| Collis, John Leighton | <person> British thoracic surgeon, *1911. See: Collis gastroplasty. (05 Mar 2000) |
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| Collis-Belsey procedure | A surgical method of treating oesophageal structure by creation of a neoesophagus and a fundoplication antireflux procedure. (05 Mar 2000) |
| collision | <physics> Refers to the close approach of two or more particles, photons, atoms, nuclei, etc, during which such quantities as energy, momentum, and charge may be altered. More-or-less synonymous with scattering, except in scattering one generally thinks of one of the particles as being at rest, and the other colliding particles scatter from their initial direction of motion due to the collision. (09 Oct 1997) |
| collision cross-section | <radiobiology> Effective surface area of a particle when it collides with another, describes probability of collisions between the two particles. (09 Oct 1997) |
| collision frequency | <chemistry> The rate at which chemical species collide, used in theories of chemical kinetics. Also, the frequency with which gaseous molecules collide. (09 Oct 1997) |
| collision theory | <chemistry> A mathematical description of the number of collisions between molecules in a sample of matter per unit time, useful for predicting rates of reaction. (09 Oct 1997) |
| collision time | <radiobiology> Typical time which passes between two consecutive collision events for a given particle. Inverse of the collision frequency, equal to the mean free path divided by the particle's velocity. In plasmas, the (Coulomb) collision time decreases with increasing density, and increases with increasing temperature. (09 Oct 1997) |
| collision tumour | Two originally separate tumour's, especially a carcinoma and a sarcoma, that appear to have developed by chance in close proximity, so that an area of mingling exists. See: carcinosarcoma. (05 Mar 2000) |
| collisionless plasma model | <radiobiology> Model of a plasma in which the density is so low, or the temperature so high, that close binary (two-body) collisions have practically no significance (on certain timescales) because the time scales of interest are shorter than the collision time. Yields valid physical results for timescales much shorter than the average collision time in a real plasma. (09 Oct 1997) |