Making of the Atomic Bomb

by Richard Rhodes

  Rutherford’s research at McGill unraveling the complex transmutations of the radioactive elements earned him, in 1908, a Nobel Prize—not in physics but in chemistry. He had wanted that prize, writing his wife when she returned to New Zealand to visit her family in late 1904, “I may have a chance if I keep going,” and again early in 1905, “They are all following on my trail, and if I am to have a chance for a Nobel Prize in the next few years I must keep my work moving.” The award for chemistry rather than for physics at least amused him.152, 153 “It remained to the end a good joke against him,” says his son-in-law, “which he thoroughly appreciated, that he was thereby branded for all time as a chemist and no true physicist.”154

  An eyewitness to the ceremonies said Rutherford looked ridiculously young—he was thirty-seven—and made the speech of the evening.155 He announced his recent confirmation, only briefly reported the month before, that the alpha particle was in fact helium.156 The confirming experiment was typically elegant. Rutherford had a glassblower make him a tube with extremely thin walls. He evacuated the tube and filled it with radon gas, a fertile source of alpha particles. The tube was gastight, but its thin walls allowed alpha particles to escape. Rutherford surrounded the radon tube with another glass tube, pumped out the air between the two tubes and sealed off the space. “After some days,” he told his Stockholm audience triumphantly, “a bright spectrum of helium was observed in the outer vessel.” Rutherford’s experiments still stun with their simplicity.157 “In this Rutherford was an artist,” says a former student. “All his experiments had style.”158

  In the spring of 1907 Rutherford had left Montreal with his family—by then including a six-year-old daughter, his only child—and moved back to England. He had accepted appointment as professor of physics at Manchester, in the city where John Dalton had first revived the atomic theory almost exactly a century earlier. Rutherford bought a house and went immediately to work. He inherited an experienced German physicist named Hans Geiger who had been his predecessor’s assistant. Years later Geiger fondly recalled the Manchester days, Rutherford settled in among his gear:

  I see his quiet research room at the top of the physics building, under the roof, where his radium was kept and in which so much well-known work on the emanation was carried out. But I also see the gloomy cellar in which he had fitted up his delicate apparatus for the study of the alpha rays. Rutherford loved this room. One went down two steps and then heard from the darkness Rutherford’s voice reminding one that a hot-pipe crossed the room at headlevel, and to step over two water-pipes. Then finally, in the feeble light one saw the great man himself seated at his apparatus.159

  The Rutherford house was cheerier; another Manchester protégé liked to recall that “supper in the white-painted dining room on Saturdays and Sundays preceded pow-wows till all hours in the study on the first floor; tea on Sundays in the drawing room often followed a spin on the Cheshire roads in the motor.” There was no liquor in the house because Mary Rutherford did not approve of drinking.160 Smoking she reluctantly allowed because her husband smoked heavily, pipe and cigarettes both.

  Now in early middle age he was famously loud, a “tribal chief,” as a student said, fond of banter and slang. He would march around the lab singing “Onward Christian Soldiers” off key. He took up room in the world now; you knew he was coming. He was ruddy-faced with twinkling blue eyes and he was beginning to develop a substantial belly. The diffidence was well hidden: his handshake was brief, limp and boneless; “he gave the impression,” says another former student, “that he was shy of physical contact.” He could still be mortified by condescension, blushing bright red and turning aside dumbstruck.161, 162, 163 With his students he was quieter, gentler, solid gold. “He was a man,” pronounces one in high praise, “who never did dirty tricks.”164

  Chaim Weizmann, the Russian-Jewish biochemist who was later elected the first president of Israel, was working at Manchester on fermentation products in those days. He and Rutherford became good friends. “Youthful, energetic, boisterous,” Weizmann recalled, “he suggested anything but the scientist. He talked readily and vigorously on every subject under the sun, often without knowing anything about it. Going down to the refectory for lunch I would hear the loud, friendly voice rolling up the corridor.” Rutherford had no political knowledge at all, Weizmann thought, but excused him on the grounds that his important scientific work took all his time.165 “He was a kindly person, but he did not suffer fools gladly.”

  In September 1907, his first term at Manchester, Rutherford made up a list of possible subjects for research. Number seven on the list was “Scattering of alpha rays.” Working over the years to establish the alpha particle’s identity, he had come to appreciate its great value as an atomic probe; because it was massive compared to the high-energy but nearly weightless beta electron, it interacted vigorously with matter.166 The measure of that interaction could reveal the atom’s structure. “I was brought up to look at the atom as a nice hard fellow, red or grey in colour, according to taste,” Rutherford told a dinner audience once.167 By 1907 it was clear to him that the atom was not a hard fellow at all but was substantially empty space. The German physicist Philipp Lenard had demonstrated as much in 1903 by bombarding elements with cathode rays.168 Lenard dramatized his findings with a vivid metaphor: the space occupied by a cubic meter of solid platinum, he said, was as empty as the space of stars beyond the earth.

