Experiments around the turn of the century had shown that not only alpha rays but several other sorts of ray were being emitted by radium. Most of the phenomena of radioactivity could be attributed to these different sorts of rays: the ability to ionize air was especially the prerogative of the alpha rays, while the ability to elicit fluorescence or affect photographic plates was more marked with the beta rays. Every radioactive element had its own characteristic emissions: thus radium preparations emitted both alpha and beta rays, where polonium preparations emitted only alpha rays. Uranium affected a photographic plate more quickly than thorium, but thorium was more potent in discharging an electroscope.
The alpha particles emitted by radioactive decay (they were later shown to be helium nuclei) were positively charged and relatively massive – thousands of times more massive than beta particles or electrons – and they traveled in undeviating straight lines, passing straight through matter, ignoring it, without any scattering or deflection (although they might lose some of their velocity in so doing). This, at least, appeared to be the case, though in 1906 Rutherford observed that there might be, very occasionally, small deflections. Others ignored this, but to Rutherford these observations were fraught with possible significance. Would not alpha particles be ideal projectiles, projectiles of atomic proportions, with which to bombard other atoms and sound out their structure? He asked his young assistant Hans Geiger and a student, Ernest Marsden, to set up a scintillation experiment using screens of thin metal foils, so that one could keep count of every alpha particle that bombarded these. Firing alpha particles at a piece of gold foil, they found that roughly one in eight thousand particles showed a massive deflection – of more than 90 degrees, and sometimes even 180 degrees. Rutherford was later to say, ‘It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you.’
Rutherford pondered these curious results for almost a year, and then, one day, as Geiger recorded, he ‘came into my room, obviously in the best of moods, and told me that now he knew what the atom looked like and what the strange scatterings signified.’
Atoms, Rutherford had realized, could not be a homogenous jelly of positivity stuck with electrons like raisins (as J.J. Thomson had suggested, in his ‘plum pudding’ model of the atom), for then the alpha particles would always go through them. Given the great energy and charge of these alpha particles, one had to assume that they had been deflected, on occasion, by something even more positively charged than themselves. Yet this happened only once in eight thousand times. The other 7,999 particles might whiz through, undeflected, as if most of the gold atoms consisted of empty space; but the eight-thousandth was stopped, flung back in its tracks, like a tennis ball hitting a globe of solid tungsten. The mass of the gold atom, Rutherford inferred, had to be concentrated at the center, in a minute space, not easy to hit – as a nucleus of almost inconceivable density. The atom, he proposed, must consist overwhelmingly of empty space, with a dense, positively charged nucleus only a hundred-thousandth its diameter, and a relatively few, negatively charged electrons in orbit about this nucleus – a miniature solar system, in effect.
Rutherford’s experiments, his nuclear model of the atom, provided a structural basis for the enormous differences between radioactive and chemical processes, the millionfold differences of energy involved (Soddy would dramatize this, in his popular lectures, by holding a one-pound jar of uranium oxide aloft in one hand – this, he would say, had the energy of a hundred and sixty tons of coal).
Chemical change or ionization involved the addition or removal of an electron or two, and this required only a modest energy of two or three electron-volts, such as could be produced easily – by a chemical reaction, by heat, by light, or by a simple 3-volt battery. But radioactive processes involved the nuclei of atoms, and since these were held together by far greater forces, their disintegration could release energies of far greater magnitude – some millions of electron-volts.
Soddy coined the term atomic energy soon after the turn of the nineteenth century, ten years or more before the nucleus was discovered. No one had known, or been able to make a remotely plausible guess, as to how the sun and stars could radiate so much energy, and continue to do so for millions of years. Chemical energy would be ludicrously inadequate – a sun made of coal would burn itself out in ten thousand years. Could radioactivity, atomic energy, provide the answer?
Supposing [wrote Soddy]… our sun… were made of pure radium… there would be no difficulty in accounting for its out-pourings of energy.
Could transmutation, which occurs naturally in radioactive substances, be produced artificially, Soddy wondered.[66] He was moved by this thought to rapturous, millennial, and almost mystical heights:
Radium has taught us that there is no limit to the amount of energy in the world… A race which could transmute matter would have little need to earn its bread by the sweat of its brow… Such a race could transform a desert continent, thaw the frozen poles, and make the whole world one smiling Garden of Eden… An entirely new prospect has been opened up. Man’s inheritance has increased, his aspirations have been uplifted, and his destiny has been ennobled to an extent beyond our present power to foretell… One day he will attain the power to regulate for his own purposes the primary fountains of energy which Nature now so jealously conserves for the future.
I read Soddy’s book The Interpretation of Radium in the last year of the war, and I was enraptured by his vision of endless energy, endless light. Soddy’s heady words gave me a sense of the intoxication, the sense of power and redemption, that had attended the discovery of radium and radioactivity at the start of the century.
But side by side with this, Soddy voiced the dark possibilities, too. These indeed had been in his mind almost from the start, and, as early as 1903, he had spoken of the earth as ‘a storehouse stuffed with explosives, inconceivably more powerful than any we know of.’ This note was frequently sounded in The Interpretation of Radium, and it was Soddy’s powerful vision that inspired H.G. Wells to go back to his early science-fiction style and publish, in 1914, The World Set Free (Wells actually dedicated his book to The Interpretation of Radium). Here Wells envisaged a new radioactive element called Carolinum, whose release of energy was almost like a chain reaction:[67]
Always before in the development of warfare the shells and rockets fired had been but momentarily explosive, they had gone off in an instant once and for all… but Carolinum… once its degenerative process had been induced, continued a furious radiation of energy and nothing could arrest it.
I thought of Soddy’s prophesies, and Wells’s, in August of 1945, when we heard the news of Hiroshima. My feelings about the atomic bomb were strangely mixed. Our war, after all, was over, V-E Day was past; unlike the Americans, we had not suffered Pearl Harbor, or the terrible struggles in Guam and Saipan; we had not been in direct combat with the Japanese. The atomic bombings seemed, in some ways, like a terrible postscript to the war, a hideous demonstration that perhaps did not need to be made.
And yet I also had, as many did, a sense of jubilation at the scientific achievement of splitting the atom, and I was enthralled by the Smyth Report, which came out in August of 1945 and gave a full description of the making of the bomb. The full horror of the bomb did not hit me until the following summer, when John Hersey’s ‘Hiroshima’ was published in a special one-article edition of The New Yorker (Einstein, it was said, bought a thousand copies of this issue) and broadcast soon after by the BBC on the Third Programme. Up to this point, chemistry and physics had been for me a source of pure delight and wonder, and I was insufficiently conscious, perhaps, of their negative powers. The atomic bombs shook me, as they did everybody. Atomic or nuclear physics, one felt, could never again move with the same innocence and lightheartedness as it had in the days of Rutherford and the Curies.
66
Soddy envisaged this artificial transmutation fifteen years before Rutherford achieved it, and imagined explosive or controlled atomic disintegrations long before fission or fusion were discovered.
67
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