In 1898, Ernest Rutherford (1871-1937), a scientifically talented young man from New Zealand, studied the radiations emitted from the elements ura­nium and thorium. Working at the Cavendish Laboratory of the Univer­sity of Cambridge, he found two distinct types of radiation, and he named them. The first seemed to have little range. It was easily stopped by air or by thin barriers of almost anything solid, and he named it alpha radiation. The second type had greater range in air and was better at penetrating shields. Rutherford named it beta radiation. A few months later, Paul Vil — lard (1860-1934), working in the chemistry department at the hcole Nor — male in Paris, identified a third, even more penetrating radiation type emitting from uranium. In keeping with Rutherford’s newly established naming convention, he called it gamma radiation.

In 1898, when he was 27 years old, Rutherford moved to Canada to become professor of physics at McGill University in Montreal. Here he had a new, well-equipped physics laboratory, generous funding, and a learned colleague in chemistry named Frederick Soddy (1877-1956). Almost immediately upon arrival, Rutherford presented Soddy with a puzzle: There was some sort of gas emanating from radioactive thorium. What might it be? A chemical analysis was in order.

Soddy analyzed the sample and found that the gas had no chemical characteristics whatsoever. There was only one conclusion possible, that the gas was an inert chemical such as argon. Odd as it seemed, the ele­ment thorium was apparently transmuting itself into argon gas, slowly but steadily. This discovery of the spontaneous disintegration of radioactive elements was a major discovery, and Rutherford and Soddy immediately investigated the known radioactive elements to discover what was hap­pening. By literally counting the number of radioactive particles emit­ted from a sample during a given time, they found that each radioactive

substance was decaying exponentially, at a characteristic rate, or that the source of the radiation would drop by half in a predictable passage of time. Soddy named the characteristic time half-life, meaning the time required for the radioactivity to decrease by half. For a given sample of a radioactive substance, the radiation level would drop by half in one half — life. In another half-life, what was left of the radioactivity would drop by another half, and so on, forever. The radioactivity would never technically disappear, but it would drop by halves in a predictable time period.

Rutherford suspected that beta rays were, in fact, a naturally occurring form of cathode rays being generated by many of his colleagues using elec­trically stimulated vacuum tubes. He was correct, and he demonstrated it using magnetic and electrostatic fields to bend beta rays in a vacuum tube. Instead of using high-voltage electricity between electrodes in the tube, he simply put a sample of uranium at one end. He also suspected that alpha rays were actually helium atoms stripped of their electrons, and he was able to test that theory in a most elegant way at the University of Man­chester in England in 1908.

Rutherfords proof of the nature of alpha rays was stunning for its sim­plicity and almost artistic style. He had a glassblower make him a tube with walls thin enough for alpha rays to penetrate. The tube was evacu-

ated, filled with radon gas (a known alpha-ray emitter), and sealed off at the end. This tube was then put inside another, larger tube with thick walls, which was pumped down and flame-sealed at the end. Rutherford used a light spectrometer to detect anything in the vacuum between the tubes. There was nothing there. He waited a few days and tried again. The space between the tubes had become filled with helium. Therefore, the alpha rays were actually positively charged helium ions, broken free of the much heavier radon and thrown through the thin glass of the inner tube. The name of the radiation was adjusted, from alpha rays to alpha particles, and Rutherford noted that this demonstration explained why helium is found trapped in the crystalline spaces in thorium and uranium ores. He announced the triumphant finding to the audience in Stockholm as he accepted his Nobel Prize in chemistry. Soddy had been almost right about his analysis of the mysterious decay product of thorium. It was not argon. It was another inert gas, helium.