Chapter II: Part 2

One of the most talked-about age measurements in recent years was the determination of the unexpectedly great age of fossil ancestors of man, found by the British anthropologist, Dr. L. S. B. Leakey, in Olduvai Gorge in Tanzania. The measurements were made by Garniss H. Curtis and Jack F. Evernden at the University of California in Berkeley by the potassium-argon method. The age came out a little less than 2 million years, about twice as old as it “should be” in the view of many scientists. Human remains of such great antiquity had never been found before, and much doubt was raised about the validity of the figures.

Time periods as short as two million years are not easy to measure by potassium-argon. The amount of argon produced in that time is extremely small, and contamination by argon from the air is a serious problem. Still, the measurements were repeated, the rocks were studied again, and the result did not change: The fossils were still about 2 million years old.

In cases like this, one tries to find some other method to check the results in an independent way. After many attempts it was discovered that the same rock strata dated by potassium-argon also contained some pumice—a porous volcanic glass—and that this glass was suitable for uranium fission-track dating. The measurements were made in the General Electric Research Laboratory. What was the result? Just about 2 million years!

When such altogether different techniques give the same number, one can have some confidence that the number is exact. It would be difficult to imagine a disturbance in nature that would cause these unrelated methods to give the same wrong number—in both cases by a factor of two. The double check simply means the Olduvai man is 2 million years old. There is not much doubt about it.

The Geologic Time Scale

Much of historical geology is based on a relationship called the LAW OF SUPERPOSITION. This simply means that when some rock formation was placed on top of some other formation by natural processes (sedimentation or volcanic eruption, for example), the layer on top must be younger than the one on the bottom. Such a conclusion may now seem obvious, but the concept was not even expressed until the very end of the eighteenth century and was still a matter of scientific controversy when Abraham Lincoln was a boy. It was the law of superposition, however, that led the early geologists to establish the first geologic time scales and to realize the enormous extent of geologic time.

In essence the system of establishing age by this concept is this: Somewhere a large and easily recognized layer of sedimentary rock was known. It had a characteristic color, texture, gross composition, and overall appearance. Let us call it _bed M_. This bed could be traced across the countryside until a place was reached where it could be seen that _bed M_ rested on another, different layer of rock, which we might call _bed L_. _Bed L_ could also be traced some distance and ultimately could be observed resting on a still different stratum, which we shall call _bed K_. Some of these beds had fossils in them, and it was eventually realized that rocks with the same kinds of fossils are of the same age, even though they may differ in other respects—in color or composition, for instance.

If _bed M_ was followed in another direction, perhaps a point was reached where it dipped down a little, and here, was found still another layer—call it _bed N_—on top of _M_. Obviously, the sequence of beds from oldest to youngest was _K-L-M-N_. Their relative ages were now established. Over the years, hundreds of geologists described various rock layers and identified the fossils in them. By the middle of the nineteenth century, this “layer-cake” structure of sedimentary rocks was well-known in western Europe. (One may say, parenthetically, that America was geologically a vast unknown at that time. Today we know much more about the geology of Antarctica than anyone a hundred years ago knew about the geology of the United States.)

A system of nomenclature for the “layer-cake” was developed and refined. Gradually this nomenclature was accepted internationally. Long-range correlations between beds of the same age, distant halfway around the globe from each other, were made possible as the science of PALEONTOLOGY developed. The relative age of almost any rock containing even poorly preserved fossils could be determined anywhere in the world with precision. That is, the age of rock layers in relation to one another was known. But the real age—the absolute age—remained unknown until knowledge of radioactivity provided the necessary clocks.

Even this process wasn’t entirely without problems. The difficulty lay in the fact that ordinary sedimentary rocks (shale, sandstone, and limestone) cannot be dated by the usual nuclear methods because they present no suitable closed systems. Only some volcanic sediments can be reliably dated by the mica, feldspar, and zircon they contain; but these ancient ashfalls are rare, usually are only a few inches thick, and are not easy to identify at the surface because they weather quickly to clay. Only about a dozen volcanic beds have been accurately dated in North America. The time points they established are mainstays of the present geologic time scale, but they have had to be supplemented by indirect information.