  But if there was empty space in atoms—void within void—there was something else as well. In 1906, at McGill, Rutherford had studied the magnetic deflection of alpha particles by projecting them through a narrow defining slit and passing the resulting thin beam through a magnetic field. At one point he covered half the defining slit with a sheet of mica only about three thousandths of a centimeter thick, thin enough to allow alpha particles to go through. He was recording the results of the experiment on photographic paper; he found that the edges of the part of the beam covered with the mica were blurred. The blurring meant that as the alpha particles passed through, the atoms of mica were deflecting—scattering—many of them from a straight line by as much as two degrees of angle. Since an intense magnetic field scattered the uncovered alpha particles only a little more, something unusual was happening. For a particle as comparatively massive as the alpha, moving at such high velocity, two degrees was an enormous deflection. Rutherford calculated that it would require an electrical field of about 100 million volts per centimeter of mica to scatter an alpha particle so far.169 “Such results bring out clearly,” he wrote, “the fact that the atoms of matter must be the seat of very intense electrical forces.” It was just this scattering that he marked down on his list to study.170

  To do so he needed not only to count but also to see individual alpha particles. At Manchester he accepted the challenge of perfecting the necessary instruments. He worked with Hans Geiger to develop an electrical device that clicked off the arrival of each individual alpha particle into a counting chamber. Geiger would later elaborate the invention into the familiar Geiger counter of modern radiation studies.

  There was a way to make individual alpha particles visible using zinc sulfide, the compound that coated the tube of radium solution Pierre Curie had carried into the night garden in Paris in 1903. A small glass plate coated with zinc sulfide and bombarded with alpha particles briefly fluoresced at the point where each particle struck, a phenomenon known as “scintillation” from the Greek word for spark. Under a microscope the faint scintillations in the zinc sulfide could be individually distinguished and counted. The method was tedious in the extreme. It required sitting for at least thirty minutes in a dark room to adapt the eyes, then taking counting turns of only a minute at a time—the change signaled by a timer that rang a bell—because focusing the eyes consistently on a small, dim screen was impossible for much longer than that.171 Even through the microscope the scintillations hovered at the edge of visibility; a counter who expected an experiment to produce a certain number
of scintillations sometimes unintentionally saw imaginary flashes. So the question was whether the count was generally accurate. Rutherford and Geiger compared the observation counts with matched counts by the electric method. When the observation method proved reliable they put the electric counter away. It could count, but it couldn’t see, and Rutherford was interested first of all in locating an alpha particle’s position in space.

  Geiger went to work on alpha scattering, aided by Ernest Marsden, then an eighteen-year-old Manchester undergraduate. They observed alpha particles coming out of a firing tube and passing through foils of such metals as aluminum, silver, gold and platinum. The results were generally consistent with expectation: alpha particles might very well accumulate as much as two degrees of total deflection bouncing around among atoms of the plum-pudding sort. But the experiment was troubled with stray particles.172 Geiger and Marsden thought molecules in the walls of the firing tube might be scattering them. They tried eliminating the strays by narrowing and defining the end of the firing tube with a series of graduated metal washers. That proved no help.

  Rutherford wandered into the room. The three men talked over the problem. Something about it alerted Rutherford’s intuition for promising side effects. Almost as an afterthought he turned to Marsden and said, “See if you can get some effect of alpha particles directly reflected from a metal surface.” Marsden knew that a negative result was expected—alpha particles shot through thin foils, they did not bounce back from them—but that missing a positive result would be an unforgivable sin.173 He took great care to prepare a strong alpha source. He aimed the pencil-narrow beam of alphas at a forty-five degree angle onto a sheet of gold foil. He positioned his scintillation screen on the same side of the foil, beside the alpha beam, so that a particle bouncing back would strike the screen and register as a scintillation. Between firing tube and screen he interposed a thick lead plate so that no direct alpha particles could interfere.

  Arrangement of Ernest Marsden’s experiment: A-B, alpha particle source. R-R, gold foil. P, lead plate. S, zinc sulfide scintillation screen. M, microscope.

  Immediately, and to his surprise, he found what he was looking for. “I remember well reporting the result to Rutherford,” he wrote, “ . . . when I met him on the steps leading to his private room, and the joy with which I told him.”174

  A few weeks later, at Rutherford’s direction, Geiger and Marsden formulated the experiment for publication. “If the high velocity and mass of the α-particle be taken into account,” they concluded, “it seems surprising that some of the α-particles, as the experiment shows, can be turned within a layer of 6 × 10−5 [i.e., .00006] cm. of gold through an angle of 90°, and even more. To produce a similar effect by magnetic field, the enormous field of 109 absolute units would be required.” Rutherford in the meantime went off to ponder what the scattering meant.175

  He pondered, in the midst of other work, for more than a year. He had a first quick intuition of what the experiment portended and then lost it.176 Even after he announced his spectacular conclusion he was reluctant to promote it. One reason for his reluctance might be that the discovery contradicted the atomic models J. J. Thomson and Lord Kelvin had postulated earlier. There were physical objections to his interpretation of Marsden’s discovery that would require working out as well.

