Frederick Soddy subsequently established the transformation theory, linking the process to nuclear transmutation. Key developments continued with the work of Lise Meitner, Otto Hahn, and Fritz Strassmann on nuclear fission.

Types of radioactive decay

The primary modes are characterized by the particles emitted. Alpha decay involves the ejection of a helium-4 nucleus, commonly observed in heavy elements like uranium-238 and radium-226. Beta decay encompasses processes where a neutron converts to a proton (beta-minus, emitting an electron and antineutrino) or a proton converts to a neutron (beta-plus, emitting a positron and neutrino), as seen in isotopes like carbon-14 and sodium-22. Gamma emission is the release of high-energy photons from an excited nucleus, often following other decay types, such as in the decay chain of cobalt-60. Other rarer types include spontaneous fission, cluster decay, and internal conversion.

Radioactive decay law and half-life

The process follows a statistical exponential decay law, where the number of nuclei decreases exponentially over time. The rate is characterized by the decay constant. A more intuitive measure is the half-life, the time required for half of a sample to decay, which is a unique property for each radionuclide. Half-lives range from fractions of a second for isotopes like francium-223 to billions of years for uranium-238. The concept of radiometric dating, such as carbon dating and uranium-lead dating, relies on measuring remaining quantities of parent isotopes like potassium-40 to determine the age of materials.

Measurement and units

Activity, the number of decays per second, is measured in becquerel (Bq) in the SI system, or the historical unit the curie (Ci). The absorbed dose of radiation energy is measured in gray (Gy), while the biological effect, accounting for different radiation types, is measured in sievert (Sv) or the older rem. Detection instruments include the Geiger counter, scintillation counter, and ionization chamber. Major regulatory bodies like the International Atomic Energy Agency and the Nuclear Regulatory Commission set safety standards based on these units.

Sources and occurrence

Radioactive materials occur naturally and are artificially produced. Primordial isotopes like uranium-235, thorium-232, and potassium-40 have existed since the formation of the Solar System and are found in the Earth's crust, contributing to background radiation. Cosmic ray interactions produce isotopes such as carbon-14 and tritium. Artificial radioactivity is created in reactors like those at Oak Ridge National Laboratory and accelerators such as the Large Hadron Collider, yielding isotopes like technetium-99m and plutonium-239.

Applications

Applications are vast and critical to modern technology and medicine. In nuclear medicine, isotopes like iodine-131 and technetium-99m are used for diagnosis and therapy. Industrial radiography employs sources like iridium-192 to inspect welds. Smoke detectors often use americium-241. Nuclear power generation relies on the fission of uranium-235 or plutonium-239 in reactors like the Pressurized Water Reactor. Other uses include food irradiation, radiocarbon dating in archaeology, and as power sources in RTGs for spacecraft like Voyager.

Biological effects and safety

Ionizing radiation can damage biological molecules like DNA, leading to radiation sickness, increased cancer risk, or mutations. Acute effects were tragically documented in survivors of the atomic bombings of Hiroshima and Nagasaki and the Chernobyl disaster. Safety principles—time, distance, and shielding—are paramount. Lead and concrete are common shielding materials. International guidelines from the International Commission on Radiological Protection inform regulations enforced by bodies like the Environmental Protection Agency. Handling protocols for materials like cobalt-60 are strictly controlled in facilities such as Los Alamos National Laboratory. Category:Nuclear physics Category:Radiation