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Internal Conversion - The process in which a nuclear transition, which would otherwise result in γ-ray emission, imparts the full available energy to one of the electrons in the atom (termed a conversion electron) which is ejected from the atom with the emission of an X-ray photon or Auger electron.
Internal Friction: Damping Capacity - The ability of a material to dissipate mechanical energy internally (i.e. excluding the overcoming of external forces such as friction). A common measure is the fractional energy lost per cycle of oscillation. Others are the logarithmic decrement and the reciprocal of the Q-value.
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Internal Pressure - (1) Of a liquid: (∂E/∂V}r where E is the internal energy and V the volume of the liquid. Also known as the cohesion pressure. (2) Of a solid:
International System of Unis: Systeme International D'unites: SI Units - A coherent system of units formed by the addition of two basic units (the degree Kelvin and the candela) to the rationalized MKSA system. The basic units are thus the metre, kilogramme, second, ampere, degree Kelvin (now referred to only as the Kelvin) and candela. See also: Units, prefixes for.
Interplanar Spacing - In a crystal: the perpendicular distance between adjacent members of any family of planes drawn through the three-dimensional structure of a crystal. These distances are of the order of 1 and can be accurately determined by X-ray diffraction methods. (Note: 1Å = 10-10 µm).
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Ionizing Particle - An elementary particle which produces ion pairs in its passage through a medium. It may be directly ionizing (e.g. an electron, proton or photon) or may achieve ionization only by the production of directly ionizing particles, as does a neutron, when it is described as indirectly ionizing. Photons have sometimes been classified, erroneously, as indirectly ionizing particles, presumbly since they are uncharged.
Ionizing Radiation - Any radiation, either electromagnetic or particulate, which is capable of producing ions. directly or indirectly, in its passage through a medium. For some purposes, e.g. in connection with nuclear energy legislation, the term is not held to include visible or ultraviolet radiation.
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Ionosphere - The region of the Earth's upper atmosphere which exhibits appreciable electrical conductivity due to the ionization of the air. This ionization is caused mainly by the effect of ultraviolet radiation from the Sun on the constituent gases of the upper atmosphere. The ionosphere consists of two main strata, the lower being the E-layer (or Kennelly-Heaviside layer) at about 100-150 km and the upper the F-layer (or Appleton layer) above about 150 km. There is a lower stratum (the D region) at 70-90 km which occurs only during the daytime.
Ion Pump - A pump for the production of a very low vacuum in which the residual gas is ionized (usually by electron bombardment) and the positive ions removed to the cathode.
Ion Spectrum - The distribution in energy, momentum, or velocity of a particular species of ion in an ion beam or ion atmosphere.
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Isopycnic Char - A chart showing the different heights in the atmosphere at which, at a given time, the atmospheric density is the same. Surfaces of equal specific volume are called isosteres.
Isotherm - A line joining points on the Earth's surface at which the temperature is the same.
Isothermal - Qualifies anything (e.g. lines on a graph, processes, layers of the atmosphere), for which the corresponding temperature is constant.
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Isotones - Nuclides having the same neutron number but different atomic numbers, e.g. 30 19 K and 4020 Ca, each of which has twenty neutrons.
Isotopes - Nuclides having the same atomic number but different mass numbers, e.g.23592U and 23592U
Isotope Separation. The separation of isotopes of the same element from each other. As they are chemically the same, physical methods have to be employed in almost all instances. The principal methods involve gaseous diffusion, electromagnetic separation, thermal diffusion and centrifugation. The one chemical method of note involves isotopic exchange reactions.
Isotopic Abundance - Of a particular isotope in a mixture of isotopes of the same element: the fractional amount of that isotope. When expressed as a percentage it is known as the relative abundance.
Isotopically Labelled Compounds - Labelled Compounds. Compounds containing isotopic tracer (s).
Isotopic Incoherence In Neutron Diffraction - The background incoherent scattering arising from the difference in scattering length between different isotopes of the same element.
Isotropy - The exhibition by a medium of uniformity in physical properties whatever the direction of measurement. A medium may be isotropic in respect of some physical properties but anisotropic in respect of others. See also: Anisotropy.
Iteration Method - A method of successive approximation used in the numerical solution of algebraic equations, transcendental equations and differential equations, and for interpolation, etc.
Iterative Structure. A number of identical passive networks connected in series, e.g. a wave filter or artificial transmission line.
Iridescence - The exhibition of rainbow-like colours by a surface composed of thin layers, e.g. mother-of-pearl, which arise from the interference of light reflected from the front and back surfaces of the layers.
Iris - A mechanism in the eye permitting the pupil to alter in aperture.
Iron Losses - In a.c. and d.c. machinery: the losses due to hysteresis and to eddy currents.
Irradiance - Irradiancy - The radiant flux per unit area, also known as radiant flux density.
Irradiation - Exposure to radiation, either by accident or intent. The term is often restricted to ionizing radiation, as in the field of radiotherapy, radiation protection and nuclear energy.
