Diana Preston - Before the Fallout

Lawrence was an extrovert of overpowering drive and energy, much like Rutherford as a young man. He also had some of Rutherford’s intuition, and this had helped him conceive the cyclotron. In 1929, the year of the Wall Street crash, Lawrence came across an article by Rolf Wideroe, a Norwegian engineer working in Germany, describing a linear device that would accelerate charged particles down a straight tube—similar to the approach being pursued at the Cavendish Laboratory. Lawrence’s German was not good enough for him to understand everything Wideroe had written, but as he studied the accompanying diagram, an inspirational thought struck him. If he could confine particles with electromagnets within a circular track, rather than pushing them along a straight line, he could accelerate them indefinitely, causing them to whizz faster after each burst of voltage. It would, in his words, be a “proton merry-go-round.” He told his friends, confidently—and accurately as it turned out—“I’m going to bombard and break up atoms!” “I’m going to be famous.”

Lawrence’s first machine was “a four-inch pillbox sprouting arms like an octopus.” When he demonstrated it to the U.S. National Academy of Sciences, he secured it in place on a kitchen chair with a clothes hanger. Despite its absurd appearance, its potential caused a sensation. Newspapers hailed the invention of a device “to break up atoms,” and they were right. So good was his progress that by the end of the 1930s Lawrence would build a cyclotron with a magnet weighing 200 tons. Inspired by the desire to explore one of the tiniest things in existence, the nucleus of the atom, big science was coming.

While the creators of the new atom-smashing machines honed their early designs, quantum mechanics continued to forge bridges between Europe and the United States. Just as young Americans eager to understand the new theories were flocking to Arnold Sommerfeld in Munich, Max Born in Gottingen, Werner Heisenberg in Leipzig, and Niels Bohr in Copenhagen, European scientists were touring the United States to spread the word. The big names like Einstein were eagerly sought, but so too were younger scientists. The Hungarians John von Neumann and Eugene Wigner were invited as guest lecturers. Their task, in Wigner’s words, was “to modernise” America’s “scientific spirit.” They saw themselves as “pioneers who break new ground”; their mission, to make quantum mechanics and relativity theory a reality to people to whom it was still “an abstraction.”

The experimenters of the Cavendish Laboratory were less immediately impressed by the deluge of fresh ideas. James Chadwick recalled that “it took quite a time to absorb the meaning of the new quantum mechanics. It was rather slow…. there was no immediate application to the structure of the nucleus, which was what we were interested in.” Rutherford was frankly skeptical of the complex new mathematical theories, preferring to scent new discoveries in some unexpected experimental result rather than indulge in abstract theorizing. Only in the late 1920s did he concede somewhat grudgingly that wave mechanics might aid the understanding of the nucleus. In the meantime his laboratory remained the greatest center of experimental physics in the world. His only rivals were Lise Meitner and Otto Hahn in Berlin and Marie Curie and Irene and Frederic Joliot-Curie—as the pair chose to be known to emphasize their close collaboration—in Paris. [15] Frederic Joliot was sensitive to suggestions that their choice, highly unusual at the time, reflected a desire to retain the fame of the Curie name or anv subordinate status for him. All the other major players were theorists.

Rutherford had been convinced for many years that an undetected particle at the heart of the nucleus, the “neutron,” as he called it, was the great, unclaimed prize. As early as June 1920 he had talked to the Royal Society of the possible existence of such a particle. His discovery, the year before, of the positively charged proton, residing in the nucleus of every atom, had provided tantalizing clues. For example, the simplest, lightest atom—hydrogen—had one single, positively charged proton counterbalanced by one external, negatively charged electron. The next-heaviest atom—helium—had two protons and two orbiting electrons. However, its mass, or atomic weight, was not, as might have been expected, double that of hydrogen. It was quaduple. This could only mean that it had to have one or more electrically neutral particles, equivalent in mass to, and complementing, the two protons. Rutherford speculated intuitively that the missing piece of the jigsaw, his “neutron,” consisted of electrons and protons parceled together.

Although Rutherford continued to think about the neutron throughout the 1920s and undertook experiments when he could, he was frequently distracted by other work, including the pressures of university administration and serving on national public committees. His ennoblement in 1931 by King George V as Baron Rutherford only added to the commitments of a man who was still considerably shaken by the sudden death in 1930 from a blood clot of his only child, his daughter, Eileen. She had left four children, to whom Rutherford was deeply attached, from her marriage to a Cavendish mathematician.

Realizing that domestic pressures and public duties would continue to hamper his search for the neutron, Rutherford entrusted more and more of the hunt to James Chadwick, who had already been working on the topic for him since the mid-1920s and who, in his own words, “just kept on pegging away” and “did quite a number of quite silly experiments” just in case they turned something up. In fact, he worked obsessively. His efforts attracted affectionate satire from junior members of the laboratory, who staged a show raucously lampooning the hunt for the elusive “Fewtron.”

Chadwick made his breakthrough in January 1932, precipitated by a paper by the Joliot-Curies in the French journal Comptes Rendus. This described how, building on work by the German scientist Walther Bothe, they had bombarded the light element beryllium—a hard, silvery, toxic metal—with an intense source of polonium, causing an unusually penetrating radiation to stream out of the beryllium. The Joliot-Curies experimented with various substances, including wax, to see whether they could halt the rays from the beryllium, but the rays not only passed through the barriers but appeared to get stronger. The puzzled Joliot-Curies concluded in their paper that the radiation had to consist of some particularly powerful form of gamma rays—the most penetrating of the three types of radiation emitted by radioactive substances. Rutherford read their conclusions and roared, “I don’t believe it.” Chadwick, too, “knew in his bones” that they were wrong. Their description of the pattern and path of the radiation they had observed convinced him that it consisted of uncharged or neutral particles knocked out of the nuclei of the beryllium—in other words, neutrons. Chadwick rushed to replicate their experiments.

Applying the classic “sealing wax and string” principles of the Cavendish to make his equipment the simplest fit for the purpose, an excited but careful Chadwick worked day and night. He violated Rutherford’s rule that all work in the laboratory should cease by 6 p.m., partly through irrepressible enthusiasm but also so that his sensitive counting equipment would not be affected by other work going on in the laboratory. After three weeks he had shown that radiation from bombarded beryllium was powerful enough to knock particles out of hydrogen, helium, lithium, beryllium, carbon, and argon. The particles expelled from the hydrogen were clearly protons, and the others were whole nuclei of the target substance. His measurements of their penetrating power and velocity proved that gamma rays could never have caused the ejection of particles of such energy. The only viable conclusion was that the radiation flowing so powerfully from the bombarded beryllium consisted of “particles of mass 1 and charge o”—neutrons.