Chapter I
Rays From Nowhere
In January 1896 the world learned of Röntgen's X-rays, invisible rays from the glowing glass of a cathode-ray tube that passed through flesh and fogged photographic plates. Henri Becquerel, an expert on minerals that glow after exposure to light, wondered whether they emit X-rays too. He wrapped photographic plates in black paper, laid uranium salts on top, and set them in the sun. The plates were fogged. Then a spell of cloudy weather stopped the experiment. He developed the plates anyway, expecting faint images, and found them strongly fogged. The uranium needed no sunlight. It emitted rays of its own, day after day, from no visible source of energy.
The rays made air conduct electricity, a legacy of electromagnetism that gave the best way to measure them. Marie Curie, a doctoral student in Paris, used an electrometer designed by Pierre Curie and his brother Jacques. She found that the strength of the rays depended only on how much uranium a sample held, whatever its chemical form, so the emission came from inside the atoms. Thorium did the same. The uranium ore pitchblende was more active than its uranium could explain, so it had to contain something else. Working through tonnes of ore in a leaking shed, the Curies found polonium, named after Marie's native Poland, and radium, about a million times more active than uranium. She called the property radioactivity.
Chapter II
Atoms That Change
At McGill University in Montreal, Ernest Rutherford sorted the radiation into kinds. Alpha rays were stopped by a sheet of paper, beta rays passed through thin metal, and gamma rays, found by Paul Villard in 1900, went through almost anything. The strangest finding was that thorium kept producing new radioactive substances. Harriet Brooks, Rutherford's first research student at McGill, showed that one of them, the "emanation", was a gas much lighter than thorium, so not thorium at all. With the chemist Frederick Soddy, Rutherford concluded in 1902 that radioactive atoms were changing into atoms of other elements. "Don't call it transmutation," Rutherford is said to have told Soddy, "they'll have our heads off as alchemists."
Each substance decayed at its own fixed rate. Half of any sample was gone in a characteristic time, its half-life, and heat, pressure and chemistry had no effect on it. In 1903 Pierre Curie and Albert Laborde found that radium stays warmer than its surroundings, indefinitely. The energy locked inside atoms was a million times larger than any chemical energy. Kelvin had calculated that the Earth cooled from a molten ball in a few tens of millions of years, and that the Sun could shine only as long. Neither calculation had allowed for energy inside atoms, and both were wrong. The Sun, it turned out, runs on nuclear fusion. For the Earth, Kelvin's larger error was to treat its interior as a rigid solid, but radioactive heat mattered too.
Chapter III
Probing the Atom
Alpha particles, which Rutherford showed in 1908 with Thomas Royds to be helium nuclei, became his favourite tool. Fired at gold foil by Hans Geiger and Ernest Marsden in Manchester from 1909, they revealed, in Rutherford's analysis of 1911, that each atom has a tiny, heavy nucleus, a turning point of old quantum theory. Radioactivity, it followed, happened inside the nucleus.
The same year Geiger and John Nuttall found a curious rule. Alpha emitters with more energetic particles decay much faster. The dependence was absurdly steep, and classical physics could not explain how alpha particles escaped the nucleus at all, since they lacked the energy to climb over its electrical barrier. In 1928 George Gamow, and independently Ronald Gurney and Edward Condon, explained both. Quantum mechanics lets a particle tunnel through a barrier it cannot climb, with a probability that depends extremely sharply on its energy. The same tunnelling, run in reverse, lets the Sun shine, as stellar astrophysics describes.
Chapter IV
A Closer Look: Counting Decays
The decay law is simple. After half-lives, a fraction of the original atoms is left:
| Half-lives passed | Fraction left |
|---|---|
| 1 | 1/2 |
| 2 | 1/4 |
| 3 | 1/8 |
| 5 | 1/32, about 3% |
| 10 | 1/1024, about 0.1% |
No atom ages. A radium atom has the same small chance of decaying this year, about 0.043%, whether it formed yesterday or ten thousand years ago. The half-life is a statement about crowds.
How active is a gram of radium? Radium-226 has a half-life of 1,600 years, which is seconds. The chance that one atom decays in a given second is its decay constant
A gram of radium contains atoms. Multiply, and about of them decay every second. That number became a unit, the curie, originally defined as the activity of one gram of radium.
The Geiger–Nuttall puzzle. Polonium-212 emits alpha particles of 8.8 MeV and has a half-life of 0.3 microseconds. Uranium-238 emits alpha particles of about 4.2 MeV and has a half-life of 4.47 billion years. The energies differ by a factor of about 2.1. The half-lives differ by a factor of
Nothing in classical physics varies like that. Tunnelling does: the chance of getting through a barrier falls exponentially with its effective width, so a modest drop in energy lengthens the wait enormously.
Chapter V
Into the Nucleus
Radioactivity turned atoms from permanent building blocks into things that change, and gave physics the tools, the alpha particle and the counter, to look inside them. What the nucleus is made of, and why some nuclei are stable and others are not, became the subject of nuclear structure. The steady ticking of radioactive clocks in rocks gave the Earth its age, the subject of radiometric dating. The free neutron, found in 1932, turned out to be radioactive too, and how long it lives is still disputed.