Diana Preston - Before the Fallout

Chadwick chose the Kapitza Club as the forum for revealing his findings. There was an air of keen anticipation as Chadwick, gray-faced from lack of sleep but plainly exhilarated, addressed his audience. Mark Oliphant captured the moment in the restrained language of the day: “Kapitza had taken him to dine in Trinity beforehand, and he was in a very relaxed mood. His talk was extremely lucid and convincing, and the ovation he received from the select audience was spontaneous and warm. All enjoyed the story of a long quest, carried through with persistence and vision.” [16] Trinity is one of Cambridge’s most famous colleges and the one in which Lord Byron famously kept a bear in his rooms. At the end, the exhausted Chadwick asked “to be chloroformed and put to bed for a fortnight.” In fact, he was up again the next morning writing to Niels Bohr and, a month after first reading the Joliot-Curies’ paper, sending a letter to Nature cautiously headed “The possible existence of the neutron.” His entry in the notebook recording presentations to the Kapitza Club was similarly guarded; it read, “Neutron?” Chadwick was instinctively cautious. Yet however he hedged his findings, he knew in his heart he was right.

Chadwick was not, as he freely acknowledged, the first to produce neutrons. Walther Bothe had done so in Germany in the 1920s. So had the Joliot-Curies, following in Bothe’s wake. However, none of them had interpreted their experiments correctly and established the existence of the neutron. Chadwick’s achievement, in the words of the distinguished Italian physicist Emilio Segre, was “immediately, clearly and convincingly” to recognize neutrons for what they were—the true hallmark “of a great experimental physicist.” Chadwick put it more modestly and prosaically: “The reason that I found the neutron was that I had looked, on and off, since about 1923 or 4. I was convinced that it must be a constituent of the nucleus.”

The discovery was a blow to Frederic Joliot-Curie, who wrote privately of his frustration: “It is annoying to be overtaken by other laboratories which immediately take up one’s experiments.” However, his public response was gracious and generous. It was “natural and just” that the final steps of the journey toward the neutron were undertaken at the Cavendish, since “old laboratories with long traditions have… hidden riches.”

Chadwick’s achievement marked a watershed. Nuclear physics (the study of the atom’s nucleus) as opposed to atomic physics (the study of atoms) had been in the doldrums. Scientists had faced difficulties of interpretation that arose far more swiftly than they could be resolved. Chadwick’s discovery provided the all-important clue to many unresolved problems. For example, the neutron added to the understanding of isotopes (discovered in 1913 by Frederick Soddy). Until then, no one had known exactly what differentiated isotopes from their “sister” element. The suspicion was that the difference lay in the nucleus, but it took Chadwick’s findings to prove that suspicion correct; what made isotopes different was the number of neutrons in their nuclei. But most exciting of all was the realization that the neutron, which carried no electrical charge, would not be deflected by the positive nuclear charge. It was the ideal missile with which to bombard and probe elements, as it could hurtle on until it penetrated the nucleus of the atom.

Across Europe scientists took note. In Germany the physicist Hans Bethe, later the head of theoretical physics at Los Alamos and an architect of the atomic bomb, decided that the discovery of the neutron made nuclear physics the field in which to work. In Rome the Italian scientist Enrico Fermi—yet another of the fraternity who had studied under Max Born in Gottingen in the 1920s and till then a theoretical physicist—plunged into experimental nuclear physics, setting up a small group to explore the interactions of neutrons “with any elements he could get hold of.”

What none of them yet knew was that the neutron was also the catalyst for achieving an explosive nuclear chain reaction. Curiously, though, that very year, 1932, Harold Nicolson published a novel, Public Faces, abouc a catastrophically destructive new weapon made from a powerful raw material. This substance could transmute itself with such violence that it could cause an explosion “that would destroy all matter within a considerable range and send out waves that would exterminate all life over an indefinite area.” “The experts,” Nicholson wrote in his novel, “had begun to whisper the words… ‘atomic bomb.’” They claimed it could “destroy New York.”

Neutrons were by no means the only reason 1932 would be recalled as a spectacular year in the history of science. In January, just a few weeks before Chad­wick’s coup, the American chemist Harold Urey made another discovery that Rutherford had long predicted. Working at Columbia University, Urey found that natural hydrogen consisted of 99.985 percent ordinary hydrogen but also of 0.015 percent “heavy hydrogen”—an isotope given the name “deu­terium”—which also existed naturally in combination with oxygen in water. This so-called “heavy water”—which appeared to the naked eye identical to ordinary water—boiled and froze at different temperatures and was 1 o percent heavier. A decade later it would become a substance much sought after by the Nazis, and people would die to deny it to them.

But in 1932 Urey thought of deuterium as a “delightful plaything for physicists” to use in bombarding other more complex atoms so that they could better understand nuclear structure. He speculated whether heavy water itself might be “valuable in understanding more of living processes,” perhaps even in the study of cancer since some initial research showed that yeast cells, which had some similarities to cancer cells, multiplied less quickly in heavy water than in ordinary. This proved impracticable. Nevertheless, heavy water caught the American public’s attention. In a 1935 fictional murder mystery, the villain killed by persuading the victim to enter a swimming pool filled with heavy water, which the author described as “lethal.” [17] Drinking a few glasses of heavy water would not be lethal, but the replacement of more than one third of the hydrogen in the human body’s fluids by deuterium would be fatal. In a review a scientist wrote, “It is the most expensive murder on record…. at the present cost that pool of heavy water would have cost about $ 200 million.”

On 21 April 1932, a few weeks after the neutron discovery, Rutherford reported another Cavendish triumph, writing exuberantly to Bohr, “It never rains but it pours.” John Cockcroft and Ernest Walton had just become the first scientists to split the atom using a man-made machine, an accelerator—the device Rutherford had asked them to develop some time earlier. They had created it lovingly and carefully, smoothing plasticine—an innovative new material which had replaced the sealing wax previously used for this purpose—over the joints to create a vacuum. Fearing that rivals might overtake them, Rutherford had urged them to stop perfecting it and “do what he’d told them to do months ago”—start experimenting. His bullying paid off. Cock­croft and Walton bombarded lithium with accelerated protons and succeeded in disintegrating the lithium nucleus into two helium nuclei. According to one of his colleagues, John Cockcroft, “normally about as much given to emotional display as the Duke of Wellington,” ran through Cambridge shouting, “We’ve split the atom! We’ve split the atom!” An additional excitement was that the energies of the particles measured by Cockcroft and Walton provided the first experimental confirmation of the validity of Einstein’s proposal that E = mc 2.