Serving the Reich

by Philip Ball

  Meanwhile, other researchers discovered that uranium decays into radium via an intermediate element, hitherto apparently unknown, which became called ionium. But ionium turned out to have chemical properties indistinguishable from thorium. In 1912 Frederick Soddy, now at Glasgow University, showed that the two substances emit light of the same frequencies—these frequencies were generally regarded as a fingerprint of chemical identity. Soddy therefore proposed that ionium was indeed a form of thorium, chemically the same but otherwise somehow distinct. Aston’s studies of neon strengthened Soddy’s conviction, and he proposed to call these different forms of the same element ‘isotopes’, meaning ‘same shape’. Most chemists didn’t like that idea—an element was supposed to be fundamental, not to come in different flavours—but Bohr, then with Rutherford in Manchester, endorsed it.

  Aston’s mass spectrometer now confirmed the reality of isotopes: those of neon had precisely the masses 20 and 22, as he had suspected. In 1922 Aston and Soddy were jointly awarded the Nobel Prize in Chemistry for their discovery—even though it was still unclear why isotopes differed in mass.

  But Aston’s measurements were precise enough to reveal something more: the atomic masses were still not quite exact multiples of that of hydrogen. They were always slightly less: there was a ‘mass defect’. Aston and his peers realized that in this tiny deficit resided the immense power of the nucleus. As protons come together and fuse to form a nucleus, a little bit of their mass is converted to energy, in accord with Einstein’s equivalence of mass and energy E = mc2. The release of this energy is what makes the nucleus stable: it is called the binding energy. As Einstein’s iconic equation implies, the energy equivalent of mass is enormous, being multiplied by the speed of light squared. From the minuscule mass defect of atomic nuclei, Aston could calculate how much energy was released by the fusion of hydrogen nuclei to form heavier elements. It was a phenomenal amount. ‘To change the hydrogen in a glass of water into helium’, he wrote, ‘would release enough energy to drive the Queen Mary across the Atlantic and back at full speed.’ If that source can be tapped, ‘the human race will have at its command powers beyond the dreams of science fiction’. And again, those numbers were fearful: ‘We can only hope that [man] will not use it exclusively in blowing up his next-door neighbour.’

  The nuclear mass defect increases as atoms get heavier, but only so far as masses of about 60 (around the atomic mass of iron). After that, the mass defect gets steadily smaller. This implies that nuclei lighter than iron can become more stable—can acquire more binding energy per nuclear particle—by binding more protons, while those heavier than iron can do so by shedding protons. In other words, there are two ways for atoms to decay to more stable forms: by fusing or by disintegrating. Here, then, was an explanation for radioactivity. Very heavy atoms, such as uranium, have relatively less binding energy: they are liable to shed nuclear particles (alpha particles) to lower their mass and become more stable. In much the same way that energetic chemical compounds such as nitroglycerine are liable to undergo reactions to form more stable, lower-energy substances, so radioactive elements undergo nuclear reactions to the same end. By the same token, light elements are apt to engage in nuclear fusion, releasing vast quantities of energy in the process. In the 1920s it was recognized that this could answer the long-standing puzzle of how the sun and other stars could maintain such a prodigious output of energy for so long: they were powered by nuclear fusion, particularly of their main constituent, hydrogen.

  Nuclear alchemy

  Chemists could access, control and manipulate the energy release of chemical processes, but the machinations and transmutations of nuclei seemed to be hidden away out of reach, in the unthinkably tiny volume inferred by Rutherford. And yet, there was a way to get inside. Rutherford, Geiger and Marsden had watched alpha particles being deflected as they were repelled by the positive electrical charge of other nuclei. Yet if the alpha particles had enough energy, they could push through this repulsive barrier and enter the nucleus, which should itself be a favourable outcome in the case of lighter nuclei because of the binding energy it would set free. For an alpha particle, you might say, it is hard to get inside a (light) nucleus but worth the effort. Capture of alpha particles by nuclei raised the prospect of harnessing radioactivity to effect transmutation artificially: to convert one element into another at will.

