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Topic Review
Electron Rest Mass
The electron rest mass (symbol: me) is the mass of a stationary electron, also known as the invariant mass of the electron. It is one of the fundamental constants of physics. It has a value of about 9.109×10−31 kilograms or about 5.486×10−4 daltons, equivalent to an energy of about 8.187×10−14 joules or about 0.5110 MeV.
  • 10.4K
  • 31 Oct 2022
Topic Review
Proton–Proton Chain Reaction
The proton–proton chain reaction is one of two known sets of nuclear fusion reactions by which stars convert hydrogen to helium. It dominates in stars with masses less than or equal to that of the Sun, whereas the CNO cycle, the other known reaction, is suggested by theoretical models to dominate in stars with masses greater than about 1.3 times that of the Sun. In general, proton–proton fusion can occur only if the kinetic energy (i.e. temperature) of the protons is high enough to overcome their mutual electrostatic repulsion. In the Sun, deuterium-producing events are rare. Diprotons are the much more common result of proton–proton reactions within the star, and diprotons almost immediately decay back into two protons. Since the conversion of hydrogen to helium is slow, the complete conversion of the hydrogen in the core of the Sun is calculated to take more than ten billion years. Although called the "proton–proton chain reaction", it is not a chain reaction in the normal sense. In most nuclear reactions, a chain reaction designates a reaction that produces a product, such as neutrons given off during fission, that quickly induces another such reaction. The proton-proton chain is, like a decay chain, a series of reactions. The product of one reaction is the starting material of the next reaction. There are two such chains leading from Hydrogen to Helium in the Sun. One chain has five reactions, the other chain has six.
  • 8.4K
  • 21 Oct 2022
Topic Review
Proton–Proton Chain
The proton–proton chain, also commonly referred to as the p-p chain, is one of two known sets of nuclear fusion reactions by which stars convert hydrogen to helium. It dominates in stars with masses less than or equal to that of the Sun, whereas the CNO cycle, the other known reaction, is suggested by theoretical models to dominate in stars with masses greater than about 1.3 times that of the Sun. In general, proton–proton fusion can occur only if the kinetic energy (i.e. temperature) of the protons is high enough to overcome their mutual electrostatic repulsion. In the Sun, deuterium-producing events are rare. Diprotons are the much more common result of proton–proton reactions within the star, and diprotons almost immediately decay back into two protons. Since the conversion of hydrogen to helium is slow, the complete conversion of the hydrogen initially in the core of the Sun is calculated to take more than ten billion years. Although sometimes called the "proton–proton chain reaction", it is not a chain reaction in the normal sense. In most nuclear reactions, a chain reaction designates a reaction that produces a product, such as neutrons given off during fission, that quickly induces another such reaction. The proton-proton chain is, like a decay chain, a series of reactions. The product of one reaction is the starting material of the next reaction. There are two main chains leading from Hydrogen to Helium in the Sun. One chain has five reactions, the other chain has six.
  • 3.3K
  • 10 Nov 2022
Topic Review
G-Factor
A g-factor (also called g value or dimensionless magnetic moment) is a dimensionless quantity that characterizes the magnetic moment and angular momentum of an atom, a particle or the nucleus. It is essentially a proportionality constant that relates the different observed magnetic moments μ of a particle to their angular momentum quantum numbers and a unit of magnetic moment (to make it dimensionless), usually the Bohr magneton or nuclear magneton.
  • 3.3K
  • 28 Oct 2022
Topic Review
Weak Interaction
In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, and gravitation. It is the mechanism of interaction between subatomic particles that is responsible for the radioactive decay of atoms. The weak interaction participates in nuclear fission, and the theory describing its behaviour and effects is sometimes called quantum flavourdynamics (QFD). However, the term QFD is rarely used, because the weak force is better understood by electroweak theory (EWT). The effective range of the weak force is limited to subatomic distances, and is less than the diameter of a proton.
  • 2.9K
  • 27 Oct 2022
Topic Review
Transactinide Element
In chemistry, transactinide elements (also, transactinides, or super-heavy elements) are the chemical elements with atomic numbers from 104 to 120. Their atomic numbers are immediately greater than those of the actinides, the heaviest of which is lawrencium (atomic number 103). Glenn T. Seaborg first proposed the actinide concept, which led to the acceptance of the actinide series. He also proposed the transactinide series ranging from element 104 to 121 and the superactinide series approximately spanning elements 122 to 153. The transactinide seaborgium was named in his honor. By definition, transactinide elements are also transuranic elements, i.e. have an atomic number greater than uranium (92). The transactinide elements all have electrons in the 6d subshell in their ground state. Except for rutherfordium and dubnium, even the longest-lasting isotopes of transactinide elements have extremely short half-lives, measured in seconds, or smaller units. The element naming controversy involved the first five or six transactinide elements. These elements thus used systematic names for many years after their discovery had been confirmed. (Usually the systematic names are replaced with permanent names proposed by the discoverers relatively shortly after a discovery has been confirmed.) Transactinides are radioactive and have only been obtained synthetically in laboratories. None of these elements has ever been collected in a macroscopic sample. Transactinide elements are all named after physicists and chemists or important locations involved in the synthesis of the elements. IUPAC defines an element to exist if its lifetime is longer than 10−14 seconds, which is the time it takes for the nucleus to form an electron cloud.
