Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [41]
While Rutherford was busy conducting his experiments, in Paris the Frenchman Henri Becquerel was trying to discover whether phosphorescent substances, which glow in the dark, could also emit X-rays. Instead he found that uranium compounds emitted radiation whether they were phosphorescent or not. Becquerel's announcement of his 'uranic rays' aroused little scientific curiosity and no newspapers clamoured to report his discovery. Only a handful of physicists were interested in Becquerel's rays for, like their discoverer, most believed that only uranium compounds emitted them. However, Rutherford decided to investigate the effects of 'uranic rays' on the electrical conductivity of gases. It was a decision he later described as the most important of his life.
Testing the penetration of uranium radiation using wafer-thin layers of 'Dutch metal', a copper-zinc alloy, Rutherford found that the amount of radiation detected depended on the number of layers used. At a certain point, adding further layers had little effect in reducing the intensity of radiation, but then surprisingly it began to fall once again as more layers were added. After repeating the experiment with different materials and finding the same general pattern, Rutherford could offer only one explanation. Two types of radiation were being emitted, and he called them alpha and beta rays.
When the German physicist Gerhard Schmidt announced that thorium and its compounds also emitted radiation, Rutherford compared it with alpha and beta rays. He found the thorium radiation to be more powerful and concluded that 'rays of a more penetrative kind were present'.29 These were later called gamma rays.30 It was Marie Curie who introduced the term 'radioactivity' to describe the emission of radiation and who labelled substances that emitted 'Becquerel rays' as 'radioactive'. She believed that since radioactivity was not confined to uranium alone, it must be an atomic phenomenon. It set her on the path to discovering, with her husband Pierre, the radioactive elements radium and polonium.
In April 1898, as Curie's first paper was published in Paris, Rutherford learned that there was a vacant professorship at McGill University in Montreal, Canada. Although acknowledged as a pioneer in the new field of radioactivity, Rutherford put his name forward with little expectation of being appointed, despite a glowing letter of recommendation from Thomson. 'I have never had a student with more enthusiasm or ability for original research than Mr Rutherford,' wrote Thomson, 'and I am sure if elected, he would establish a distinguished school of physics at Montreal.'31 He concluded: 'I should consider any institution fortunate that secured the services of Mr Rutherford as professor of physics.' After a stormy voyage, Rutherford, just turned 27, arrived in Montreal at the end of September and stayed for the next nine years.
Even before he left England he knew that he was 'expected to do a lot of original work and to form a research school to knock the shine out of the Yankees!'32 He did just that, beginning with the discovery that the radioactivity of thorium decreased by half in one minute and then by half again in the next. After three minutes it had fallen to an eighth of its original value.33 Rutherford called this exponential reduction of radioactivity the 'half-life', the time taken for the intensity of radiation emitted to fall by half. Each radioactive element had its own characteristic half-life. Then came the discovery that would earn him the professorship in Manchester and a Nobel Prize.
In October 1901, Rutherford and Frederick Soddy, a 25-year-old British chemist at Montreal, began a joint study of thorium and its radiation and were soon faced with the possibility