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Splitting the Atom

'The experiments started about four in the afternoon,' recalled a scientist whom Rutherford had invited one day in 1919 to see what he was doing. 'We went into his laboratory to spend a preliminary half hour in the dark to get our eyes into the sensitive state necessary for counting. Sitting there, drinking tea, in the dim light of a minute gas jet at the further end of the laboratory, we listened to Rutherford talking of all things under the sun. It was curiously intimate, yet impersonal, and all of it coloured by that characteristic of his of considering statements independently of the person who put them forward.'

Then Rutherford, in his unassuming white coat, made a last minute inspection tour round his laboratory, a high and wide room with a cement floor. There was, in one corner, the enormous column of the condenser, which went right up through the ceiling; and at the other end of the room a large tube, enthroned on top of a work-bench in the midst of a mass of entangled electric wires. There was an arc-lamp projector behind the tube, and a screen had been set up in front of it.

'You know, we might go up through the roof,' warned Rutherford, but the boyish smile under the big greying moustache belied his words. The blinds were now pulled down over the big leaded windows, and bluish-green sparks were seen to jump to and fro in the tube. The screen lit up. At first there was nothing but a thick grey mist. Then some large objects, like the shadows of enormous fish, flowed across the screen in a steady stream.

The Professor explained. Alpha particles - helium nuclei - were being hurled through the tube, in which an artificial mist had been created. It was an adaptation of Wilson's cloud chamber, filled with nitrogen gas. Suddenly a thick streak appeared on the screen, striking off at right angles at terrific speed. 'That's it,' said Rutherford. 'The atom has been split!'

The performance was repeated - once, twice, a third time at irregular intervals. Millions of alpha particles went straight through the nitrogen gas without touching any of its atoms. But now and then there came a direct hit on a nitrogen nucleus, which split it. 'Where are we going from here?' mused one of Rutherford's guests. 'Who knows?' he replied, 'We are entering no-man's land.'

What interested Rutherford in these experiments was the transmutation of one element into another-which furnished the proof that his theory of what the atom looked like was correct. When an alpha particle hit a nitrogen nucleus it drove out some of its seven protons. And each of these loose protons became the nucleus of a hydrogen atom, which has only one proton with an electron revolving around it. Thus nitrogen changed into hydrogen!

But Rutherford proved yet another theory, which was closely connected with Einstein's hotly disputed claim-that there is no real difference between mass and energy, and that the destruction of matter would free its latent energy. Already in 1905, Albert Einstein, then a young man of 26, had startled the scientific world with his Special Theory of Relativity, in which he gave the phenomenon of radio-activity an important place within the framework of his new picture of the universe. He explained that, if matter is converted into energy by the disintegration of atoms, that process would be represented by a simple little equation: E = mc�.

What does it mean? Basically it says that mass and energy are not different things which have no relation to one another, but that one can be changed into the other. Einstein's equation connects the two quantities. E is the energy in ergs released when a mass of m grams is completely disintegrated. And c is the velocity of light in centimetres per second, and therefore c� is 900 million million ergs.

This sounded completely fantastic. Even if matter could ever be converted into energy, surely the energy released in this process would not be of such unimaginable magnitude! There was, of course, no way of proving or disproving Einstein's equation - until Rutherford showed how to split the atom. In 1905 no one really believed that Einstein's equation would ever be put to the test; that man could ever release the incredible forces locked up in the atoms of matter. Today we know that, if one ounce of matter could be completely destroyed and changed into energy, it would yield as much power as we derive from burning 100,000 tons of coal in a conventional power station!

'Atom-splitting' became almost a fashion in the physical laboratories of Europe and America, and in the 1920s most of the lighter nuclei were being split up by bombarding them with alpha particles. Only beryllium - the fourth lightest of the elements resisted all attempts to break up its nucleus. Instead of releasing one of its four protons when hit, it gave off a burst of radiation more penetrating than even the hard gamma rays. Sir James Chadwick, again at the Cavendish Laboratory in Cambridge, proved that this radiation must consist of particles about as heavy as protons, but without an electric charge. 'If such a neutral particle exists,' Rutherford had said in 1920, 'it should be able to move freely through matter, and it may be impossible to contain it in a sealed vessel.' He knew that its discovery would be of great importance-an electrically neutral particle could be fired into any matter without being attracted or repelled by protons or electrons.

In 1932 the Joliot-Curies made a radio-active metal bombard beryllium - which is a non-radio-active metal - with its rays. The result was that the beryllium, too, became radio-active - even more so than the original source of the rays. Sir James Chadwick's explanation was that the beryllium nuclei had released their non-electrical particles, which he called neutrons. They were found to have slightly greater mass than the protons - but neutrons can change to protons by acquiring a positive electric charge.

The discovery of the neutron not only solved quite a number of problems which had so far defied the efforts of the scientists, but it also gave an even greater impetus to atom-splitting. The Americans, true to style, went into the business in a big way. The University of California built an enormous machine, the cyclotron, for the head of its radiation laboratory, Professor E. O. Lawrence, who had just come to the conclusion that the heavy hydrogen atom, consisting of one proton plus one neutron, would make an ideal bullet for shooting up other nuclei.

(From Atomic Energy - A Layman's Guide to the Nuclear Age by E. Larsen.)