The most important of these indirect time points are furnished by what geologists call BRACKETED INTRUSIVES. It often happens in geologic history that a mass of rock becomes molten at a great depth and forces its way up through several layers of sedimentary rocks. The sedimentary layers are usually bent and twisted (folded) by the upthrust, and where the cooler rock comes in contact with the molten mass, cooler material is burned (recrystallized). Geologists call that process CONTACT METAMORPHISM because the sedimentary rock forms are changed, or metamorphosed, to have another form or composition. Contact-metamorphosed rocks, in spite of the damage they have suffered, may contain recognizable and accurately datable fossils. Thereby the metamorphic rock establishes a lower limit for the age of the intrusion: It must be younger than the fossils in the youngest of the metamorphic rocks it touched.

Now on top of the intrusive rock, we may find another sedimentary rock, deposited on top of the intrusive after it cooled and was exposed by the erosion of overlying materials, perhaps millions of years later. This new sediment may also contain fossils and thus furnish an upper limit for the age of the intrusion. The measured age of the intrusive rock thus can be used to set upper and lower limits on the absolute ages of the two sediments. If we are lucky, the two sediments will bracket a relatively short interval, making our measurement quite precise.

Time-scale in millions of years
PERIODS Since beginning of period Duration of period
PLEISTOCENE _ca_ 1
.... _ca_ 1 ....
PLIOCENE 10
.... 11 ....
MIOCENE 14
.... 25 ....
OLIGOCENE 15
.... 40 ....
EOCENE 20
.... 60 ....
PALEOCENE 10
.... 70 ± 2 ....
Upper }
CRETACEOUS 65
Lower }
.... 135 ± 5 ....
Upper }
JURASSIC 45
Mid. & Lower }
.... 180± 5 ....
TRIASSIC 45
.... 225 ± 5 ....
PERMIAN 45
.... 270 ± 5 ....
Upper }
CARBONIFEROUS 80
Lower }
.... 350 ± 10 ....
Upper }
DEVONIAN 50
Lower }
.... 400 ± 10 ....
SILURIAN 40
.... 440 ± 10 ....
ORDOVICIAN 60
.... 500 ± 15 ....
CAMBRIAN 100
.... 600 ± 20 ....

Using measurements made in many laboratories, and interpolating between them by using the relative thicknesses of sediments, the great British geologist, Arthur Holmes, established the time scale that is in general use today. His original scale is shown on the preceding page. Small changes can be expected to be made in this scale from time to time, but major alterations are not likely, except perhaps in the Cambrian Epoch where the present data are unreliable because they are not complete. (A scale showing the epochs or periods as often given now is on page 4.)

Precambrian Stratigraphy

So far we have talked only about rocks that are of the Cambrian Epoch or younger—rocks that may contain fossils. Yet there are vast areas (most of Canada, for example) that are covered with rocks older than the Cambrian formation. Some Precambrian fossils have been found, but they are so rare that they are useless for dating the strata containing them. Long-range correlation of Precambrian rocks must rely on nuclear measurements. Therefore it has been only in the last dozen or so years that some order could be established for the Precambrian rock sequences. The elaborate Precambrian stratigraphies (arrangements of strata in sequence) proposed in the past, most of them based on superficial similarities of the rocks in one place to those in another place, now have been drastically altered and in some cases completely overturned by nuclear measurements. We are still far from understanding the sequence of all the events in that vast span of time we call the Precambrian. Many thousands of nuclear age determinations will have to be made to lighten the dark corners of our ignorance.

AND WHERE DO WE GO FROM HERE?

Perhaps we must first realize that we really haven’t come very far yet. Granted that the age of rocks in many parts of the world is now suddenly known—and that this was a total mystery some dozen years ago. Granted that enormous strides forward have been made. It’s only a beginning.

Vast areas of the world are still geologically unexplored. The geologic time scale is still fragmentary and crude. Thousands of important geologic questions remain to be defined, explored, and answered by nuclear age determination. And—as is inevitable in science—many of them will lead to new questions. It is apparent that dating techniques have barely begun to be used and understood by geologists.

But apart from geologic work, what else is in store? It is difficult to predict, but probably the most important advance in the next decade or two will come when we obtain samples of rock from the moon. Will there be young rocks there or will they all be 4550 million years old? Or will they perhaps be some other age? The chemical composition and nuclear ages of the first moon samples will probably be the most important information we can hope to obtain from them. These results from the moon will contribute enormously to our understanding of the processes that formed the earth, made the continents, and determined the major features of our world.

