How to Make an Apple Pie from Scratch

by Harry Cliff

  Between them, Rutherford and Aston had created the first truly unified theory of matter, radical in its simplicity. You only needed two ingredients to make any atom you fancied: the proton and the electron. Protons are just the nuclei of hydrogen atoms and are positively charged, while electrons are negative and have a tiny mass, two thousand times lighter than the proton. At the time, Rutherford, Aston, and most other physicists believed that the tiny nucleus must contain both protons and electrons squashed together inside it, with other “atomic” electrons orbiting the nucleus at much larger distances. You needed to have electrons in the nucleus to explain why all the elements except hydrogen had masses that were roughly twice as big as their positive charges. For example, the helium nucleus has a charge of +2 but weighs as much as four hydrogen atoms. This must mean that the helium nucleus contains four positive protons combined with two negative electrons, which cancel out those two extra positive charges. To make a helium atom you then add two extra atomic electrons in orbit around the nucleus, leaving the atom electrically neutral overall. The same goes for carbon, which was thought to have a nucleus made of twelve protons and six electrons, giving it a mass of 12 and a charge of +6, with six further electrons orbiting the nucleus to complete the carbon atom. What’s more, the fact that radioactive elements occasionally spat out electrons (known as “beta rays”) only added to the evidence that electrons must exist inside the nucleus.

  As for isotopes, these could now be understood simply by adding extra protons and electrons to the nucleus. Add two extra protons and two extra electrons to the nucleus of chlorine-35 and you’ve got chlorine-37. The electric charges of the protons and electrons cancel each other out, meaning that the total charge of the chlorine nucleus stays the same (which, after all, is what sets the atom’s chemical properties and means it’s still chlorine), but we’ve successfully added two units of mass to the nucleus to create a heavier version of the same atom.

  The theory was a triumph, neatly explaining the makeup of the chemical elements, how atoms change in radioactive decay, and why many of the elements have different isotopes. Unfortunately, it was wrong. Rutherford, Aston, and their colleagues were missing a key ingredient, the one we need to finally complete our atomic shopping list and allow us to make any chemical element we like. Finding it, though, was going to be a long and tortuous process.

  NEUTRON, WHERE ART THOU?

  In a room at the Cavendish Laboratory, two full-grown men sit hunched in what can only be described as a large box. One is Ernest Rutherford, the big-boned, booming father of nuclear physics. Squashed in beside him is James Chadwick, pale, thin, and taciturn. They make an odd couple. Outside, the lab assistant, George Crowe, has just brought down a radioactive source from the storeroom in the Cavendish’s neo-Gothic tower and is now busily setting up the apparatus for their experiment. As they sit together in the darkness waiting for their eyes to adjust, naturally they talk.

  Since his return to Cambridge to take charge of the Cavendish following J. J. Thomson’s retirement, Rutherford has been pondering the same question that we’re trying to answer: How do you make the chemical elements? He has realized that as you start to build heavier and heavier atoms by adding protons to the nucleus you soon run into a serious problem. As the nucleus gets bigger, so does its positive electric charge, which means it exerts a stronger and stronger repulsive force on any proton that tries to get close. Eventually this force becomes so enormous that for a proton to get into the nucleus it would have to be traveling at what Rutherford considered to be impossibly high speeds.

  Now Rutherford is not usually one for wild speculation, but here, sitting with Chadwick in the dark, he lets his imagination wander. If both electrons and protons exist inside the nucleus, then why shouldn’t it be possible to squeeze a single electron and a single proton together to make a nucleus with an electric charge of zero? This neutral nucleus would be unlike any particle seen so far. It wouldn’t form atoms in the traditional sense, would be totally chemically inert, and would be impossible to contain in any vessel. But this weird hypothetical particle might just hold the key to the creation of all the elements. We know it today as the neutron.

  While a positively charged proton is repelled by a positively charged nucleus, the neutron would face no such obstacle. No electric charge means no repulsive force; the neutron could simply breeze its way into any nucleus, even one with a gigantic repulsive field around it, a bit like a ghost walking through the wall of a heavily fortified castle. As they talk Rutherford and Chadwick become convinced that adding neutrons to the nucleus is the only way to build up the heavier atoms—without the neutron, most of the elements in the periodic table simply would not exist.

