Radioactive Decay Section 21.2
Vocabulary Radioactive decay: the spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by emission of particles, electromagnetic radiation, or both Nuclear radiation: particles or electromagnetic radiation emitted from the nucleus during radioactive decay
More Vocabulary Nuclide: an atom that is identified by the number of protons and neutrons in its nucleus Radioactive nuclide: an unstable nucleus that undergoes radioactive decay All nuclides beyond atomic number 83 are unstable and therefore radioactive
Types of Radioactive Decay Alpha decay Beta decay Gamma decay Nuclide emission occurs with all three
Alpha Decay Alpha particle: two protons and two neutrons bound together and is emitted from the nucleus during alpha decay They are the same as helium nuclei Have a charge of 2+ Mass is 4.0015062 amu
More about Alpha Decay Alpha emission occurs with very heavy nuclei The number of neutrons and the number of protons are reduced in order to increase the stability of the nucleus Example reaction: Mass number decreases by 4, atomic number decreases by 2
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More about Alpha Decay Can only travel a few cm Low penetrating ability Can cause damage if ingested or inhaled Can be shielded with a piece of paper Helium produced on earth (99%) is the result of the alpha decay of U or Th in underground minerals
Beta Decay Beta particle: an electron emitted from the nucleus during beta decay A neutron is converted into a proton and an electron The charge is 1- Mass is 0.0005486
More about Beta Decay This occurs when the neutron/proton ratio is too large, so the number of neutrons must be decreased Example reaction: Atomic number increases by one, the mass number stays the same
More about Beta Decay Can travel a few meters in air Travel at speeds close to the speed of light Have a penetrating ability about 100 times greater than that of alpha particles Can be shielded with lead or glass
Medical Use of Beta Decay Iodine-124 experiences beta decay This isotope of iodine is taken up by the thyroid gland The beta decay can be measured to see if the thyroid gland is working, while the iodine does not adversely affect the body
Gamma Decay Gamma rays: high-energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground energy state Usually occurs immediately following other types of decay when the other particles leave the nucleus in an excited state
More about Gamma Decay There is no charge and no mass The symbol is γ Gamma rays are the most penetrating Require shielding with thick layers of lead or concrete, or both
Medical Uses for Gamma Decay I-131 is used to treat thyroid cancer Y-90 is used to treat other cancers Ir-192 disrupts brain and breast cancer
Shielding for Radioactive Decay
Half-Life Half-life: (t1/2) is the time required for half the atoms of a radioactive nuclide to decay No two radioactive isotopes decay at the same rate It is a random process with the average being reported for a particular nuclide See page 708 for some examples