Irreversible Process - A proscess at the conclusion of which it is not possible to return the system involved to its original thermodynamic state. By the second law of thermodynamics all natural physical processes are irreversible.
Isenthalpic Process - One carried out at constant entropy.
Isentropic Change - A change that is accomplished at constant entropy.
Isobars - (1) In meteorology; contour lines on a weather chart joining points at which the atmospheric pressure is the same. (2) In atomic physics: nuclides having the same mass number but different atomic numbers, i.e. the opposite of isotopes, e.g.4019 K and 40 20Ca.
Isochore - A graph representing the state of a system as a function of two variables (e.g. temperature and pressure), the volume remaining constant.
Isoclinic Lines - Lines joining points of the Earth's surface at which the magnetic dip is the same.
Isodose Chart - For ionizing radiation in a body: a chart showing isodose lines ,i.e. lines along which the absorbed dose is the same.
Isodynamic Lines - Lines joining points on the Earth's surface at which the total magnetic intensity is the same.
Isoelectronic Sequence - In spectroscopy: a sequence of ions having the same number of extranuclear electrons, and hence having spectral terms of the same types. Such a sequence starts with a neutral atom and proceeds through the singly ionized atom which follows it in the Periodic Table, and so on.
Isogonic Lines - Lines joining points on the Earth's surface at which the magnetic declination of the same.
Isogyres - The dark brushes which appear in the interference figure of a biaxial crystal when observed in convergent light between crossed nicols in a polarizing microscope. They correspond to directions in the crystal for which the vibration dirctions are parallel to those for ,the analyser and polarizer.
Isomagnetic Lines - Lines joining points on the Earth's surface at which some given property of the Earth's magnetic field is the same.
Isomers - The nuclides having the same mass number and atomic number, but occupying different nuclear energy states.
Isometric Process: Isovolumic Process - The process of heating or cooling a gas, keeping the volume constant and allowing the pressure to change.
Isopiestic Process - The process of heating or cooling a gas, keeping the pressure constant and allowing the volume to change.
lsoporic Lines - Lines joining points on the Earth's surface at which the annual change in some given property of the Earth's magnetic field is the same.
Interstellar Gas - The gas, consisting mostly of hydrogen, which occupies interstellar space, and amounts to about 1 % of the total mass of the Galaxy.
Inverse Square Law - Describes (a) the law which states that the intensity of radiation (acoustic or electromagnetic) falls off inversely as the square of the distance from a point source of radiation; or (b) the law which states that the force between two points (e.g. the gravitational force, the force between two electrical charges or two magnetic poles) is inversely proportional to the square of the distance between them.
Inversion Axis - In crystallography: an axis characterized by rotation through a fraction of revolution (1/2,1/3,1/4 or 1/6)followed by reflection through a centre or symmetry.
Inverter: Inverted Rectifier - Any device for converting direct current to alternating current. It may be a rotary machine, a grid-controlled mercury are rectifier, or a circuit employing thermionic tubes.
lodesin Test - A test of the ability of optical glass to retain its polish and transparency. It is based on the measurement of the amount of alkali released from the glass by fracture, using iodesin. which is a very sensitive indicator.
Ion Dose - For any ionizing radiation: the total amount of electric charge produced perunit mass of air by the ions of one sign.
Ion Gun - An assembly of electrodes which produces, from a suitable ion source, a beam of ions for use in such equipment as mass spectrometers, proton microscopes, or accelerating devices for nuclear investigations.
Ionic Conduction - Strictly, any electrical conduction which is brought about by the movement of positive or negative ions. The term is, however, usually restricted to such conduction in solids, the equivalent process in molten salts or aqueous solution being more often described as electrolytic conduction.
Ionic Mobility - (1) For gaseous ions: the drift velocity per unit electric field. (2) For ions in solution the quotient of the ion conductance and the Faraday constant. It is different for positive and negative ions.
Ionic Recombination - In gases: the mechanism by which the charge of an ion is neutralized in a collision with an oppositely charged particle.
Ionization Burst - A sudden pulse of ionization recorded in an ionization chamber
Ionization by Collision - The removal of one or more electrons from an atom or particle by collision with another atom or particle.
Ionization Chamber - A gas-filled. enclosure for the detection or measurement of ionizing radiation. It contains in its simplest form two electrodes one of which may be the chamber wall. The electrodes are at different potentials and one (the collector electrode or collector) is connected to an instrument which detects or measures the ionization current which flows when ionizing radiation enters the chamber.
Ionization Gauge - For the measurement of small pressures: (a) A thermionic triode connected to the vacuum to be measured, the pressure being obtained from measurement of the grid current. (b) A wire ring anode between two metal plate cathodes, all placed within the vacuum to be measured. The pressure is obtained from measurement of the ionization current produced when the gas molecules are ionized by electrons ejected from the plates and caused to take a spiral path by the application of an outside magnetic field. This type of gauge is the Penning gauge.
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