We have a long way to go.

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 × 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

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

SUGGESTED REFERENCES

Books

_How Old Is the Earth?_, Patrick M. Hurley, Doubleday & Company, Inc.,
Garden City, New York, 1959, 160 pp., $1.25.

_Radiocarbon Dating_ (second edition), Willard F. Libby, University of
Chicago Press, Chicago, Illinois, 1955, 175 pp., $5.00 (hardback);
$1.95 (paperback).

_The Birth and Death of the Sun_, George Gamow, The Viking Press, New
York, 1949, 238 pp., $4.75 (out of print but available through
libraries), $0.60 (paperback) from the New American Library of World
Literature, Inc., New York.

_Inside the Nucleus_, Irving Adler, The John Day Company, Inc., New
York, 1963, 191 pp., $4.95 (hardback); $0.60 (paperback) from the New
American Library of World Literature, Inc., New York.

_Ages of Rocks, Planets, and Stars_, Henry Faul, McGraw-Hill Book
Company, New York, 1966, 109 pp., $2.45.

_Principles of Geochemistry_ (second edition), Brian Mason, John Wiley &
Sons, Inc., New York, 1958, 310 pp., $7.50. (New edition due in 1967.)

_Potassium Argon Dating_, J. H. Zahringer and O. A. Schaeffer,
Springer-Verlag New York, Inc., New York, 1966, 250 pp., $12.50.

Articles

A Clock for the Ages: Potassium Argon, Garniss H. Curtis, _National
Geographic Magazine_, 120: 590 (October 1961).

Exploring 1,750,000 Years into Man’s Past, L. S. B. Leakey, _National
Geographic Magazine_, 120: 564 (October 1961).

Five-Billion-Year Clock, Patrick M. Hurley, _Saturday Evening Post_,
234: 26 (March 18, 1961).

Geologic Time Scale, J. Laurence Kulp, _Science_, 133: 1105 (April 14,
1961).

Geology, Reginald A. Daly, _Scientific American_, 183: 36 (September
1950).

Tracks of Charged Particles in Solids, R. L. Fleischer, P. B. Price, and
R. M. Walker, _Science_, 149: 383 (July 23, 1965).

How Old Is It?, Lyman J. Briggs and Kenneth F. Weaver, _National
Geographic Magazine_, 114: 234 (August 1958).

Moving Picture of the Last Ice Age, Richard Foster Flint, _Natural
History_, 66: 188 (April 1957).

Modern Methods for Measurement of Geologic Time, E. J. Zeller, _Mineral
Information Service_, 18: 9 (January 1965). Single copies are $0.10
from Division of Mines and Geology, Ferry-Building, San Francisco,
California 94111.

Fluted Projectile Points: Their Age and Dispersion, C. Vance Haynes,
Jr., _Science_, 145: 1408 (September 25, 1964).

Unraveling the Age of Earth and Man, E. L. Simons, _Natural History_,
76: 52 (February 1967).

Radiocarbon Dating and Archeology in North America, F. Johnson,
_Science_, 155: 165 (January 13, 1967).

Fission-track Dating of Bed I, Olduvai Gorge, R. L. Fleischer and
others, _Science_, 146: 72 (April 2, 1965).

Lead Isotopes and the Age of the Earth, G. R. Tilton and R. H. Steiger,
_Science_, 150: 1805 (December 31, 1965).

Strontium-Rubidium Age of an Iron Meteorite, G. J. Wasserburg and
others, _Science_, 150: 1814 (December 31, 1965).

U. S. Geological Survey scientists prepare acetylene gas, made from the
carbon-14 in a geological specimen, in a vacuum line. This gas will be
fed into a proportional counter to determine the age of the specimen
by its ¹⁴C count. (See “Carbon-14 Counting” beginning on page 12.)

