Subject:
All Disciplines Arts & Humanities Biology & Life Sciences Business & Economics Chemistry & Materials Science Computer Science & Mathematics Engineering Environmental & Earth Sciences Medicine & Pharmacology Physical Sciences Public Health & Healthcare Social Sciences
Sort:
Hottest Latest Alphabetical (A-Z) Alphabetical (Z-A)
Type:
All Topic Review Biography
Topic Review
Cross Section
In physics, the cross section is a measure of the probability that a specific process will take place when some kind of radiant excitation (e.g. a particle beam, sound wave, light, or an X-ray) intersects a localized phenomenon (e.g. a particle or density fluctuation). For example, the Rutherford cross-section is a measure of probability that an alpha particle will be deflected by a given angle during an interaction with an atomic nucleus. Cross section is typically denoted σ (sigma) and is expressed in units of area, more specifically in barns. In a way, it can be thought of as the size of the object that the excitation must hit in order for the process to occur, but more exactly, it is a parameter of a stochastic process. In classical physics, this probability often converges to a deterministic proportion of excitation energy involved in the process, so that, for example, with light scattering off of a particle, the cross section specifies the amount of optical power scattered from light of a given irradiance (power per area). It is important to note that although the cross section has the same units as area, the cross section may not necessarily correspond to the actual physical size of the target given by other forms of measurement. It is not uncommon for the actual cross-sectional area of a scattering object to be much larger or smaller than the cross section relative to some physical process. For example, plasmonic nanoparticles can have light scattering cross sections for particular frequencies that are much larger than their actual cross-sectional areas. When two discrete particles interact in classical physics, their mutual cross section is the area transverse to their relative motion within which they must meet in order to scatter from each other. If the particles are hard inelastic spheres that interact only upon contact, their scattering cross section is related to their geometric size. If the particles interact through some action-at-a-distance force, such as electromagnetism or gravity, their scattering cross section is generally larger than their geometric size. When a cross section is specified as the differential limit of a function of some final-state variable, such as particle angle or energy, it is called a differential cross section (see detailed discussion below). When a cross section is integrated over all scattering angles (and possibly other variables), it is called a total cross section or integrated total cross section. For example, in Rayleigh scattering, the intensity scattered at the forward and backward angles is greater than the intensity scattered sideways, so the forward differential scattering cross section is greater than the perpendicular differential cross section, and by adding all of the infinitesimal cross sections over the whole range of angles with integral calculus, we can find the total cross section. Scattering cross sections may be defined in nuclear, atomic, and particle physics for collisions of accelerated beams of one type of particle with targets (either stationary or moving) of a second type of particle. The probability for any given reaction to occur is in proportion to its cross section. Thus, specifying the cross section for a given reaction is a proxy for stating the probability that a given scattering process will occur. The measured reaction rate of a given process depends strongly on experimental variables such as the density of the target material, the intensity of the beam, the detection efficiency of the apparatus, or the angle setting of the detection apparatus. However, these quantities can be factored away, allowing measurement of the underlying two-particle collisional cross section. Differential and total scattering cross sections are among the most important measurable quantities in nuclear, atomic, and particle physics.
  • 571
  • 18 Nov 2022
Topic Review
Crux
Crux, commonly known as the Southern Cross, is a small but distinctive constellation located in the southern celestial hemisphere. Recognized for its iconic shape resembling a cross, Crux holds cultural significance across various civilizations and is one of the most recognizable asterisms in the night sky. Positioned close to the South Celestial Pole, Crux serves as a navigational aid for travelers in the southern hemisphere and has been used for centuries by sailors and explorers to determine directions.
  • 278
  • 08 Mar 2024
Topic Review
Cryotropic Gelation and Macroporous Scaffolds
Cryogels obtained by the cryotropic gelation process are macroporous hydrogels with a well-developed system of interconnected pores and shape memory. Biodegradable cryogels have been prepared from natural polymers (biopolymers) and synthetic polymers with biodegradable/bioresponsive bonds containing bio-resembling or artificial units. The cryogels could be composed of one type of polymer or a combination of different materials. Polymers from natural origins (plants or animals) attract considerable attention for their intrinsic biocompatibility and potential ability to mimic ECM, favoring cell-matrix interactions for tissue-engineering and regeneration.
  • 1.3K
  • 12 Jul 2021
Topic Review
Crystallization of LiNbO3
Due to its piezoelectric, ferroelectric, nonlinear optics, and pyroelectric properties, LiNbO3 crystal has found its wide applications in surface acoustic wave (SAW) devices, optical waveguides, optical modulators, and second-harmonic generators (SHG). LiNbO3 crystallized as R3c space group below Curie temperature shows spontaneous polarization that leads to its ferroelectric and piezoelectric properties. Physical and chemical characteristics of LiNbO3 are mainly determined by Li/Nb ratio, impurity cations, vacancies in a cation sublattice. Different sizes of LiNbO3 ranging from nanoscale and microscale to bulk size have been synthesized by solid state method, hydrothermal/solvothermal method, Czochralski (Cz) growth method, etc. Most basic and applied studies of LiNbO3 focus on its bulk single crystal.
