NEUTRONS Elementary particles in the nucleus having no electric charge and the mass of one atomic mass unit. NUCLIDE A species of atom characterized by the constitution of its nucleus. ORIGINAL (strontium, lead, etc.) Common strontium, lead, etc., taken into a system at the time of its formation. PALEONTOLOGY The study of fossil remains. PARENT The radioactive element from which a daughter nuclide is produced by radioactive decay. PLUMBOLOGY The study of the uranium and thorium-lead decay systems. The name is derived from the Latin name for lead, _plumbum_. PRIMORDIAL Present at the time of the formation of the earth. PROPORTIONAL COUNTER An instrument for detecting radiation by producing pulses of electrical charge that are proportional to the energy of the radiation being measured. The design permits use of radiation of a desired energy level (within limits), and discrimination against other radiation, especially background radiation. PROTONS Elementary particles with a single positive electrical charge and a mass approximately 1837 times that of the electron. The atomic number of an atom is equal to the number of protons in its nucleus. RADIOACTIVE DECAY The change of one nuclide to another by the emission of charged particles from the nucleus of its atom. RADIOACTIVITY The property of some nuclides to decay by themselves into others. RADIOGENIC Formed as the result of radioactive decay. RARE EARTH Any of the elements from atomic number 57 (lanthanum) to 71 (lutetium). SAMPLE COUNTER An instrument into which a sample of material can be placed to have its radiation measured. SECULAR EQUILIBRIUM The production of a radioactive substance at a rate equal to its decay. SPECIFIC ACTIVITY The number of atoms decaying per unit time per unit weight of the total amount being tested. SPIKE A known amount of an element of unusual isotopic composition used in isotope-dilution analysis. STATISTICAL ERROR The error associated with nuclear measurements and arising from the random distribution of nuclear events. STRATA Plural of stratum. A sheet or mass of sedimentary rock (formed by deposits of sediments, as from ancient seas) of one kind, usually in layers between beds or layers of other kinds. APPENDIX Radioactive Decay When a radioactive nucleus disintegrates or decays, the resultant remaining nucleus may still be radioactive, and sooner or later it also will disintegrate and become still another kind of atom. This process continues through a series of distinct steps until a stable atom—one that is not radioactive—is formed. All natural radioactivity in the heavy elements proceeds by such a series of steps, and the series finally ends with a stable form of lead as its end product. In other words, any naturally radioactive heavy element eventually becomes nonradioactive lead.