Geophysicist Henry Faul, an authority on nuclear dating, is professor of
Geophysics and chairman of the Department of Geology at the University
of Pennsylvania, Philadelphia. He holds a doctoral degree from the
Massachusetts Institute of Technology, and has taught at the
Universities of Strasbourg and Bern in Europe. Dr. Faul was a
geophysicist with the Manhattan Project during World War II, and was
formerly chief of the Radiation Laboratory, U. S. Geological Survey,
Denver, Colorado. For many years he was engaged in geological age
determination work with the Geological Survey and the Carnegie
Institution of Washington in Washington, D. C.

PHOTO CREDITS

Frontispiece courtesy Anthropology Department, Southern Methodist University (SMU)

Cover photo courtesy U. S. Geological Survey (USGS)

Page
11 USGS
12 USGS
14, 16, 17 Willard F. Libby
18 University of Arizona
21 Massachusetts Institute of Technology
22 Graduate Research Center of the Southwest
23 Graduate Research Center of the Southwest
25 Robert L. Fleischer
26 Perkin-Elmer Corporation
30 University of Texas
32 Graduate Research Center of the Southwest
36 Massachusetts Institute of Technology
40 Des Bartlett, Armand Denis Productions
43 Anthropology Department, SMU
44 USGS
47 USGS
59 Author’s photo courtesy Graduate Research Center of the
Southwest

Footnotes

[1]Words appearing in SMALL CAPITAL LETTERS are defined in the Glossary
beginning on page 49.

[2]The process of natural radioactive decay is described in the Appendix
beginning on page 52.

[3]The U. S. War Department program during World War II that developed
the first nuclear weapons.

[4]Drawn to scale, the whole age of man is represented by less than the
width of the line.

[5]For more information on the structure of atoms, see _Our Atomic
World_, a companion booklet in this series.

[6]From the _Chart of the Nuclides_, prepared by David T. Goldman,
Knolls Atomic Power Laboratory, August 1964.

[7]This decay process proceeds in a series of steps, during which 6
alpha particles and 4 beta particles are emitted. (See Appendix.)

[8]Named after their creator, John Napier, a Scottish mathematician
(1550-1617), who also invented the decimal point.

[9]It is difficult to determine the half-life of ¹⁴C exactly. In the
early days of ¹⁴C dating, in order not to delay continued work, an
arbitrary value of 5568 years was chosen and this value is still
used in calculations.

[10]This means that uranium decays through successive steps in which the
entire series emits eight alpha particles. (See Appendix.)

[11]Remember, this enormous period of time is a measure of the _rate_ of
spontaneous fission, _not_ of the age of ²³⁸U.

[12]The rhenium-osmium scheme is shown below the dotted line because the
method is still in an early experimental stage and its general
utility is not yet established.

[13]For more on this family of elements, see _Rare Earths, The Fraternal
Fifteen_, a companion booklet in this series.

[14]For a fuller explanation of the fission process, see _Our Atomic
World_, another booklet in this series.

[15]Neutrons that have had their speed reduced by passing through a
moderator (graphite, for example) which is built into every reactor
to accomplish this very thing. For more about how this is done, see
_Nuclear Reactors_ and _Research Reactors_, companion booklets in
this series.

[16]Note that some radionuclides sometimes decay by one method,
sometimes by another. For example, 98.8% of the nuclei of
actinium-227 emit a beta particle to form thorium-227; the remaining
1.2% emit an alpha particle to form francium-223; both of these
daughter products decay to radium-223.

[17]Some of the protactinium (0.12%) changes by an intermediate step,
known as isomeric transition, in which its nucleus shifts to a lower
energy state. The process does not alter the remaining
parent-daughter progression in the series.

[18]Undergoes both alpha and beta decay, in definite proportion of decay
events, as shown.

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Complete sets of the series are available to school and public librarians, and to teachers who can make them available for reference or for use by groups. Requests should be made on school or library letterheads and indicate the proposed use.

Students and teachers who need other material on specific aspects of nuclear science, or references to other reading material, may also write to the Oak Ridge address. Requests should state the topic of interest exactly, and the use intended.

In all requests, include “Zip Code” in return address.

Printed in the United States of America
USAEC Division of Technical Infection Extension, Oak Ridge, Tennessee

Transcriber’s Notes

--Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.

--In the text versions only, text in italics is delimited by
_underscores_.

--In the text versions only, superscript text is preceded by ^caret.

--In the text versions only, subscript text is preceded by _underscore.