Geoscience Reference
In-Depth Information
which we know them today. (The French chemist Paul Villard discovered gamma
rays in 1900 while studying radium.) The beta rays turned out to be the electrons
that J. J. Thomson had discovered in 1897.
One of Rutherford's McGill colleagues found that air currents could waft the ra-
dioactivity emitted by thorium about the laboratory: the thorium rays were a gas.
Rutherford drew the gaseous rays, which they called “Thorium Emanation,” in-
to a tube and, using an electrometer like the one that Marie Curie had employed,
found that no matter how much of the thorium gas was present at the start, every
54.5 seconds its radioactive emissions would decline by one-half. In the next 54.5
seconds, the activity would decline by half again, and so on. Other radioactive
substances also lost half their activity in a given amount of time, but the amount
differed for each. Thus was born the concept of half-life: the amount of time it
takes for half of any starting number of radio active atoms to decay. Plotted on a
graph, radioactive decay is exponential. (“Thorium emanation” turned out to be an
isotope of radon: Rn-220.)
Another component of thorium, which they called Thorium X, decayed with a
half-lifeof3.6days,buttheradioactivitybuiltupagainatthesamerate.(“Thorium
X” turned out to be Ra-224.) Rutherford and his research partner Frederick Soddy
(1857-1956)deducedtheprocess:“Thenormalorconstantradioactivitypossessed
by thorium is an equilibrium value, where the rate of increase of radioactivity due
to the production of fresh active material is balanced by the rate of decay of radio-
activity of that already formed.” 2
Soddy went on to discover that elements are composed of varieties with the
same chemical properties but different atomic weights, which he named isotopes.
For this discovery Soddy won the 1921 Nobel Prize in Chemistry. In his remarks
at the ceremony, Soddy gave a succinct description of isotopes: “Put colloquially,
their atoms have identical outsides but different insides.” 3
Rutherford and Soddy had found a strange new world: substances decay so as to
lose exactly half their original activity in a fixed amount of time, but in so doing
they transmute themselves into other substances, which also die away but with a
different half-life. Atoms, far from being eternal, immutable, and the smallest sub-
division of matter, may spontaneously split into pieces and vanish while atoms of
an entirely different element arise in their place. In a sense, the ancient alchemists
were right: one element can be transformed into another. But although lead cannot
be changed into gold, as the ancients had hoped, lead did turn out to be the most
important element in discovering the age of the Earth.
In a series of classic papers, Rutherford and Soddy explained that radioactive
decay obeys the law of probability. 4 If we could observe a particular atom, say
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