Nuclear energy was in the air! And the discovery of the nucleus was another half-dozen years in the future.
Rutherford, a towering figure of this period, quite naturally wanted to understand the interior of the atom, which he now assumed to be complex and to include moving electric charges, probably electrons. He knew, too, that the atom must contain enough positive charge to balance the negative charge of the electrons, but what carried the positive charge and how it might be distributed within the atom no one knew. Some other physicists at the time constructed models of what an atom might look like.{10} Rutherford did experiments. And he had atomic bullets available. The alpha particles shot out by radioactive nuclei could serve as projectiles to be fired at targets. If the target was a thin sheet of metal, most of the alpha particles fired at it emerged on the other side, with varying small deflections away from their original flight direction. This was no surprise. The alpha particles passed through or quite near many atoms in the thin sheet, and Rutherford assumed that at each encounter the alpha particle suffered some small deflection, which could add to or subtract from a previous deflection. A large total deflection was not expected because of the random nature of the individual deflections. It’s as if you threw baseballs one after another through a stand of wheat. At each encounter with a stalk of wheat, a baseball would suffer a tiny deflection—left, right, up, down. If the stand were thin enough to allow most of the baseballs to get through, they would fan out on the other side, but only through a small range of angles. For one ball to get “reflected” and come back toward the thrower would require an incredibly improbable series of deflections, one after the other, all bending the trajectory in the same way to produce one large deflection.
This was the situation with alpha particles and metal foils in Rutherford’s Manchester laboratory in 1908-1909. (Rutherford had moved in 1907 from Montreal to Manchester, UK.{11} His 1908 Nobel Prize in Chemistry did not seem to slow him down at all.) Beginning in 1909, Rutherford’s associates Hans Geiger (yes, of the Geiger counter) and Ernest Marsden were beginning to see some larger-than-expected angles of deflections of alpha particles passing through gold foils.{12} This understandably caught Rutherford’s attention, for it was unexpected. He initiated a series of alpha-particle scattering experiments that culminated in 1911 with his announced discovery of the atomic nucleus.{13}
Geiger and Marsden were able to measure just what fraction of the incoming alpha particles were deflected through various angles, from zero degrees all the way to nearly 180 degrees. From these results, analyzed mathematically, Rutherford drew two conclusions. First, the deflection of an alpha particle was the result not of many accumulating small deflections, but of a single encounter within an atom. Second, the force causing the deflection was an electric force resulting from the presence within the atom of a massive nugget of electric charge.{14}, {15} From the experiments alone, it was not possible to tell whether this charge was positive or negative, but Rutherford assumed, correctly, that it was positive, given the evidence that negative electrons also existed in atoms. It was also not possible to tell how large this charged “nugget” (soon to be called a nucleus) was. From the number of alpha particles that “bounced” back, almost reversing course, Rutherford could conclude that this central nucleus was less than a thousandth the size of the atom (less than a billionth of the volume).{14}
It is in fact even a good deal smaller than that. Writing later about these findings, Rutherford said, “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back to hit you.”{15}
This discovery, described as introducing the “planetary model” of the atom, gave rise, over the next fifteen years, to a string of discoveries in atomic physics culminating in quantum mechanics and all of its wonders. Our concern here, however, is just with the nucleus itself and its energy. It was immediately evident to Rutherford and others that the atomic nucleus must be the site of radioactivity. Evident that unstable nuclei can emit alpha, beta, and gamma particles; that for alpha and beta emission, the nucleus is transmuted into that of a different element; and that the mass of a nucleus measures its energy content.
But could humankind control and harness the enormous energy stored within the nucleus, not just observe it? It was a writer, not a physicist, who first suggested that possibility. In his book The World Set Free, published in 1914,{16} H. G. Wells imagined an “atomic bomb,” a device for which scientists had figured out a way to make radioactive elements release their stored energy much more rapidly than the normal pace of spontaneous decay in nature. His hypothesized new element, carolinium, had a half life of seventeen days instead of the 1,600-year half life of radium or the even longer half life of uranium. Its energy release was activated by an “inductive” applied just after the bomb, a two-foot-diameter sphere, was dropped by hand from an airplane. Then, for weeks, the bomb continued to spew out its great store of energy on unlucky combatants on the ground.
Nearly twenty years later, in 1933, Leo Szilard, a Hungarian physicist then in London, came up with another idea for a nuclear bomb, still hypothetical but far better grounded than H. G. Wells’ fascinating fantasy. His thinking was based on two important discoveries of the preceding year. One was the discovery of the neutron by James Chadwick in Cambridge, England.{17} The neutron produced an immediate “aha” moment for physicists, who recognized at once that this neutral particle, about as massive as a proton, must be a constituent of atomic nuclei. Suddenly, nuclei could be imagined as collections of protons and neutrons rather than protons and electrons (a view that had been problematic for some time since no one could see how electrons could be confined within a nucleus). The other discovery of 1932 that influenced Szilard came from an experiment by John Cockcroft and Ernest Walton, also in Cambridge. They used an early-model accelerator to fire protons at a lithium target, and observed alpha particles emerging from the collision, each with an energy greater than the energy of an incident proton. Specifically, the isotope lithium-7 took part in the reaction, which can be written p + Li7 → 2α + energy.[28] Cockcroft and Walton knew that the combined mass of a proton and a lithium-7 nucleus was greater than the mass of two alpha particles, and indeed this known mass difference appeared as the energy of motion (the kinetic energy) of the emerging alpha particles.{18}
28
This reaction can be viewed as a kind of fission process. The combination of a proton and a Li7 nucleus creates, momentarily, a beryllium-8 nucleus (containing four protons and four neutrons), which then splits into two alpha particles, each with two protons and two neutrons. This fission process is an exception to the rule explained later in this chapter that fission releases energy for heavy nuclei, whereas fusion releases energy for light nuclei. That rule applies only for stable or long-lived nuclei. The highly unstable, and very short-lived Be8 nucleus