  That is what Rutherford achieved at the Cavendish in 1919. He showed that alpha particles emitted by polonium and passing through nitrogen gas could transmute some nitrogen atoms to oxygen. Alpha particles, being helium nuclei, have two protons, while oxygen has only one more proton than nitrogen. So the alpha particle is first subsumed by the nitrogen nucleus, and one proton is then spat back out alone.

  This brand of nuclear alchemy had its limits, because as a nucleus gets more massive and more highly charged, the electrostatic barrier impeding an alpha particle’s entry and fusion gets higher, and greater energy is needed to overcome it. The answer was to speed up the projectiles, granting them more energy. In 1929 the American physicist Ernest Lawrence at the University of California at Berkeley began to use electrodes charged to high voltages to accelerate particles—be they alpha particles or lone protons—so that they could push more forcefully at the electrical barriers of nuclei. Particle accelerators like Lawrence’s could be used to induce transmutation in heavy elements, and were thereby tools for making new elements.

  There was still something missing from the picture, and in retrospect it seems surprising that the nuclear physicists got so far without it. The proton or hydrogen nucleus could not be the sole constituent of the nucleus, for the mass of nuclei typically exceeded that of their protons. An alpha particle or helium nucleus, say, has twice the proton’s charge but four times its mass. What produced this extra mass, typically about equal to the total mass of the nucleus’ protons?

  The answer is a second nuclear constituent, the neutron: a particle with no charge but a mass essentially equal to the proton’s. Rutherford speculated in 1920 that such an entity—an ‘atom’ of mass 1, but without nuclear charge, as he expressed it—might exist. If so, what a boon it would be. Feeling no electrical repulsion from protons, it could enter other nuclei easily and thus be highly useful as a probe of the interior. But Rutherford did not regard this putative neutron as a fundamental particle; rather, it was a composite, a close association of a proton and an electron. His student James Chadwick was taken with the idea, and set out to find the neutron.

  It was, however, first sighted in Germany. In the late 1920s, Walther Bothe and Herbert Becker in Heidelberg were using alpha particles from polonium to bombard light elements such as lithium and beryllium. Heavier elements such as boron and magnesium are disintegrated by this treatment, spitting out protons just as in Rutherford’s experiments on nitrogen. But although beryllium was not disintegrated, nevertheless it emitted a kind of ‘radiation’ with intense penetrating power. Chadwick was intrigued, and so were Irène Curie, first daughter of Marie and Pierre, and her husband Frédéric Joliot in Paris. The French scientists found that Bothe’s ‘rays’ could knock protons out of hydrogen-rich substances such as water and paraffin wax. They thought that the rays were gamma rays, although others doubted that gamma rays would have enough energy to eject protons. This was their hypothesis in a paper that the Joliot-Curies presented to the French Academy of Sciences in January 1932. Chadwick saw it the following month, and didn’t believe it. He was convinced that in these findings ‘there was something quite new as well as strange’, and later admitted that ‘my thoughts were on the neutron’. He applied himself to his own experiments, showing in short order that all could be explained on the assumption that Bothe’s rays were indeed composed of particles like those proposed by Rutherford twelve years earlier, with zero charge and the same mass as the proton.

  The neutron sets a great deal in order. It accounts for the rest of the nuclear mass: a nucleus consists of a number of protons (which determines the a
tomic number and chemical identity of the element) combined with a comparable number of neutrons. The carbon atom, for example, has six of each, and thus atomic mass 12. But the neutron count for a given element is not fixed: different atoms of the same element may have different numbers of neutrons. These differences account for isotopes, which have identical atomic number but different atomic mass. Carbon atoms can, for example, also have five, seven or eight neutrons, and so masses of 11, 13 and 14. Carbon-13 is stable and accounts for a little over 1 per cent of naturally occurring carbon, but carbon-11 and carbon-14 are radioactive and decay by transmutation into other elements. They are therefore relatively short-lived, being produced in nuclear reactions. In particular, carbon-14 is formed from nitrogen in the atmosphere in a nuclear process induced by collisions of cosmic rays. It decays back into nitrogen by emitting a beta particle, with a half-life of around 5,730 years, and this decay supplies the basis of radiocarbon dating.

