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3 definitions found
 for radioactivity
From The Collaborative International Dictionary of English v.0.48 :

  radioactivity \ra`di*o*ac*tiv"i*ty\
     (r[=a]`d[i^]*[-o]*[a^]k*t[i^]v"[i^]*t[y^]), n. (Physics)
     a form of instability which is a property of the atomic
     nuclei of certain isotopes, which causes a spontaneous change
     in the structure of the nucleus, accompanied by emission of
     energetic radiation. The radiation emitted is usually
     sufficient to cause ionization in matter through which it
     passes, and is therefore called ionizing radiation. The
     radiation emitted by most radioactive substances is one of
     three types: alpha rays, beta rays, or gamma rays. Some
     chemical elements have no stable isotopes, and these are
     referred to as radioactive elements, and the element itself
     is said to possess radioactivity.
  
     Note: The changes in radioactive nuclei which cause radiation
           in most cases cause the chemical identity of the
           nucleus itself to change, as when tritium (an isotope
           of hydrogen) emits a beta ray and converts to helium.
           The radioactive decay process is a first-order
           reaction, and the rate of decay of a particular isotope
           can therefore be expressed as the half life of the
           isotope, which is the time it takes for one half of the
           remaining undecayed isotope to decay, and is a constant
           independent of the proportion of original material
           which has already decayed. The half life of tritium,
           for example, is 12.3 years.
           [PJC]

From WordNet (r) 3.0 (2006) :

  radioactivity
      n 1: the spontaneous emission of a stream of particles or
           electromagnetic rays in nuclear decay [syn: radiation,
           radioactivity]

From Moby Thesaurus II by Grady Ward, 1.0 :

  19 Moby Thesaurus words for "radioactivity":
     activity, artificial radioactivity, contamination, curiage,
     decontamination, fallout, half-life, natural radioactivity,
     nuclear radiation, radiant energy, radiation,
     radioactive radiation, radiocarbon dating, radiolucency,
     radiopacity, radiosensibility, radiosensitivity, saturation point,
     specific activity
  
  

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