FALLOUT, RADIOACTIVE  The deposition on the ground of radioactive substances from nuclear explosions and other injections of radioactive material  into the atmosphere. IUPAC82..

FAMILIES, RADIOACTIVE  The naturally occurring decay chains associated with uranium-238, uranium-235, and thorium-232, and the artificial decay chain associated with neptunium-235.

FARADAY CUP  A hollow collector, open at one end and closed at the other, used to collect and measure, beams of ions.

FAST  Fast or forward alpha scattering technique .

<FAST ALPHA SCATTERING TECHNIQUE>

FAST NEUTRON ACTIVATION ANALYSIS  Activation analysis  using fast neutrons , usually from accelerator sources, in the MeV energy range, most frequently 14 MeV.

FAST NEUTRONS  See neutrons, fast .

FEATHER ANALYSIS  A technique for determining the range  of a nuclide's beta rays in aluminum by comparing the absorption curve to that of a reference beta-emitting species. M.

FEATHER RULE  A rule or equation for the maximum range  of beta particles as a function of particle maximum energy. The equation is usually a linear one and when expressed in terms of mass per unit area does not vary widely with the atomic number of the absorber.

FENA  Filtered epithermal neutron activation.

FERMI  A unit of length useful on the nuclear scale and defined as 1 fermi = 1 fm = 10-15 meters. (Not a SI unit.) Synonymous with femtometer.  

FERTILE  1. Of a nuclide : capable of being transformed, directly or indirectly, into a fissile nuclide  by neutron capture . 2. Of a material: containing one or more fertile nuclides . IUPAC82..

FET AMPLIFIER  A field-effect transistor amplifier.

FIGURE OF MERIT  A measure of performance of a detection system that combines sensitivity and ability to record rapid or discontinuous spatial changes in source intensity. NM.

FILM BADGE  A package containing one or more small photographic films for approximate measurement of radiation exposure , used for the purpose of monitoring personnel. IUPAC82..

FILTER (OF A RADIATION)  Material interposed in the path of heterogeneous radiation  to modify the spectral distribution of the radiation . IUPAC82..  

FILTERED EPITHERMAL NEUTRON ACTIVATION ANALYSIS  Epithermal neutron activation analysis  in which various filters  are used, frequently to change the influence of resonances  on reaction probabilities.

FINGERPRINT, NUCLEAR  A pattern of distinct radiation energies and type suitable to identify the emitting substance.

FISSILE  1. Of a nuclide : capable of undergoing fission  by interaction with (slow) neutrons . 2. Of a material: containing one or more fissile nuclides . IUPAC82..

FISSION  Spontaneous or induced Nuclear transformation  in which a nucleus splits into two or more relatively large fragments and can be referred to in more detail as binary, ternary,... fission.  

FISSION CHAMBER or COUNTER  An ionization chamber , used to detect slow neutrons, the inside of which is coated with uranium from which highly ionizing, recoiling fission products may be produced.

FISSION FRAGMENTS  Nuclei  resulting from fission  and possessing kinetic energy acquired from the fission process. IUPAC82..

FISSION ISOMER  A highly deformed metastable nuclear state of a fissile nuclide.

FISSION NEUTRONS  See neutrons, fission .

FISSION, NUCLEAR  The spontaneous or induced division of a nucleus  into two or more major parts, usually accompanied by the emission of neutrons, gamma radiation  and, rarely, small charged nuclear fragments.  

FISSION PRODUCTS  Nuclides  produced by fission  and the daughter products  of these nuclides. IUPAC82..

FISSION, SPONTANEOUS  Nuclear fission  which occurs without the addition of particles  or energy to the nucleus . IUPAC82..

FISSION, THERMAL  Fission  caused by thermal neutrons . IUPAC82..

FISSION YIELD  The fraction of fissions  giving rise to a particular nuclide  before any nuclear decay  has occurred. IUPAC82..

FISSION YIELD, CHAIN  The fraction of fissions  giving rise to nuclei of particular mass number . IUPAC82..

FISSION YIELD, CUMULATIVE  The fraction of fissions  which have resulted in the production of a nuclide  either directly or indirectly, up to a specified time. If no time is specified, the yield is considered to be the asymptotic value. IUPAC82..

FISSION YIELD, DIRECT  The fraction of fissions  giving rise to a particular nuclide  before any nuclear decay  has occurred. IUPAC82..

FISSION YIELD, INDEPENDENT  The fraction of fissions  which have resulted in the direct production of a given fission product nuclide .

FISSIONABLE  See fissile .

FIXATION  The incorporation of radioactive elements, usually fission products, into solid materials in such a way as to insure no significant release over long periods of exposure to the natural environment. NM.

FLOW COUNTER  A radiation counter in which an appropriate atmosphere is maintained in a counter tube  by allowing a suitable gas to flow through the volume. Such counters are operated in either the Geiger-Mller  region or the proportional  region. NM. 

FLUENCE  A measure of the time-integrated particle flux  expressed in particles per unit area. M.

FLUOR  A liquid or solid that is used in scintillation  counters and that emits a flash of light when it is excited by radioactive or other radiation. B.

FLUORESCENCE  Luminescence  which occurs essentially only during the irradiation  of a substance by electromagnetic radiation . IUPAC94.

FLUORESCENCE YIELD  For a given excited state  of a specified atom , the ratio of the number of excited atoms which emit a photon  to the total number of excited states. IUPAC82..