  But finding the neutron, if it exists, is going to be devilishly hard. Every method of detecting particles that exists at the time relies on the particles’ electric charge to make them visible in one way or another. Protons and alpha particles only produce flashes when they hit a zinc-sulfide screen, thanks to their electric charge. The neutron, on the other hand, would leave no trace at all.

  The first experiment they try is like something out of Frankenstein. Rutherford supposed that if you could pass an extremely powerful electric arc through a tube of hydrogen gas, then perhaps the huge electrical forces would drive electrons and protons together and neutrons would come flying out. They try it, at not inconsiderable risk to their safety, but with no success. In fact, every experiment they try ends in failure.

  As the 1920s wore on, Rutherford and Chadwick cooked up ever-more-desperate schemes to trap the elusive neutron. As Chadwick later put it, “I did quite a number of quite silly experiments, when it comes to that. I must say the silliest were done by Rutherford.” For the first time in his life, Rutherford was firing blanks. The indefatigable bloodhound of physics became increasingly frustrated and disillusioned, spending less and less time in the lab and devoting more of his energy to his growing national and international role as a scientific leader. Appointed assistant director of the Cavendish, Chadwick was now responsible for the day-to-day running of the lab, setting projects for the researchers and fighting a continual battle against the lack of equipment and space. By the mid-1920s the Cavendish was beginning to show its age, while Rutherford stubbornly insisted on filling the creaking building with as many research students as possible.

  Although he was undoubtedly an inspiring director, Rutherford’s view that any experiment worth its salt could be done on a shoestring budget was beginning to hamstring the Cavendish’s work. After all, who needs fancy apparatus? He had unraveled the mysteries of radioactivity, discovered the atomic nucleus and broken it apart using startlingly basic equipment that could fit easily on a lab bench. When one student complained that he didn’t have the equipment necessary to make progress, Rutherford bellowed, “Why, I could do research at the North Pole!” This attitude was now also starting to put a strain on his relationship with Chadwick.

  Chadwick was by no means unresourceful—during the First World War he had managed to run a makeshift laboratory while a prisoner at the famous Ruhleben camp in Germany—but even he struggled to meet the needs of his researchers. He later recalled how one young Australian physicist, Mark Oliphant, once came to him almost in tears, unable to make any progress at all without the right sort of pump. The only way Chadwick could placate the distraught young man was to “borrow” a pump from Rutherford’s personal research room, which he kept strictly reserved for his own public demonstrations.

  Nonetheless, Chadwick soldiered on. He was convinced the neutron must be out there; it was only a matter of finding the right experiment. “I just kept on pegging away,” he later recalled. “I didn’t see any other way of building up nuclei.”

  When Rutherford had returned to Cambridge in 1919, most of the global physics community was caught up in the quantum revolution that was shaking physics to its foundations. Studying the atomic nucleus, on the other
hand, was a bit of a fringe pursuit. Rutherford had taken the Cavendish out on a limb as the only lab dedicated almost entirely to nuclear physics, but by the end of the 1920s researchers in Vienna, Berlin, and Paris were starting to challenge Rutherford’s lab for the crown.

  A new way to peer into the nucleus had caught their interest. When nuclei of lighter elements were bombarded with alpha particles, they often emitted high-energy particles of light, known as “gamma rays.” The idea was that when an alpha particle smacked into a nucleus it briefly kicked its constituent protons and electrons out of their usual positions into an “excited” state. Almost immediately, these electrons and protons would then fall back into a more stable arrangement, giving off gamma rays in the process. Physicists realized that these gamma rays could act as messengers from deep within the nucleus, potentially carrying precious information about its internal structure. By studying them they hoped to discover a true theory of the nucleus, including an understanding of the mysterious forces that hold it together.

