GLOSSARY
AEON One billion (10⁹) years.
ALPHA DECAY Radioactive decay with emission of an alpha particle.
ALPHA PARTICLE Essentially the nucleus of helium, composed of two neutrons and two protons with double positive charge.
ANTICOINCIDENCE RING A ring of counters connected to exclude outside radiation.
BACKGROUND COUNT The number of impulses per unit time registered on a counting instrument when no sample is present.
BETA DECAY Radioactive decay with emission of a beta particle.
BETA PARTICLE An electron emitted by a nucleus.
BRACKETED INTRUSIVE Igneous rock extending into sedimentary rocks that are datable by their fossils.
CLOSED SYSTEM A system in which the parent material radioactively decays into its daughter products and nothing is added or removed.
COMMON (strontium, lead, etc.) The ordinary element present in nature at any one time as distinguished from that produced by radioactive decay.
CONCORDIA ANALYSIS A mathematical technique to determine graphically the age of a material containing radiogenic lead by comparing its uranium-to-lead ratio with the similar ratio in a closed uranium-lead system.
CONTACT METAMORPHISM A metamorphism genetically related to the intrusion of molten masses of rock and taking place at or near the contact.
COSMIC RAYS High-energy particles moving in our galaxy.
CRYSTAL A periodic or regularly repeating arrangement of atoms, formed from a single element or compound.
DAUGHTER A nuclide formed from the radioactive decay of another nuclide.
DECAY CONSTANT The number of atoms decaying per atom per unit of time (0.693/half-life).
ELECTRON CAPTURE A nuclear process in which the nucleus of an atom captures an electron from one of the inner shells.
ELECTRONS Elementary particles with a unit negative electrical charge and a mass 1/1837 that of the proton, or 9.12 × 10⁻²⁷ gram. Electrons surround the atom’s positively charged nucleus and determine the atom’s chemical properties.
GAMMA RAYS Electromagnetic radiation from an atomic nucleus.
GEIGER COUNTERS Instruments that count pulses produced by radioactivity, consisting of a counting tube with a central wire anode, usually filled with a mixture of argon and organic vapor.
HALF-LIFE The time it takes for half the atoms in a radioactive substance to decay.
ION An atom or molecule that has lost or gained one or more electrons and is thus electrically charged.
ISOTOPE DILUTION An analytical technique involving addition of a known amount of an isotopic mixture of abnormal composition to the unknown amount of an element of normal or known isotopic composition.
ISOTOPES Nuclides of the same atomic number but different atomic weight. Isotopes of a given element have an identical number of protons but different numbers of neutrons in their nuclei.
LAW OF SUPERPOSITION Statement that overlying strata must be younger than underlying strata if there has been no inversion.
MASS SPECTROMETER An instrument for separation and measurement of isotopes by their mass.
NET COUNTING RATE Sample counting rate minus background counting rate.
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.
The nucleus of every atom (except hydrogen) contains one or more neutrons and one or more protons. The instability of the nuclei of the heavy atoms is related to the ratio of the number of neutrons to the number of protons in the nuclei. Radioactive decay is, in fact, a way of adjusting these ratios. The adjustment can occur in various ways. The most common is the emission of alpha particles or beta particles.
An alpha particle is identical with the nucleus of a helium atom and has two neutrons and two protons bundled together. Loss of an alpha particle from a nucleus lowers the mass number (the total of protons and neutrons) of the parent nucleus by four and the atomic number (the number of protons) by two; the number of neutrons also is reduced by two.
A beta particle is an electron and has a negative electric charge. When a beta particle is emitted from a nucleus, the nucleus is changed so that it has one more proton (which has a positive charge) and one less neutron (which has no charge); in effect, a neutron has changed into a proton as the nucleus lost a negative charge. Beta decay occurs in nuclei with a greater proportion of neutrons than is normal for the number of protons. Since beta emission increases the proportion of protons, the process raises the atomic number of the parent nucleus by one and leaves the mass number the same.
Gamma rays are a form of electromagnetic radiation. They are emitted when a nucleus shifts from one energy state to a lower energy state—the energy difference emerging as the gamma radiation. Gamma emission often accompanies alpha or beta emission, but the production of gamma rays does not itself alter the atomic number nor the mass number of the parent.
Nuclei also can decay by emission of a positron, which is a positively charged electron. When this occurs, the new nucleus has one more neutron and one less proton than its parent; in effect a proton has become a neutron as the nucleus loses a positive charge. Positrons usually are emitted by nuclei that have a greater proportion of protons than is normal for the number of neutrons.
