Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [42]
The pair were soon convinced that radioactivity was the transformation of one element into another through the emission of radiation. Their heretical theory was met with widespread scepticism but the experimental evidence quickly proved decisive. Their critics had to discard long-cherishe beliefs in the immutability of matter. No longer an alchemist's dream, but a scientific fact: all radioactive elements did spontaneously transform into other elements, the half-life measuring the time it took for half the atoms to do so.
'Youthful, energetic, boisterous, he suggested anything but the scientist', is how Chaim Weizmann, later the first president of Israel but then a chemist at Manchester University, remembered Rutherford. 'He talked readily and vigorously on any subject under the sun, often without knowing anything about it. Going down to the refectory for lunch, I would hear the loud, friendly voice rolling up the corridor.'35 Weizmann found Rutherford 'devoid of any political knowledge or feelings, being entirely taken up with his epoch-making scientific work'.36 At the centre of that work lay his use of the alpha particle to probe the atom.
But what exactly was an alpha particle? It was a question that had long vexed Rutherford after he discovered that alpha rays were in fact particles with a positive charge that were deflected by strong magnetic fields. He believed that an alpha particle was a helium ion, a helium atom that had lost two electrons, but never said so publicly because the evidence was purely circumstantial. Now, almost ten years after discovering alpha rays, Rutherford hoped to find definitive proof of their true character. Beta rays had already been identified as fast-moving electrons. With the help of another young assistant, this time 25-year-old German Hans Geiger, Rutherford confirmed in the summer of 1908 what he had long suspected: an alpha particle was indeed a helium atom that had lost two electrons.
'The scattering is the devil', Rutherford had complained as he and Geiger tried to unmask the alpha particle.37 He had first noticed the effect two years earlier in Montreal when some alpha particles that had passed through a sheet of mica were slightly deflected from their straight-line trajectory, causing fuzziness on a photographic plate. Rutherford made a mental note to follow it up. Soon after arriving in Manchester, he had drawn up a list of potential research topics. Rutherford now asked Geiger to investigate one of those items – the scattering of alpha particles.
Together they devised a simple experiment that involved counting scintillations, tiny flashes of light produced by alpha particles when they strike a paper screen coated with zinc sulphide, after passing through a thin sheet of gold foil. Counting scintillations was an arduous task, with long hours spent in total darkness. Luckily, according to Rutherford, Geiger was 'a demon at the work and could count at intervals for a whole night without disturbing his equanimity'.38 He found that alpha particles either passed straight through the gold foil or were deflected by one or two degrees. This was as expected. However, surprisingly, Geiger also reported finding a few alpha particles 'deflected through quite an appreciable angle'.39
Before he could fully consider the implications, if any, of Geiger's results, Rutherford was awarded the Nobel Prize for chemistry for discovering that radioactivity was the transformation of one element into another. For a man who regarded 'all science as either physics or stamp collecting', he appreciated the funny side of his own instant transmutation from physicist to chemist.40 After returning from Stockholm with his prize, Rutherford learnt to evaluate the probabilities associated with different degrees of alpha particle scattering. His calculations