  Rutherford had been genuinely astonished by Marsden’s results. “It was quite the most incredible event that has ever happened to me in my life,” he said later. “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration I realised that this scattering backwards must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greatest part of the mass of the atom was concentrated in a minute nucleus.”177

  “Collision” is misleading. What Rutherford had visualized, making calculations and drawing diagrammatic atoms on large sheets of good paper, was exactly the sort of curving path toward and away from a compact, massive central body that a comet follows in its gravitational pas de deux with the sun.178 He had a model made, a heavy electromagnet suspended as a pendulum on thirty feet of wire that grazed the face of another electromagnet set on a table.179 With the two grazing faces matched in polarity and therefore repelling each other, the pendulum was deflected into a parabolic path according to its velocity and angle of approach, just as the alpha particles were deflected. He needed as always to visualize his work.

  When further experiment confirmed his theory that the atom had a small, massive nucleus, he was finally ready to go public. He chose as his forum an old Manchester organization, the Manchester Literary and Philosophical Society—“largely the general public,” says James Chadwick, who attended the historic occasion as a student on March 7, 1911, “ . . . people interested in literary and philosophical ideas, largely business people.”180

  The first item on the agenda was a Manchester fruit importer’s report that he had found a rare snake in a consignment of Jamaica bananas.181 He exhibited the snake. Then it was Rutherford’s turn. Only an abstract of the announcement survives, but Chadwick remembers how it felt to hear it: it was “a most shattering performance to us, young boys that we were. . . . We realized this was obviously the truth, this was it.”182

  Rutherford had found the nucleus of his atom. He did not yet have an arrangement for its electrons. At the Manchester meeting he spoke of “a central electric charge concentrated at a point and surrounded by a uniform spherical distribution of opposite electricity equal in amount.” That was sufficiently idealized for calculation, but it neglected the significant physical fact that the “opposite electricity” must be embodied in electrons.183 Somehow they would have to be arranged around the nucleus.

  Another mystery. A Japanese theoretical physicist, Hantaro Nagaoka, had postulated in 1903 a “Saturnian” model of the atom with flat rings of electrons revolving like Saturn’s rings around a “positively charged particle.”184 Nagaoka adapted the mathematics for his model from James Clerk Maxwell’s first triumphant paper, published in 1859, “On the stability of motion of Saturn’s rings.” All Rutherford’s biographers agree that Rutherford was unaware of Nagaoka’s paper until March 11, 1911—after the Manchester meeting—when he heard about it by postcard from a physicist friend: “Campbell tells me that Nagaoka once tried to deduce a big positive centre in his atom in order to account for optical effects.” He thereupon looked up the paper in the Philosophical Magazine and added a discussion of it to the last page of the full-length paper, “The scattering of a and β particles by matter and the structure of the atom,” that he sent to the same magazine in April.185 He described Nagaoka’s atom in that paper as being “supposed to consist of a central attracting mass surrounded by rings of rotating electrons.”186

  But it seems that Nagaoka had recently visited him, because the Japanese physicist wrote from Tokyo on February 22, 1911, thanking him “for the great kindness you showed me in Manchester.”1 Yet the two physicists seem not to have discussed atomic models, or Nagaoka would probably have continued the discussion in his letter and Rutherford, a totally honest man, would certainly have acknowledged it in his paper.187

  One reason Rutherford was unaware of Nagaoka’s Saturnian model of the atom is that it had been criticized and abandoned soon after Nagaoka introduced it because it suffered from a severe defect, the same theoretical defect that marred the atom Rutherford was now proposing.188 The rings of Saturn are stable because the force operating between the particles of debris that make them up—gravity—is attractive. The force operating between the electrons of Nagaoka’s Saturnian electron rings, however—negative electric charge—was repulsive. It followed mathematically that whenever two or more electrons equally spaced on an orbit rotated around the nucleus, they would drift into modes of oscillation—instabilities—that would quickly tear the atom apart.

  What was tr
ue for Nagaoka’s Saturnian atom was also true, theoretically, for the atom Rutherford had found by experiment. If the atom operated by the mechanical laws of classical physics, the Newtonian laws that govern relationships within planetary systems, then Rutherford’s model should not work. But his was not a merely theoretical construct. It was the result of real physical experiment. And work it clearly did. It was as stable as the ages and it bounced back alpha particles like cannon shells.

  Someone would have to resolve the contradiction between classical physics and Rutherford’s experimentally tested atom. It would need to be someone with qualities different from Rutherford’s: not an experimentalist but a theoretician, yet a theoretician rooted deeply in the real. He would need at least as much courage as Rutherford had and equal self-confidence. He would need to be willing to step through the mechanical looking glass into a strange, nonmechanical world where what happened on the atomic scale could not be modeled with planets or pendulums.

  As if he had been called to the cause, such a person abruptly appeared in Manchester. Writing to an American friend on March 18, 1912, Rutherford announced the arrival: “Bohr, a Dane, has pulled out of Cambridge and turned up here to get some experience in radioactive work.” “Bohr” was Niels Henrick David Bohr, the Danish theoretical physicist.189 He was then twenty-seven years old.