Knocking on Heaven's Door - Lisa Randall [52]
The discovery of the nucleus, the other part of the atom, was more surprising still. In an experiment analogous to particle experiments today, Ernest Rutherford and his students discovered the nucleus by shooting Helium nuclei (then called alpha particles since nuclei hadn’t been discovered) at a thin gold foil. The alpha particles turned out to have enough energy for Rutherford to identify the structure inside the nucleus. He and his colleagues found that the alpha particles they shot at the foil sometimes scattered at much greater angles than they would have anticipated. (See Figure 20.) They expected scatterings like those from tissue paper and instead discovered ones seeming more like they were ricocheting off marbles inside. In Rutherford’s own words:
[ FIGURE 20 ] Rutherford’s experiment scattered alpha particles (which we now know to be Helium nuclei) off gold foil. The unexpectedly large deflections of some of the alpha particles demonstrated the existence of concentrated masses at the centers of the atoms—atomic nuclei.
“It was quite the most incredible event that has ever happened to me in my life. 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 realized that this scattering backward 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 greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre carrying a charge.” 32
The experimental discovery of quarks inside protons and neutrons used methods in some respects similar to Rutherford’s but required even higher energies than that of the alpha particles he had used. Those higher energies required a particle accelerator that could accelerate electrons and the photons they radiated to sufficiently high energies.
The first circular particle accelerator was named a cyclotron, due to the circular paths along which the particles were accelerated. Ernest Lawrence built the first cyclotron at the University of California in 1932. It was less than a foot in diameter and was very feeble by modern standards. It produced nowhere near the energy needed to discover quarks. That milestone could happen only with a number of improvements in accelerator technology (that nicely gave rise to a couple of important discoveries along the way).
Well before quarks and the inner structure of the nucleus could be explored, Emilio Segre and Owen Chamberlain received the 1959 Nobel Prize for their discovery of antiprotons at Lawrence Berkeley Laboratory’s Bevatron in 1955. The Bevatron was a more sophisticated accelerator than a cyclotron and could raise the protons to energy more than six times their rest mass—more than enough to create proton-antiproton pairs. The proton beam at the Bevatron bombarded targets and (via the magic of E = mc2) produced exotic matter, which includes antiprotons and antineutrons.
Antimatter plays a big role in particle physics, so let’s take a brief detour to explore this remarkable counterpart to the matter we observe. Because the charges of matter and antimatter particles add up to zero, matter can annihilate with its associated antimatter when they meet. For example, antiprotons—one