  • 1.7K
  • 17 Dec 2021
Topic Review
Cubic Mile of Oil
The cubic mile of oil (CMO) is a unit of energy, aiming to give the general public an understanding of large quantities of energy. It is approximately equal to 1.6×1020 joule. It was created by Hew Crane of SRI International to aid in public understanding of global-scale energy consumption and resources. Large scale sources of energy include wind, solar photovoltaic, solar thermal, nuclear, hydroelectric, oil, coal, natural gas, geothermal, and biomass (primarily the burning of wood). Traditionally, many different units are commonly used to measure these sources (e.g., joules, BTUs, kilowatt hours, therms) but only some of them are familiar to a global general public, and some argue that fewer are needed and a standard should be chosen. Still, these common energy units are mainly sized for everyday activities, for example a joule is the energy required to lift a small apple one metre vertically. For regional, national, and global scales, larger energy units, such as exajoule, terawatt-hour, billion barrels of oil equivalent (BBOE) and quad are used. Derived by multiplying the small common units by large powers of ten these larger units pose additional conceptual difficulties for many citizens. Crane intended the cubic mile of oil to provide a visualizable scale for comparing the contributions of these diverse energy components as a percentage of total worldwide, energy use. In 2005, the global economy was consuming approximately 30 billion barrels (4.8 billion cubic metres; 1.3 trillion US gallons) of oil each year. Numbers of this magnitude are difficult to conceive by most people. The volume occupied by 1 trillion US gallons (3.8 billion cubic metres) is about 1 cubic mile (4.2 billion cubic metres). Crane felt that a cubic mile would be an easier concept for the general public than a trillion gallons.
  • 1.1K
  • 28 Oct 2022
Topic Review
Current Status of The Celestial Reference Frame
The International Celestial Reference Frame (ICRF) is based on the currently accepted International Celestial Reference System (ICRS) and forms the basis for all positional astronomy in the radio domain. The first three generations of this frame have been built from high precision Very Long Baseline Interferometry (VLBI) astrometric measurements of positions of extragalactic radio sources (quasars and other radio-loud Active Galactic Nuclei, AGN), with each successive realization of the ICRF becoming more precise. VLBI angular position accuracy has now improved to near the 100 micro-arcsecond (μas) level. Catalogues of positions of extragalactic radio sources with the highest precision, such as the ICRF, are crucial to many applications, such as determining the Earth’s orientation in space, providing calibrator sources for astronomy, studying the motion of tectonic plates, and in spacecraft navigation. The ICRF also contributes towards the realization of a Global Geodetic Reference Frame (GGRF) for sustainable development, a resolution adopted by the United Nations in 2015.
  • 506
  • 28 Jul 2022
Topic Review
Cydonia (Region of Mars)
Cydonia (/sɪˈdoʊniə/, /saɪˈdoʊniə/) is a region on the planet Mars that has attracted both scientific and popular interest. The name originally referred to the albedo feature (distinctively coloured area) that was visible from Earthbound telescopes. The area borders plains of Acidalia Planitia and the Arabia Terra highlands. The area includes the regions: "Cydonia Mensae", an area of flat-topped mesa-like features, "Cydonia Colles", a region of small hills or knobs, and "Cydonia Labyrinthus", a complex of intersecting valleys. As with other albedo features on Mars, the name Cydonia was drawn from classical antiquity, in this case from Kydonia, a historic polis (or "city-state") on the island of Crete. Cydonia contains the "Face on Mars", located about halfway between Arandas Crater and Bamberg Crater.
  • 4.0K
  • 25 Oct 2022
Topic Review
Cygnus
Cygnus, known as the Swan, is a prominent constellation in the northern celestial hemisphere. Its distinctive shape resembles a flying bird with outstretched wings, earning it the nickname "the Northern Cross." Cygnus is rich in celestial wonders, including the famous binary star system Albireo, the North America Nebula, and the Veil Nebula complex.
  • 271
  • 08 Mar 2024
Topic Review
D0 Experiment
The DØ experiment (sometimes written D0 experiment, or DZero experiment) consists of a worldwide collaboration of scientists conducting research on the fundamental nature of matter. DØ was one of two major experiments (the other is the CDF experiment) located at the world's second highest-energy accelerator, the Tevatron Collider at the Fermilab in Batavia, Illinois, USA. The research is focused on precise studies of interactions of protons and antiprotons at the highest available energies. It involves an intense search for subatomic clues that reveal the character of the building blocks of the universe.
  • 656
  • 11 Oct 2022
Topic Review
Deep Learning and Human Monkeypox
The DNA virus responsible for monkeypox, transmitted from animals to humans, exhibits two distinct genetic lineages in central and eastern Africa. Beyond the zoonotic transmission involving direct contact with the infected animals’ bodily fluids and blood, the spread of monkeypox can also occur through skin lesions and respiratory secretions among humans. Both monkeypox and chickenpox involve skin lesions and can also be transmitted through respiratory secretions, but they are caused by different viruses. The key difference is that monkeypox is caused by an orthopox-virus, while chickenpox is caused by the varicella-zoster virus.
  • 94
  • 27 Feb 2024
  • Page
  • of
  • 118
Featured Entry Collections
>>

Featured Books

>>
Encyclopedia of Social Sciences
Encyclopedia of COVID-19
Encyclopedia of Fungi
Encyclopedia of Digital Society, Industry 5.0 and Smart City
Video Production Service
Feedback