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Term
  
  
The mean radiation energy imparted by ionizing radiation to a unit mass of the irradiated material. Expressed in rads  grays or joules per kilogram (see units).  The SI derived unit of absorbed dose is the gray (Gy); 1 Gy = 100 rad = 1 J/kg (also commonly called "dose").
  
Pressure measured with respect to zero pressure  in units of  force per unit of area.
  
A temperature measured with respect absolute zero  i.e. the zero of the Kelvin thermodynamic scale of temperature  a scale which cannot take negative values. See Kelvin.
  
Units derived directly from the fundamental units of a system and not arbitrary numerical definitions. The differences between absolute and international units were small;  both are now superseded by the definitions of the SI.
  
The least possible temperature for all substances. At this temperature the molecules of any substance possess no  heat energy.  A figure of -273.15 °C is generally accepted as the value of absolute zero.
  
The layer of matter inserted between the source and the detector  thereby causing a reduction in the radiation transmitted.
  
An interaction  such as photoelectric absorption  the Compton effect or pair production  by which photon energy is transferred to matter.
  
The relative concentration of the individual isotope in a mixture of isotopes of the same  chemical element.
  
A method for determining trace amounts of a chemical element  based on the assay of one or more radioisotopes produced by neutron bombardment (see also neutron activation equation).
  
The amount of a radioactive substance expressed as the number of disintegrations taking place per unit time  expressed in becquerels or curies (see units). The count rate is a non-absolute measure of activity dependent on the counting efficiency.
  
An alpha particle comprises two neutrons and two protons (a helium nucleus)  ejected at high speed from a disintegrating alpha active nucleus.
  
A device whose output is a magnified function of its input signal  drawing its power from a source other than the signal. In counting contexts  it is usually an electronic unit.
  
The reaction of a particle and its anti-particle  whereby they both cease to exist as such. For instance  a positron is annihilated together with an electron  and two photons of 0.51 MeV energy are created in their place.
  
An electronic circuit that produces a pulse only when an anti-coincidence occurs.
  
An integer that expresses the positive charge on the nucleus of an atom and hence defines the position of a chemical element in the periodic table. Z is equal to the number of protons in the nucleus of any atom that belongs to element number Z.
  
The fractional decrease in the number of particles or photons per unit distance traversed in a medium as a result of interactions in that medium. This concept includes absorption and scattering.
  
The ratio of the photon intensity after traversing a layer of matter to its intensity before  for example  matter placed in the path of a radiation beam for radiation protection purposes (a reduction factor).
  
The fast electron ejected as the result of the interaction between an X ray photon and an orbital (valence) electron  resulting in non-radiative transition of an atom to a lower excited electronic energy state (see isomeric transition).
  
The method for recording the two dimensional distribution of radioactive material in an object (or a thin section thereof). Performed by placing the surface of the object in contact with photographic emulsion which  by blackening on development  indicates where particles have been emitted by radioactive nuclei.
  
See turnover time.
  
Signals not emanating from the tracer but from the surroundings  sample holder  etc. which the measuring system does not discriminate from the tracer signals. The background count rates may be determined with a blank (zero tracer) sample in place. The net count rate of the tracer in any sample is obtained by subtracting the background from the gross sample count rate.
  
The optical spectra of molecules appear as groups of closely spaced and partly overlapping lines; these groups are called bands. At one end  the band rises to a relatively high intensity peak that falls off sharply to zero; this is called the bandhead. The synonym is band edge.
  
The SI derived unit of activity, being one radioactive disintegration per second. Supersedes the curie (Ci). 1 Bq= 2.7027 x 10<SUP>-11</SUP> Ci
  
A (b)-particle is a high speed (negatively charged) electron ejected from a nucleus during radioactive decay. A (b)+ particle is a high speed positron (positively charged electron) ejected from a nucleus during radioactive decay.
  
See half-life  biological.
  
The maximum acceptable amount of a given radionuclide that may be continuously contained in the human body.
  
The photon radiation emitted by fast moving charged particles that are sharply decelerated or deflected by an electric or magnetic field. (The original German word means  literally  braking radiation.)
  
(1) A given substance added in a ponderable amount to a trace amount of test substance to facilitate chemical or physical separation or manipulation; and (2) a ponderable amount of stable element mixed with a radioisotopic tracer of that element.
  
The designation for a radioisotope of a given element prepared by transmutation and which is free  or practically free  of any stable isotope of that element.
  
The light emitted when a charged particle travels through Z medium at a speed that exceeds the speed of light in that medium.
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