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: ''For decay rate in a more general context see Particle Decay ''. Radioactive decay is the process in which an unstable '' process on the atomic level, in that it is impossible to predict when a '''particular''' atom will decay, but given a large number of similar atoms, the decay rate, on average, is predictable. is used to indicate radioactive material.]] The SI unit of radioactive decay (the phenomenon of natural and artificial radioactivity) is the Becquerel (Bq). One Bq is defined as one transformation (or decay) per second. Since any reasonably-sized sample of radioactive material contains many atoms, a Bq is a tiny measure of activity; amounts on the order of TBq (terabecquerel) or GBq (gigabecquerel) are commonly used. Another unit of (radio)activity is the Curie , Ci, which was originally defined as the activity of one gram of pure Radium , isotope Ra-226. At present it is equal (by definition) to the activity of any radionuclide decaying with a disintegration rate of 3.7 × 1010 Bq. The use of Ci is presently discouraged by SI. EXPLANATION The Neutron s and Proton s that constitute nuclei, as well as other particles that may approach them, are governed by several interactions. The Strong Nuclear Force , not observed at the familiar Macroscopic scale, is the most powerful force over subatomic distances. The Electrostatic Force is also significant, while the Weak Nuclear Force is responsible for Beta Decay . The interplay of these forces is simple. Some configurations of the particles in a nucleus have the property that, should they shift ever so slightly, the particles could fall into a lower- between the snow crystals can support the snow's weight, the system is inherently unstable with regards to a lower-potential-energy state, and a disturbance may facilitate the path to a greater entropy state (i.e., towards the ground state where heat will be produced, and thus total energy is distributed over a larger number of quantum states). Thus, an Avalanche results. The total energy does not change in this process, but because of entropy effects, avalanches only happen in one direction, and the end of this direction, which is dictated by the largest number of chance-mediated ways to distribute available energy, is what we commonly refer to as the "ground state." Such a collapse (a ''decay event'') requires a specific , in contrast to Chemical Reaction s, which also are driven by entropy, but which involve changes in the arrangement of the outer Electron s of atoms, rather than their nuclei. Some Nuclear Reaction s do involve external sources of energy, in the form of collisions with outside particles. However, these are not considered ''decay''. Rather, they are examples of induced Nuclear Reaction s. Nuclear Fission and Fusion are common types of induced nuclear reactions. DISCOVERY Radioactivity was first discovered in 1896 by the French scientist Henri Becquerel while working on Phosphorescent materials. These materials glow in the dark after exposure to light, and he thought that the glow produced in Cathode Ray Tube s by X-ray s might somehow be connected with phosphorescence. So he tried wrapping a photographic plate in black paper and placing various phosphorescent Mineral s on it. All results were negative until he tried using Uranium Salts . The result with these compounds was a deep blackening of the plate. However, it soon became clear that the blackening of the plate had nothing to do with phosphorescence because the plate blackened when the mineral was kept in the dark. Also non-phosphorescent salts of uranium and even metallic uranium blackened the plate. Clearly there was some new form of radiation that could pass through paper that was causing the plate to blacken. At first it seemed that the new radiation was similar to the then recently discovered X-rays. However further research by Becquerel, Marie Curie , Pierre Curie , Ernest Rutherford and others discovered that radioactivity was significantly more complicated. Different types of decay can occur, but Rutherford was the first to realize that they all occur with the same mathematical approximately exponential formula (see below). As for types of radioactive radiation, it was found that an Electric or Magnetic Field could split such emissions into three types of beams. For lack of better terms, the rays were given the Alphabetic names Alpha , Beta , and Gamma , names they still hold today. It was immediately obvious from the direction of Electromagnetic forces that Alpha Rays carried a positive charge, Beta Rays carried a negative charge, and Gamma Ray s were neutral. From the magnitude of deflection, it was also clear that Alpha Particles were much more massive than Beta Particles . Passing alpha rays through a thin glass membrane and trapping them in a Discharge Tube allowed researchers to study the Emission Spectrum of the resulting gas, and ultimately prove that alpha particles are in fact Helium nuclei. Other experiments showed the similarity between beta radiation and Cathode Ray s; they are both streams of Electrons , and between gamma radiation and X-rays, which are both high energy Electromagnetic Radiation . Although alpha, beta, and gamma are most common, other types of decay were eventually discovered. Shortly after discovery of the Neutron in 1932, it was discovered by Enrico Fermi that certain rare decay reactions give rise to neutrons as a decay particle. Isolated Proton Emission was also eventually observed in some elements. Shortly after the discovery of the Positron in cosmic ray products, it was realized that the same process that operates in classical Beta Decay can also produce positrons ( Positron Emission ), analogously to negative electrons. Each of the two types of beta decay acts to move a nucleus toward a ratio of neutrons and protons which has the least energy for the combination. Finally, in a phenomenon called Cluster Decay , specific combinations of neutrons and protons other than alpha particles were