The Making of the Atomic Bomb

high-energy but nearly weightless beta electron, it interacted vigorously with matter. 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. 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. 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. “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.

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 cricked 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. 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. 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. 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.”

A few weeks later, at Rutherford's direction, Geiger and Marsden formulated the experiment for publication. “If the high velocity and mass of the ?-particle be taken into account,” they concluded, “it seems surprising that some of the ?-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.

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. 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.”

“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. 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. 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.”

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. 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.”

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. 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.” Nagaoka adapted the mathematics for his model from James Clerk Maxwell's first triumphant paper, pubUshed in