Another process—internal electron conversion—sometimes occurs in connection with gamma-ray emission, usually in heavy elements when the gamma-ray energy is low. Instead of being emitted directly, the gamma ray strikes an orbital electron, knocking the electron out of the atom; the gamma ray then disappears. Another electron jumps into the “hole” in the orbit from which the first electron was emitted, and this jump—from a higher to a lower energy level—results in the emission of an X ray (which is similar to a gamma ray, but originates in the electron orbit region of the atom, not in the nucleus).
Finally, a nucleus may be altered by electron capture. In a nucleus with a low ratio of neutrons to protons, the nucleus captures one of its own orbital electrons. This immediately combines with a proton to form a new neutron and emit a neutrino (a high-energy particle with neither mass nor charge). The process increases the neutron-to-proton ratio of the nucleus; the daughter has the same mass number as the parent, but has an atomic number one less than the parent.
There are three series by which naturally radioactive nuclei decay to stable ones: The Uranium Series, the Thorium Series, and the Actinium Series. Man-made radioactive nuclei decay similarly, with bismuth as the end product, via the Neptunium Series. These can be illustrated in tabular form and diagrammatically. The Actinium Series (Uranium-235 Series), for example, proceeds like this:
| THE URANIUM-235 SERIES | ||||
|---|---|---|---|---|
| Element | Symbol | Radiation Emitted | Half-life | |
| Uranium | ²³⁵U | α | 7.13 × 10⁸ years | |
| Thorium | ²³¹Th | β | 25.6 hours | |
| Protactinium | ²³¹Pa | α | 3.25 × 10⁴ years | |
| Actinium[16] | ²²⁷Ac | β (98.8%) and α (1.2%) | 21.2 years | |
| Thorium | ²²⁷Th | α | 18.17 days | |
| Francium | ²²³Fr | β | 22 minutes | |
| Radium | ²²³Ra | α | 11.7 days | |
| Radon | ²¹⁹Rn | α | 4.0 seconds | |
| Polonium[16] | ²¹⁵Po | α (~100%) and β (~5 × 10⁻⁴%) | 1.83 × 10⁻³ second | |
| Lead | ²¹¹Pb | β | 36.1 minutes | |
| Astatine | ²¹⁵At | α | ~10⁻⁴ second | |
| Bismuth[16] | ²¹¹Bi | α (99.7%) and β (0.3%) | 2.15 minutes | |
| Polonium | ²¹¹Po | α | 0.25 second | |
| Thallium | ²⁰⁷Tl | β | 4.78 minutes | |
| Lead | ²⁰⁷Pb | — | Stable | |
The Uranium-235 Series
The Uranium (Uranium-238) Series proceeds like this:
| THE URANIUM-238 SERIES | |||
|---|---|---|---|
| Element | Symbol | Radiation | Half-life |
| Uranium | ²³⁸U | α | 4.51 × 10⁹ years |
| Thorium | ²³⁴Th | β | 24.1 days |
| Protactinium[17] | ²³⁴Pa | β | 1.18 minutes |
| Uranium | ²³⁴U | α | 2.48 × 10⁵ years |
| Thorium | ²³⁰Th | α | 7.6 × 10⁴ years |
| Radium | ²²⁶Ra | α | 1.62 × 10³ years |
| Radon | ²²²Rn | α | 3.82 days |
| Polonium[18] | ²¹⁸Po | α (99.98%) and β (0.02%) | 3.05 minutes |
| Lead | ²¹⁴Pb | β | 26.8 minutes |
| Astatine | ²¹⁸At | α | 1.3 seconds |
| Bismuth[18] | ²¹⁴Bi | β (99.96%) and α (0.04%) | 19.7 minutes |
| Polonium | ²¹⁴Po | α | 1.6 × 10⁻⁴ second |
| Thallium | ²¹⁰Tl | β | 1.32 minutes |
| Lead | ²¹⁰Pb | β | 22 years |
| Bismuth[18] | ²¹⁰Bi | β (~100%) and α (~2 × 10⁻⁴%) | 5.0 days |
| Polonium | ²¹⁰Po | α | 138.4 days |
| Thallium | ²⁰⁶Tl | β | 4.30 minutes |
| Lead | ²⁰⁶Pb | ... | Stable |
The Uranium-238 Series