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Movements, leaders, victories and the continuing fight for equality.
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In August 1908, a white mob attacked Springfield, Illinois’s Black community, destroying homes and businesses and lynching two Black men. National outrage over the violence helped spur the movement that created the NAACP the following year.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
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A particle beam is a stream of charged or neutral particles other than photons. In particle accelerators, these particles can move with a velocity close to the speed of light.[1] There is a difference between the creation and control of charged particle beams and neutral particle beams, as only the first type can be manipulated to a sufficient extent by devices based on electromagnetism. The manipulation and diagnostics of charged particle beams at high kinetic energies using particle accelerators are main topics of accelerator physics.
Charged particles such as electrons, positrons, and protons may be separated from their common surroundings. This can be accomplished by processes such as thermionic emission or arc discharge. The following devices are commonly used as sources for particle beams:
Charged beams may be further accelerated by use of high resonant, sometimes also superconducting, microwave cavities. These devices accelerate particles by interaction with an electromagnetic field. Since the wavelength of hollow macroscopic, conducting devices is in the radio frequency (RF) band, the design of such cavities and other RF devices is also a part of accelerator physics.
More recently, plasma acceleration has emerged as a possibility to accelerate particles in a plasma medium, using the electromagnetic energy of pulsed high-power laser systems or the kinetic energy of other charged particles. This technique is under active development, but cannot provide reliable beams of sufficient quality at present.
In all cases, the beam is steered with dipole magnets and focused with quadrupole magnets. With the end goal of reaching the desired position and beam spot size in the experiment.
High-energy particle beams are used for particle physics experiments in large facilities; the most common examples being the Large Hadron Collider and the Tevatron.
Electron beams are employed in synchrotron light sources to produce X-ray radiation with a continuous spectrum over a wide frequency band which is called synchrotron radiation. This X-ray radiation is used at beamlines of the synchrotron light sources for a variety of spectroscopies (XAS, XANES, EXAFS, μ-XRF, μ-XRD) in order to probe and to characterize the structure and the chemical speciation of solids and biological materials.
Energetic particle beams consisting of protons, neutrons, or positive ions (also called particle microbeams) may also be used for cancer treatment in particle therapy.
Linear accelerators generate MeV electron beams, which can be used to treat surface cancers in patients. A tungsten/molybdenum target can also be moved into the beam to generate x-rays to treat deeper cancers.[citation needed]
Many phenomena in astrophysics are attributed to particle beams of various kinds.[3] Solar Type III radio bursts, the most common impulsive radio signatures from the Sun, are used by scientists as a tool to better understand solar accelerated electron beams.[4] Additionally, particle beams cause instabilities when interacting with plasma, which may lead to conditions causing electrostatic solitary waves.[5]
The U.S. Advanced Research Projects Agency started work on particle beam weapons in 1958.[6] The general idea of such weaponry is to hit a target object with a stream of accelerated particles with high kinetic energy, which is then transferred to the atoms, or molecules, of the target. The power needed to project a high-powered beam of this kind surpasses the production capabilities of any standard battlefield powerplant,[6] thus such weapons are not anticipated to be produced in the foreseeable future.
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
A particle beam is a stream of charged or neutral particles other than photons. In particle accelerators, these particles can move with a velocity close to the speed of light. There is a difference between the creation and control of charged particle beams and neutral particle beams, as only the first type can be manipulated to a sufficient extent by devices based on electromagnetism. The manipulation and diagnostics of charged particle beams at high kinetic energies using particle accelerators are main topics of accelerator physics.
A charged particle beam is a spatially localized group of electrically charged particles that have approximately the same position, kinetic energy (resulting in the same velocity), and direction. The kinetic energies of the particles are much larger than the energies of particles at ambient temperature. The high energy and directionality of charged particle beams make them useful for many applications in particle physics (see Particle beam#Applications and Electron-beam technology). Such beams can be split into two main classes: unbunched beams (coasting beams or DC beams), which have no longitudinal substructure in the direction of beam motion. bunched beams, in which the particles are distributed into pulses (bunches) of particles. Bunched beams are most common in modern facilities, since the most modern particle accelerators require bunched beams for acceleration. Assuming a normal distribution of particle positions and impulses, a charged particle beam (or a bunch of the beam) is characterized by the species of particle, e.g. electrons, protons, or atomic nuclei the mean energy of the particles, often expressed in electronvolts (typically keV to GeV) the (average) particle current, often expressed in amperes the particle beam size, often using the so-called β-function the beam emittance, a measure of the area occupied by the beam in one of several phase spaces. These parameters can be expressed in various ways. For example, the current and beam size can be combined into the current density, and the current and energy (or beam voltage V) can be combined into the perveance K = I V−3/2. The charged particle beams that can be manipulated in particle accelerators can be subdivided into electron beams, ion beams and proton beams.
A particle-beam weapon uses a high-energy beam of atomic or subatomic particles to damage the target by disrupting its atomic and/or molecular structure. A particle-beam weapon is a type of directed-energy weapon, which directs focused energy toward a target using atomic scale particles. Some particle-beam weapons have potential practical applications, e.g. as an antiballistic missile defense or detection system. They have been known by several names: particle accelerator guns, ion cannons, proton beams, lightning rays, rayguns, etc. The concept of particle-beam weapons comes from sound scientific principles and experiments. One process is to simply overheat a target until it is no longer operational. However, after decades of research and development, particle-beam weapons remain at the research stage, and it remains to be seen if or when they will be deployed as practical, high-performance military weapons. Particle accelerators are a well-developed technology used in scientific research. They use electromagnetic fields to accelerate and direct charged particles along a predetermined path, and a magnetic lens system to focus these streams on a target. The cathode-ray tube in many twentieth-century televisions and computer monitors is a very simple type of particle accelerator. More powerful versions include synchrotrons and cyclotrons used in nuclear research. A particle-beam weapon is a weaponized version of this technology. It accelerates charged particles (in most cases electrons, positrons, protons, or ionized atoms, but very advanced versions can accelerate other particles such as mercury nuclei) to near-light speed and then directs them towards a target. The particles' kinetic energy is imparted to matter in the target, inducing near-instantaneous and catastrophic superheating at the surface, and when penetrating deeper, ionization effects can upset or destroy electronics. However, many accelerators used for high-energy nuclear physics are quite large (sometimes on the order of kilometers in length, such as the LHC), with highly constrained construction, operation, and maintenance requirements. If an accelerator is to be deployed in space, it has to be lightweight and robust.
A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A laser beam is an archetypical example. A perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction prevents the creation of any such beam. Light can be approximately collimated by a number of processes, for instance by means of a collimator. Perfectly collimated light is sometimes said to be focused at infinity. Thus, as the distance from a point source increases, the spherical wavefronts become flatter and closer to plane waves, which are perfectly collimated. Other forms of electromagnetic radiation can also be collimated. In radiology, X-rays are collimated to reduce the volume of the patient's tissue that is irradiated, and to remove stray photons that reduce the quality of the x-ray image ("film fog"). In scintigraphy, a gamma ray collimator is used in front of a detector to allow only photons perpendicular to the surface to be detected. The term collimated may also be applied to particle beams – a collimated particle beam – where typically shielding blocks of high density materials (such as lead, bismuth alloys, etc.) may be used to absorb or block peripheral particles from a desired forward direction, especially a sequence of such absorbing collimators. This method of particle collimation is routinely deployed and is ubiquitous in every particle accelerator complex in the world. An additional method enabling this same forward collimation effect, less well studied, may deploy strategic nuclear polarization (magnetic polarization of nuclei) if the requisite reactions are designed into any given experimental applications.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Madam C.J. Walker