  Neutrons are the glue that binds protons together in the nucleus. In effect, neutrons and protons attract one another via a so-called ‘strong nuclear force’ that overwhelms the electrostatic repulsion one proton feels for another. Without neutrons, the nucleus would burst apart. It gradually became clear that neutrons are the key to beta decay too. This decay process looked peculiar: electrons are ejected from nuclei that apparently don’t contain any. But Rutherford’s and Chadwick’s composite notion of the neutron has some validity: in beta decay, a neutron can decay into a proton and an electron, under the auspices of a second nuclear force (the ‘weak force’). The electron leaves as a beta particle; the proton remains, transmuting the atom into the element one position to the right in the periodic table.*5

  It was Rutherford’s intuition about the value of the neutron for experimental nuclear physics that most excited many of his contemporaries. Without any charge, neutrons could burrow into a nucleus at energies far below those required by protons and alpha particles. No accelerator was needed. Chadwick’s colleague Norman Feather at the Cavendish soon showed that this was so, using neutrons to transmute nitrogen into boron. At the Kaiser Wilhelm Institute for Chemistry in Berlin (which Chadwick visited in June 1932), Lise Meitner and her assistant Kurt Philipp followed suit by transforming oxygen into carbon with neutrons.

  The neutron changed everything, and at the Solvay conference on the ‘Structure and Properties of the Atomic Nucleus’ in Brussels in October 1933 their nature was debated intensively. Hans Bethe, another Sommerfeld protégé who worked at the University of Munich before emigrating to England in 1933, has asserted that everything in nuclear physics before Chadwick’s discovery in 1932 was ‘prehistory’.†6 The real history, according to Bethe, started with the neutron.

  The world set free?

  It was also in 1932 that the itinerant Jewish Hungarian physicist Leo Szilard, sometime collaborator of Einstein in Berlin, first read H. G. Wells’ book The World Set Free (1914). Here Wells looked into the future forecast by Soddy, Rutherford and Aston, in which humankind had learnt how to liberate nuclear energy. Wells wrote of a war between England, France and America on one side, and Germany and Austria on the other, beginning in 1956. It would use what Wells called ‘atomic bombs’, which would destroy all the major cities of the world.

  Szilard had gone to Berlin in 1919, where he studied under Laue, Planck and Einstein before becoming Laue’s assistant and then a lecturer at the university. But in April 1933 he fled to Vienna, leaving just before the train became crammed with refugees from the Nazis. By September Szilard was in London, just another unemployed refugee himself. Here he read in The Times about a talk Rutherford had delivered to the annual meeting of the British Association for the Advancement of Science on ‘breaking down the atom’ and the ‘transformation of the elements’. Rutherford had mentioned the recent experiments by his colleagues John Cockcroft and Ernest Walton at the Cavendish, who had used a particle accelerator to fire protons at lithium atoms and split them into fragments, releasing a tremendous amount of energy. Rutherford doubted that this was a practical way to generate energy, rather hastily asserting that ‘anyone who looked for a source of power in the transformation of atoms was talking moonshine’.

  Szilard had a healthy disdain for proclamations that such and such was impossible, all the more so if they came from ‘experts’. Could Rutherford be proved wrong?

  It was while walking through the London streets that Szilard saw how. The answer would surely have occurred to any nuclear physicist, Rutherford included, sooner or later, and the neutron was the key. Bothe’s experiments had demonstrated that nuclear reactions could produce neutrons. Feather, Meitner and Philipp had shown that neutrons could induce them. What if there was an atomic disintegration that could both be triggered by the absorption of a neutron, and expel a neutron during the decay? Then the process could, once triggered, be sustained spontaneously, all the time releasing energy. If it happened slowly, this would create a continual source of heat which might be tapped for the generation of power. But such a self-perpetuating nuclear reaction could become a runaway process if the number of neutrons produced exceeded the number stimulating their production.*7 What if there was an element that absorbed one neutron and decayed to emit two? Then it would develop a chain reaction, a cascade of disintegrations that would bloom in an instant into a tremendous output of energy: an explosion. In his mind’s eye, Szilard saw the apocalyptic fury of Wells’ atomic bombs. As writer Richard Rhodes has put it, ‘time cracked open before him and he saw a way to the future, death into the world and all our woes, the shape of things to come’.

