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Radioactive materials have certain characteristics, such as the types of radiations emitted and the rate of emission. Knowledge of these characteristics is helpful in establishing protective controls for the use of the material. A nuclide is an atom with a particular number of protons and neutrons in its nucleus. A radionuclide is a nuclide that has the property of spontaneously converting part of its mass into energy and emitting this energy in the form of energetic particles and electromagnetic radiation. The radionuclide emits radiation. This property is called radioactivity and the actual event is referred to as radioactive decay, disintegration, or transformation. For example, Hydrogen-1 (1H) is composed of one proton and one electron. This is normal, stable hydrogen. Hydrogen-2 (2H), also called deuterium or "heavy" hydrogen consists of one proton, one electron and one neutron. Deuterium is also stable. Hydrogen-3 (3H), tritium, is composed of one proton, one electron, and two neutrons. Tritium is not stable, it is a radionuclide and a radioisotope of hydrogen. When 3H spontaneously converts part of its mass into energy and emits this energy in the form of energetic nuclear particles it yields helium-3, which is stable, plus energetic particles. All radionuclides eventually decay to stable nuclides. Strontium-90 is a radionuclide that decays into yttrium-90, also a radionuclide. Yttrium-90 subsequently decays into the stable nuclide zirconium-90. This is a series radioactive decay with 90Sr being the "parent" and 90Y the "daughter." HALF-LIFE The process of radioactive decay is spontaneous and the time when any particular atom will decay is not known. However, when large numbers of radioactive atoms are present, the fraction of atoms that will decay in a given time span (the decay rate) can be specified. A quantity that uniquely identifies the rate of decay is the half-life of the radionuclide. This is the time required for one-half of the atoms present to decay. The half-life is a useful measure because no two radionuclides have exactly the same half-life. Also, the half-life is unaffected by the chemical or physical environment of the atom. ACTIVITY The quantity of radioactive material present at a given time is usually expressed in terms of the rate of decay at that time. Two basic units used to describe the amount of activity in a sample are the Curie and Becquerel.1
curie (Ci) = 3.7 x 1010 disintegrations/sec. (dps) The quantity of activity left after any time interval is given by the following equation:
SPECIFIC ACTIVITY The
concentration of radioactivity, or in general, the number of curies (or
mCi or Ci) per unit mass or volume is defined by the specific activity. Examples: Ci/g, Ci/mg, mCi/ml, Bq/g, etc B. FORMS OF IONIZING RADIATION The manner in which a radionuclide will emit radiation is well defined and quite characteristic. The term "manner" refers to the type, energy and intensity of the radiation. alpha particles - massive charged particles, identical in mass and charge with 4He nuclei, that are emitted from the nucleus with discrete energies (for example, 238U emits alpha particles) beta particles - light charged particles that come in positive (positron) and negative (negatron) forms, have the same mass as an electron, and are emitted from the nucleus with a continuous range of energies up to some maximum energy, (for example, 22Na emits positrons, 32P, 3H, 14C, 35S, and 131I all emit negatrons) gamma rays - electromagnetic radiation emitted from the nucleus with discrete energies (for example, 131I, 125I,57Co, 51Cr, 137Cs) x-rays - electromagnetic radiation emitted from the electron shells of an atom with discrete energies (for example,131I, 125I) Two
other types of radiation are generated in the material surrounding the
radioactive atoms rather than by the radioactive atoms themselves.
These are external bremsstrahlung and annihilation radiation. external bremsstrahlung - consists of photons created by the acceleration of charged particles in the electromagnetic field of the nucleus. The photons are emitted with a continuous range of energies up to the maximum energy of the charged particle. For example, when phosphorous-32 (32P) beta particles interact with certain materials, lead for example, significant external bremsstrahlung radiation fields can be generated. annihilation radiation - consists of two 0.511 MeV photons formed by the mutual annihilation of a positive beta particle and an electron. For example, when sodium-22 (22Na) positive beta particles interact with matter, annihilation radiation is emitted. The energy of a radiation is typically given in units of electron volts (eV), kiloelectron volts (keV) or megaelectron volts (MeV). When the energy of a radiation is discrete or non-continuous, as in the case of gamma rays, it may be correctly said that a radionuclide "emits a 1 MeV gamma ray." A similar statement about radiation with a continuous range of energies, such as beta particles, is ambiguous because it could be referring to the mean or maximum energy of the radiation. Therefore, when specifying the energy of continuous radiation, it must be said that the radionuclide "emits a beta particle with a maximum energy of 1 MeV. The intensity of a radiation is the fraction of all decays that emit a particular radiation. For example, every time an 3H atom decays, it emits a beta particle with a range of energies up to a maximum energy of 18 keV. The intensity of this beta is, therefore, 100%.
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Radiation Safety Office Webmaster Revised February 13, 2006 |
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