found to occasionally spontaneously be emitted from atoms. Still other types of radioactive decay were found which emit previously seen particles, but by different mechanisms. An example is Internal Conversion , which results in electron and sometimes high energy photon emission, even though it involves neither beta nor gamma decay. The early researchers also discovered that many other Chemical Element s besides uranium have Radioactive Isotope s. A systematic search for the total radioactivity in uranium ores also guided Marie Curie to isolate a new element Polonium and to separate a new element Radium from Barium ; the two elements' chemical similarity would otherwise have made them difficult to distinguish. The dangers of radioactivity and of radiation were not immediately recognized. Acute effects of radiation were first observed in the use of X-rays when the Serbo-Croatian-American electric engineer Nikola Tesla intentionally subjected his fingers to X-rays in 1896. He published his observations concerning the burns that developed, though he attributed them to ozone rather than to the X-rays. Fortunately his injuries healed later. The genetic effects of radiation, including the effects on cancer risk, were recognized much later. It was only in 1927 that Hermann Joseph Muller published his research that showed the genetic effects. In 1946 he was awarded the Nobel Prize for his findings. Before the biological effects of radiation were known, many physicians and corporations had begun marketing radioactive substances as Patent Medicine and Radioactive Quackery ; particularly alarming examples were radium Enema treatments, and radium-containing waters to be drunk as tonics. Marie Curie spoke out against this sort of treatment, warning that the effects of radiation on the human body were not well understood (Curie later died from Aplastic Anemia assumed due to her own work with radium, but later examination of her bones showed that she had been a careful laboratory worker and had a low burden of radium; a better candidate for her disease was her long exposure to unshielded X-ray tubes while a volunteer medical worker in WW I). By the 1930s, after a number of cases of bone-necrosis and death in enthusiasts, radium-containing medical products had nearly vanished from the market. MODES OF DECAY Radionuclides can undergo a number of different reactions. These are summarized in the following table. A nucleus with positive charge (atomic number) ''Z'' and atomic weight ''A'' is represented as (''A'', ''Z''). Radioactive decay results in a reduction of summed rest Mass , which is Converted To Energy (the ''disintegration energy'') according to the formula . This energy is released as kinetic energy of the emitted particles. The energy remains associated with a measure of mass of the decay system Invariant Mass , inasmuch the kinetic energy of emitted particles contributes also to the total Invariant Mass of systems. Thus, the sum of rest masses of particles is not conserved in decay, but the ''system'' mass or system Invariant Mass (as also system total energy) is conserved. DECAY CHAINS AND MULTIPLE MODES The daughter nuclide of a decay event is usually also unstable, sometimes even more unstable than the parent. If this is the case, it will proceed to decay again. A sequence of several decay events, producing in the end a stable nuclide, is a '' Decay Chain ''. Many radionuclides have several different observed modes of decay. Bismuth -212, for example, has three. Thus a given nuclide may lead to several different decay chains. Of the commonly occurring forms of radioactive decay, the only one that changes the number of aggregate protons and neutrons ('' Nucleon s'') contained in the nucleus is alpha emission, which reduces it by four. Thus, the number of nucleons Modulo 4 is preserved across any decay chain. OCCURRENCE AND APPLICATIONS According to the Big Bang Theory , radioactive isotopes of the lightest elements ( H , He , and traces of Li ) were produced very shortly after the emergence of the universe. However, these nuclides are so highly unstable that virtually none of them have survived to today. Most radioactive nuclei are therefore relatively young, having formed in Star s (particularly Supernova e) and during ongoing interactions between stable isotopes and energetic particles. For example, Carbon-14 , a radioactive nuclide with a half-life of only 5730 years, is constantly produced in Earth's upper atmosphere due to interactions between cosmic rays and nitrogen. Radioactive decay has been put to use in the technique of Radioisotopic Labeling , used to track the passage of a chemical substance through a complex system (such as a living Organism ). A sample of the substance is synthesized with a high concentration of unstable atoms. The presence of the substance in one or another part of the system is determined by detecting the locations of decay events. On the premise that radioactive decay is truly Random (rather than merely Chaotic ), it has been used in Hardware Random-number Generator s. Because the process is not thought to vary significantly in mechanism over time, it is also a valuable tool in estimating the absolute ages of certain materials. For geological materials, the radioisotopes and certain of their decay products become trapped when a rock solidifies, and can then later be used (subject to many well-known qualifications) to estimate the date of the solidification. These include checking the results of several simultaneous processes and their products against each other, within the same sample. RADIOACTIVE DECAY RATES The decay rate, or '''activity''', of a radioactive substance are characterized by: ''Constant'' quantities:
::(Note that although these are constants, they are associated with statistically random behavior of substances, and predictions using these constants are less accurate for small number of atoms.) ''Time-variable'' quantities:
These are related as follows: : : |
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