FLUORIMETRY  A method of analysis involving fluorescence by an analyte. M.

FLUOROGRAPHY  Autoradiography involving the additional use of scintillation fluid or solid. Jones. Also known as photofluorography. NM.

FLUOROMETRY  See fluorimetry .

FLUX  The amount of a substance passing through an area per unit time. See also flux density .  

FLUX DENSITY  In a multicomponent mixture, the flux density NB of a species B is a vector which indicates the direction in which the species moves and the amount of substance of B passing through a plane perpendicular to the vector, divided by the time and by area. Sometimes the flux density is simply called flux . However, this is not consistent with the usage in vector theory. C.

FLUX DENSITY, ENERGY  For mono-directional radiation , the energy traversing in a time interval a small area perpendicular to the direction of the energy flow, divided by that time interval and by that area. IUPAC82..

FLUX DENSITY, NEUTRON  See flux density, particle .

FLUX DENSITY, PARTICLE (OR, PHOTON)  At a given point in space, the number of particles  or photons  incident in a time interval on a suitably small sphere centered at that point, divided by the cross-sectional area of that sphere and by that time interval. The particle flux density  is identical with the product of the particle  density and the average speed of the particles. IUPAC82..

FLUX DENSITY, 2200-METRE-PER-SECOND  A fictitious flux density  expressed as the product of the total number of neutrons per cubic metre and a neutron speed of 2200 metres per second. IUPAC82..

FLUX DEPRESSION  The lowering of the particle flux density  in the neighborhood of a sample due to absorption  of particles  in the sample. IUPAC82..

FLUX MONITOR  A known amount of activatable  material irradiated together with a sample; the induced radioactivity  is used as a measure for a particular flux density  during the irradiation. IUPAC82..

FLUX PERTURBATION  The lowering of the flux density  in a sample as a result of both flux depression  and self shielding . Sometimes a specified flux density can increase as a result of the change of the energy  of the particles  in the sample, e.g., the increase of the thermal neutron flux density  in hydrogen containing material. IUPAC82..

FNAA  Fast neutron activation analysis .

FOIL DETECTOR  A small piece of foil used to measure flux densities  by activation . IUPAC82..

FORCE, NUCLEAR  The force acting between nucleons. Synonymous with strong force.

FORWARD ALPHA SCATTERING TECHNIQUE  Determination of elastically scattered alpha particles with a detector at forward angles; the method uses the energy of the detected particle to deduce the scattering mass for light target elements.

FOUR- COUNTING  Measuring radiation  from a source with a detector or detector system that covers all directions from the source; that is, with a 100% geometry factor .

FRACTION, BOUND  In radioassays , the fraction of the incubation mixture which, after separation, contains the analyte bound to the binding reagent.

FRACTION, FREE  In radioassays , the fraction of the incubation mixture which, after separation, does not contain the bound analyte.

FRACTIONAL CRYSTALLIZATION  A class of chemical separation by fractionation which takes advantage of slight differences in solubility of different compounds in a solvent, the least soluble crystallizing first and removed from the mother liquor .

FREE RADICAL  See radical .  

FRENKEL DEFECT  A crystal defect consisting of a vacancy and an interstitial atom which arises when an atom is moved out of a normal lattice site and forced into an interstitial position. M.

FREQUENCY OF EMISSION  In imaging with gamma rays, it is the branching fraction  of each gamma ray. NM.

FRICKE DOSIMETER  A dosimeter  based on the oxidation of aqueous Fe+2 to Fe+3 by ionizing radiation. Ch.

ft VALUE  The product of the half-life of a beta-decaying nuclide and a function (the Fermi function) that depends on the decay energy and transition type. M.

FUEL CYCLE  The sequence of steps, such as mining, processing, fabrication, utilization, reprocessing, and refabrication, through which nuclear fuel  may pass.

FUEL ELEMENT  The smallest structurally discrete part of a reactor which has fuel as its principal constituent. IUPAC82..

FUEL, NUCLEAR  Material containing fissile nuclides , which when placed in a reactor enables a chain reaction to be achieved.

FUEL REPROCESSING  The processing of nuclear fuel , after its use in a reactor, to remove fission products  and recover fissile  and fertile  material. IUPAC82..

FULL ENERGY PEAK  In a radiation spectrum , the part of the spectral response curve corresponding to the total energy of the detected radiation .

FULL ENERGY PEAK EFFICIENCY, ABSOLUTE  For a radiation spectrometer  viewing a source of radiation, the fraction of events of a given energy radiation that are registered in that radiation's full energy peak .  

FULL ENERGY PEAK EFFICIENCY, INTRINSIC  The detector efficiency  when considering only events where the total energy of the radiation  is absorbed in the sensitive volume of the detector . IUPAC82..

FULL WIDTH AT HALF MAXIMUM  (FWHM) In a distribution curve comprising a single peak, the width of that peak at half its maximum amplitude.  

FULL WIDTH AT TENTH MAXIMUM  (FWTM) In a distribution curve comprising a single peak, the width of that peak at one-tenth its maximum amplitude.

FUSION, NUCLEAR  The process in which nuclei  undergo nuclear fusion reactions . IUPAC82..

FUSION REACTION, NUCLEAR  A reaction between any two nuclei  which results in their complete coalescence to give a compound nucleus equal to the sum of the two constituent nuclides.

FWHM  See full width at half maximum .

FWTM  See full width at tenth maximum .