Richard Rhodes - The Making of the Atomic Bomb

An eyewitness to the ceremonies said Rutherford looked ridiculously young—he was thirty-seven—and made the speech of the evening. 155He announced his recent confirmation, only briefly reported the month before, that the alpha particle was in fact helium. 156The 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. 160Smoking 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, 163With 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. 166The 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. 167By 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. 168Lenard 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. 171Even 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.