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radiation (ra´de-a´shun)

1. The act or condition of diverging in all directions from a center. 2. The sending forth of light, short radio waves, ultraviolet or x-rays, or any other rays for treatment or diagnosis or for other purpose. Cf. irradiation (2). 3. radiatio 4. A ray. 5. Radiant energy or a radiant beam. [L. radiatio, fr. radius, ray, beam]
acoustic r. the fibers that pass from the medial geniculate body to the transverse temporal gyri of the cerebral cortex by way of the sublentiform part of the internal capsule.radiatio acustica [NA];
alpha r. an emission of a nucleus of high kinetic energy from the nucleus of an atom undergoing radioactive decay or fission.
annihilation r. the r. resulting when a positron from beta positive decay comes to rest. It encounters an electron, and they annihilate each other and convert their rest mass into two 0.51-MeV gamma rays emitted in exactly opposite directions.
anterior thalamic r.'s r.'s formed by fibers interconnecting, via the anterior limb of the internal capsule, the anterior and medial thalamic nuclei and the cerebral cortex of the frontal lobe (excluding the precentral gyrus bordering on the central sulcus).
background r. irradiation from environmental sources, including the earth's crust, the atmosphere, cosmic rays, and ingested radionuclides in the body.Natural sources account for the largest amount of radiation received by most people each year (average annual dose, 3.00 mSv), with medical and occupational sources accounting for only a fraction (on average, less than.60 mSv). It is currently believed that radon, a gas produced by radium decay within crustal rock, constitutes the major source of background radiation throughout many parts of the U.S. Radon buildup in inadequately ventilated homes may pose a long-term health hazard. The deleterious effects of background radiation, estimated as causing 1-6% of spontaneous genetic mutations, rise with dose.
beta r. radiant energy from a source of beta rays.
central thalamic r.'s r.'s formed by fibers interconnecting, through the posterior limb of the internal capsule, the ventral lateral, ventral posterolateral and posteromedial, lateral dorsal, and lateral posterior nuclei and the precentral gyrus and parietal lobe of the cerebral cortex.
Cerenkov r. light given off by a transparent medium when a high energy particle speeds through it at a velocity greater than that of light in that medium.
characteristic r. monochromatic r. that is produced when an electron is ejected from an atom and another takes its place by jumping from another shell; the energy of the photon is the difference between that of the two shell positions.characteristic emission;
r. of corpus callosum the spreading out of the fibers of the corpus callosum in the centrum semiovale of each cerebral hemisphere.radiatio corporis callosi [NA];
corpuscular r. r. consisting of streams of subatomic particles such as protons, electrons, neutrons, etc.
electromagnetic r. r. originating in a varying electromagnetic field; e.g., long and short radio waves; light, visible and invisible; x-radiation and gamma rays.
gamma r. ionizing electromagnetic r. resulting from nuclear processes, such as radioactive decay or fission.
geniculocalcarine r. optic r
Gratiolet's r. optic r
heterogeneous r. r. consisting of different frequencies, various energies, or a variety of particles.
homogeneous r. r. consisting of a narrow band of frequencies, the same energy, or a single type of particle.
ionizing r. corpuscular (e.g., neutrons, electrons) or electromagnetic (e.g., gamma) r. of sufficient energy to ionize the irradiated material.
K-r. usually a very penetrating form of x-r. excited by cathode rays (high speed electrons) impinging upon a metal anode such as tungsten; the energy of the r. is a function of the binding energy of the K-shell electrons of the metal anode.
L-r. an x-r. of slight penetrating power excited by cathode rays (high speed electrons) impinging on a metal anode; the energy of the r. is a function of the binding energy of the L-shell electrons of the metal anode.
neutron r. an emission of neutrons from the nucleus of an atom by decay or fission.
occipitothalamic r. optic r
optic r. the massive, fanlike fiber system passing from the lateral geniculate body of the thalamus to the visual cortex (striate or calcarine cortex, area 17 of Brodmann); the fibers follow the retrolenticular and sublenticular limbs of the internal capsule into the corona radiata but they curve back along the lateral wall of the temporal and occipital horns of the lateral ventricle to the striate cortex on the medial surface and pole of the occipital lobe.radiatio optica [NA], geniculocalcarine r., geniculocalcarine tract, Gratiolet's fibers, Gratiolet's r., occipitothalamic r., Wernicke's r;
posterior thalamic r.'s r.'s formed by fibers interconnecting through the retrolenticular part of the posterior limb of the internal capsule, the pulvinar complex and lateral geniculate nucleus and the posterior parietal and occipital lobes of the cerebral cortex.
primary r. an incident x-ray beam.
pyramidal r. corticospinal fibers passing from the cortex into the pyramid.radiatio pyramidalis;
scattered r. secondary r. emitted from the interaction of x-rays with matter; generally lower in energy, with a directional distribution which depends on the energy of the incident r.secondary r;
secondary r. scattered r
Wernicke's r. optic r


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