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  1. 1. Angel Mae G. Bolaño BSRT 1-A What is Radioactive Decay/ Radioactivity? Radioactive decay, also known as nuclear decay or radioactivity, is the process by which a nucleus of an unstable atom loses energy by emitting particles of ionizing radiation. A material that spontaneously emits this kind of radiation—which includes the emission of energetic alpha particles, beta particles, and gamma rays—is considered radioactive. Modes of Radioactive Decay 1. Alpha Decay - Alpha decay, or α-decay, is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms (or 'decays') into an atom with a mass number 4 less and atomic number 2 less. Ex: 238U → 231Pa + α 2. Negatron Emission (Beta Decay)-A beta particle is often an electron, but can also be a positron, a positively-charged particle that is the anti-matter equivalent of the electron. If an electron is involved, the number of neutrons in the nucleus decreases by one and the number of protons increases by one. Ex: 60Co → 60Ni + e− + ν¯ 3. Positron Emission - Nuclides that are imbalanced in their ratio of protons to neutrons undergo decay to correct the imbalance. Nuclei that are rich in protons relative to their number of neutrons can decay by conversion of a proton to a neutron, emitting a positron (01e+) and a neutrino (ν). Positrons are the antiparticles of electrons, therefore a positron has the same mass as an electron but with the opposite (positive) charge. In positron emission, the atomic number Z decreases by 1 while the mass number A remains the same. Ex:22Na → 22Ne + e+ + ν 4. Electron Capture - is a type of radioactive decay where the nucleus of an atom absorbs a K or L shell electron and converts a proton into a neutron. This process reduces the atomic number by 1 and emits gamma radiation and a neutrino. Ex: 147Dy → 146Tb + e+ + ν + p
  2. 2. 5. Isomeric Transition - is a form of radioactive decay where a gamma photon is emitted by a nucleus in an excited metastable state. The photon is emitted when the energy of the excited nucleus drops to the lower, ground state. Ex: 137mBa → 137Ba + γ(662 keV) 6. Internal Conversion- a transition from a higher to a lower electronic state in a molecule or atom. It is sometimes called "radiationless de-excitation", because no photons are emitted. It differs from intersystem crossing in that, while both are radiationless methods of de-excitation, the molecular spin state for internal conversion remains the same, whereas it changes for intersystem crossing. The energy of the electronically excited state is given off to vibrational modes of the molecule or phonons. The excitation energy is transformed into heat. Internal conversion is another electromagnetic process which can occur in the nucleus and which competes with gamma emission. Sometimes the multipole electric fields of the nucleus interact with orbital electrons with enough energy to eject them from the atom. This process is not the same as emitting a gamma ray which knocks an electron out of the atom. It is also not the same as beta decay, since the emitted electron was previously one of the orbital electrons, whereas the electron in beta decay is produced by the decay of a neutron.
  3. 3. Properties of a Radioactive Material 1. Decay Constant (λ) - The constant ratio for the number of atoms of a radionuclide that decay in a given period of time compared with the total number of atoms of the same kind present at the beginning of that period. Also called disintegration constant, radioactive constant. 2. Half-life - is the amount of time required for a quantity to fall to half its value as measured at the beginning of the time period. While the term "half-life" can be used to describe any quantity which follows an exponential decay, it is most often used within the context of nuclear physics and nuclear chemistry—that is, the time required, probabilistically, for half of the unstable, radioactive atoms in a sample to undergo radioactive decay. The original term, dating to Ernest Rutherford's discovery of the principle in 1907, was "half-life period", which was shortened to "half-life" in the early 1950s.Rutherford applied the principle of a radioactive elements' half-life to studies of age determination of rocks by measuring the decay period of radium to lead-206. Half-life is used to describe a quantity undergoing exponential decay, and is constant over the lifetime of the decaying quantity. It is a characteristic unit for the exponential decay equation. The term "half-life" may generically be used to refer to any period of time in which a quantity falls by half, even if the decay is not exponential. The table on the right shows the reduction of a quantity in terms of the number of half-lives elapsed. 3. Activity - The quantity which expresses the degree of radioactivity or the radiation producing potential of a given amount of radioactive material is activity. The curie was originally defined as that amount of any radioactive material that disintegrates at the same rate as one gram of pure radium. The curie has since been defined more precisely as a quantity of radioactive material in which 3.7 x 1010 atoms disintegrate per second. The International System (SI) unit for activity is the Becquerel (Bq), which is that quantity of radioactive material in which one atom is transformed per second. The radioactivity of a given amount of radioactive material does not depend upon the mass of material present. For example, two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source. Radioactivity is expressed as the number of curies or becquerels per unit mass or volume. The concentration of radioactivity, or the relationship between the mass of radioactive material and the activity, is called "specific activity." Specific activity
  4. 4. is expressed as the number of curies or becquerels per unit mass or volume. Each gram of cobalt-60 will contain approximately 50 curies. Iridium-192 will contain 350 curies for every gram of material. The shorter half-life, the less amount of material that will be required to produce a given activity or curies. The higher specific activity of iridium results in physically smaller sources. This allows technicians to place the source in closer proximity to the film while maintaining geometric unsharpness requirements on the radiograph. These unsharpness requirements may not be met if a source with a low specific activity were used at similar source to film distances. Sources: http://en.wikipedia.org/ http://physics.bu.edu/py106/notes/RadioactiveDecay.html http://chemistry.about.com/od/chemistryglossary/g/Electron-Capture-Definition.htm http://ie.lbl.gov/education/decmode.html http://dictionary.reference.com http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radact2.html