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 × 1.83 × 10⁻³ second
10⁻⁴%)
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
[Illustration: 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 3.05 minutes
(0.02%)
Lead ²¹⁴Pb β 26.8 minutes
Astatine ²¹⁸At α 1.3 seconds
Bismuth[18] ²¹⁴Bi β (99.96%) and α 19.7 minutes
(0.04%)
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 × 5.0 days
10⁻⁴%)
Polonium ²¹⁰Po α 138.4 days
Thallium ²⁰⁶Tl β 4.30 minutes
Lead ²⁰⁶Pb ... Stable
[Illustration: The Uranium-238 Series]