  • 2.6K
  • 01 Dec 2022
Topic Review
Transmission Electron Microscopy
Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a grid. An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen. The image is then magnified and focused onto an imaging device, such as a fluorescent screen, a layer of photographic film, or a sensor such as a scintillator attached to a charge-coupled device. Transmission electron microscopes are capable of imaging at a significantly higher resolution than light microscopes, owing to the smaller de Broglie wavelength of electrons. This enables the instrument to capture fine detail—even as small as a single column of atoms, which is thousands of times smaller than a resolvable object seen in a light microscope. Transmission electron microscopy is a major analytical method in the physical, chemical and biological sciences. TEMs find application in cancer research, virology, and materials science as well as pollution, nanotechnology and semiconductor research, but also in other fields such as paleontology and palynology. TEM instruments have multiple operating modes including conventional imaging, scanning TEM imaging (STEM), diffraction, spectroscopy, and combinations of these. Even within conventional imaging, there are many fundamentally different ways that contrast is produced, called "image contrast mechanisms". Contrast can arise from position-to-position differences in the thickness or density ("mass-thickness contrast"), atomic number ("Z contrast", referring to the common abbreviation Z for atomic number), crystal structure or orientation ("crystallographic contrast" or "diffraction contrast"), the slight quantum-mechanical phase shifts that individual atoms produce in electrons that pass through them ("phase contrast"), the energy lost by electrons on passing through the sample ("spectrum imaging") and more. Each mechanism tells the user a different kind of information, depending not only on the contrast mechanism but on how the microscope is used—the settings of lenses, apertures, and detectors. What this means is that a TEM is capable of returning an extraordinary variety of nanometer- and atomic-resolution information, in ideal cases revealing not only where all the atoms are but what kinds of atoms they are and how they are bonded to each other. For this reason TEM is regarded as an essential tool for nanoscience in both biological and materials fields. The first TEM was demonstrated by Max Knoll and Ernst Ruska in 1931, with this group developing the first TEM with resolution greater than that of light in 1933 and the first commercial TEM in 1939. In 1986, Ruska was awarded the Nobel Prize in physics for the development of transmission electron microscopy.
  • 2.4K
  • 05 Dec 2022
Topic Review
Void Coefficient
In nuclear engineering, the void coefficient (more properly called void coefficient of reactivity) is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids (typically steam bubbles) form in the reactor moderator or coolant. Net reactivity in a reactor is the sum total of all these contributions, of which the void coefficient is but one. Reactors in which either the moderator or the coolant is a liquid typically will have a void coefficient value that is either negative (if the reactor is under-moderated) or positive (if the reactor is over-moderated). Reactors in which neither the moderator nor the coolant is a liquid (e.g., a graphite-moderated, gas-cooled reactor) will have a void coefficient value equal to zero. It is unclear how the definition of 'void' coefficient applies to reactors in which the moderator/coolant is neither liquid nor gas (supercritical water reactor).
  • 2.2K
  • 22 Nov 2022
Topic Review
Inertial Confinement Fusion
Inertial confinement fusion (ICF) is a type of fusion energy research that attempts to initiate nuclear fusion reactions by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium. Typical fuel pellets are about the size of a pinhead and contain around 10 milligrams of fuel. To compress and heat the fuel, energy is delivered to the outer layer of the target using high-energy beams of laser light, electrons or ions, although for a variety of reasons, almost all ICF devices (As of 2015) have used lasers. The heated outer layer explodes outward, producing a reaction force against the remainder of the target, accelerating it inwards, compressing the target. This process is designed to create shock waves that travel inward through the target. A sufficiently powerful set of shock waves can compress and heat the fuel at the center so much that fusion reactions occur. ICF is one of two major branches of fusion energy research, the other being magnetic confinement fusion. When it was first proposed in the early 1970s, ICF appeared to be a practical approach to power production and the field flourished. Experiments during the 1970s and '80s demonstrated that the efficiency of these devices was much lower than expected, and reaching ignition would not be easy. Throughout the 1980s and '90s, many experiments were conducted in order to understand the complex interaction of high-intensity laser light and plasma. These led to the design of newer machines, much larger, that would finally reach ignition energies. The largest operational ICF experiment is the National Ignition Facility (NIF) in the US, designed using the decades-long experience of earlier experiments. Like those earlier experiments, however, NIF has failed to reach ignition and is, as of 2015, generating about ​1⁄3 of the required energy levels.
  • 2.1K
  • 29 Sep 2022
Topic Review
Triple-Alpha Process
The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon.
  • 1.8K
  • 22 Nov 2022
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