  On 4 July 1934—coincidentally the day Marie Curie died—Szilard filed a proposal with the British Patent Office for a method to harness nuclear energy based on a chain reaction of neutron-induced atomic disintegration. He never imagined that the vision would be his alone, and it wasn’t.

  However, when that same year Max Planck dangled before Goebbels the promise of ‘revolutionary innovations’ in nuclear physics, he had as yet nothing so concrete in mind. There was no talk of weapons, nor even a clear intimation of a source of unlimited power. All the physicists understood that atomic energy might be tapped one day, but most concurred with Rutherford that the prospect was indefinitely remote. Planck’s gambit was not much more than a ploy to garner state support for basic science: to wring money from the Nazis.

  The German Marie Curie

  Like Marie Curie, Lise Meitner always knew that she had to achieve more than her male colleagues if she was going to forge a career in science. When she arrived from Vienna to study in Berlin in 1907, Prussia still did not admit women to its universities. That changed the following year, but attitudes did not. Many academics were convinced that women would undermine the social and intellectual character of the universities. Planck, who Meitner revered for his ‘inner rectitude’, struggled to accept the idea: while a woman might be admitted into his discipline if she ‘possess[es] a special gift for the tasks of theoretical physics and also the drive to develop her talent’, he felt that ‘this does not happen often’. In general, he said, ‘Amazons are abnormal, even in intellectual fields . . . Nature itself had designated for woman her vocation as mother and housewife.’ Marie Curie was familiar with such wearisome views; when Einstein called Meitner ‘our Madame Curie’, he was saying more than he intended.

  Otto Hahn, based at the KWIC after his return from Rutherford’s laboratory in Montreal, met Meitner in September 1907 and they decided that, sharing an interest in nuclear chemistry, they would work together. But women were not permitted inside the institute, allegedly because its director Emil Fischer was convinced they would set fire to their hair in the laboratories. As a compromise, Meitner was given a room in the basement, but forbidden, rather symbolically, to come upstairs even to talk to Hahn. The partnership was immediately productive, and by the end of 1908 the pair had published several major papers in the field. Even Rutherford had heard of Meitner when he visited Berlin on his return journey from the Nobel c
eremony in Stockholm. Clearly, however, he did not know much else about her. ‘Oh, I thought you were a man!’ he confessed with his characteristic Antipodean bluntness.

  By the 1930s Meitner had established her credentials sufficiently to join Marie Curie and her daughter Irène as an honorary man in the ranks of physics, her sex tacitly ignored. There the three of them sit, surrounded by starched collars and ties, in a photograph of the Solvay conference in October 1933, Meitner in particular looking small and frail, her eyes directed to another part of the room. But by then she will have had other things on her mind besides nuclear physics.

  In 1934 Meitner and Hahn began to study neutron bombardment of uranium at the KWIC, where Hahn was now the director. This was basic science, not a quest to make nuclear power practical. They wanted to understand the sequence of transmutations that uranium underwent, stimulated by Fermi’s claim to have (perhaps) found new elements heavier than uranium by neutron irradiation. If uranium were to absorb a neutron and then undergo beta decay, it would gain a proton and so become the next element in the row of the periodic table: element 93, which was not known to exist in nature. In that year, Fermi suspected he might have found it, and perhaps even element 94 too. These so-called trans-uranic elements were given provisional names based on their presumed chemical similarities with the elements that would sit above them in the periodic table: eka-rhenium, eka-osmium. To sift through the products of these nuclear reactions required chemical adroitness, which was Hahn’s forte. To understand them, one needed Meitner’s physics.

  Lise Meitner, seated second from right at the 1933 Solvay conference. Irène Joliot-Curie is seated second from left, and her mother Marie Curie fifth.

  Fermi was wrong about his trans-uranics, but the principle of making elements by neutron capture was sound. Hahn and Meitner, assisted by a young German chemist named Fritz Strassmann, began to gather evidence for new types of radioactive substances created from uranium: maybe other isotopes of that element, maybe new elements in the uncharted territory beyond. There was something here, but it was hard to interpret.