  But there was a problem. Radium, which had been physicists’ favorite source of alpha particles since Marie Curie had discovered it back in 1898, also gave off large numbers of gamma rays. This made it very hard for an experimenter to know whether a gamma ray had come from a nucleus that had been knocked out of whack by an alpha particle or directly from the radium source itself. What was needed was a different source of alpha particles, one that gave off far fewer gamma rays. Fortunately, Marie Curie had discovered just such an element back in 1898, which she had named “polonium” after her home country of Poland. Research at the Cavendish had long been hampered by a shortage of the rare element. The lab with the largest source of polonium in the world by far was Marie Curie’s own Institut du Radium in Paris.

  The great Marie Curie herself was now an international scientific leader, head of the Paris institute and two-time Nobel Prize winner. Increasingly her work took her away from frontline research, but another Curie was ready to step into her shoes: her daughter Irène.

  In the autumn of 1931, Irène’s interest was piqued by a paper written by two physicists in Berlin, Walther Bothe and Herbert Becker. Bothe and Becker had been bombarding light atoms (all the elements from lithium to oxygen along with magnesium, aluminum, and silver) with alpha particles produced by polonium and studying the gamma rays that came flying out. However, when they got to beryllium,*3 they had seen something strange: gamma rays that were able to punch through a 7-centimeter-thick iron plate. Normally that amount of iron would have stopped a gamma ray dead. Stranger still, far more gamma rays came flying out from beryllium than from the other elements they’d tested.

  Irène had a big advantage over the Berlin team—a polonium source that was ten times more intense. Working with her husband and scientific partner, Frédéric Joliot, she quickly repeated Bothe and Becker’s experiments, finding that the gamma rays emitted by beryllium were even more penetrating than her German colleagues had thought. However, most surprisingly, they found that when these gamma rays were fired at paraffin wax, protons came whizzing out at tremendous speeds.

  Think of it like a trick shot in a game of nuclear billiards: one ball smacks into another, which collides with another, and so on. We start with radioactive polonium, which fires out alpha particles. These alpha particles smack into beryllium nuclei, which then emit some kind of highly penetrating radiation that Irène and Frédéric assume to be gamma rays. These gamma rays then fly into a sample of paraffin wax, which is a compound containing lots of hydrogen atoms. The gamma rays knock some of the hydrogen nuclei out of the paraffin, which emerge as high-energy protons.

  The most surprising thing about all of this was the incredible energies that the protons got accelerated to when they were struck by one of these gamma rays. To explain the next bit I’m going to have to introduce the idea of something called an “electron volt.” An electron volt is just a unit of energy like the (possibly) more familiar joule or calorie, but while calories are great if you want to talk about the energy in a slice of apple pie, they’re not very convenient for dealing with subatomic particles. Compared to an atom, a calorie is a stupidly huge amount of energy. Using calories to talk about the energy of a subatomic particle would be like giving your body weight in solar masses.*4 So instead we use units more appropriate to the atomic world—the electron volt (eV)—which is just the energy of an electron that’s been accelerated using a 1-volt battery.

  Curie calculated that to accelerate the protons to the speeds she measured, the gamma rays had to have absolutely enormous energies—around 50 million electron volts (MeV)! This was very difficult to make sense of; the alpha particles emitted by polonium had a maximum energy of around 5.3 MeV. Even if a beryllium nucleus swallowed the alpha particle whole, how on earth could it then release a gamma ray with ten times more energy than it had absorbed? Something very strange indeed was going on.

  On a cold January morning a few days after Irène Curie had presented her extraordinary results to the French Academy of Sciences, James Chadwick was leafing through the latest scientific journals in his office at the Cavendish Laboratory. Opening a newly arrived copy of the Comptes rendus, he read Curie’s paper on beryllium radiation with increasing astonishment. A few minutes later, the young physicist Norman Feather burst into Chadwick’s office, just as astounded as he was. At around eleven o’clock Chadwick went to tell Rutherford the news from Paris. As he listened, Rutherford’s eyes slowly widened in amazement, until he eventually thundered, “I don’t believe it!” Chadwick had never seen his boss get himself into such a state over a scientific paper. They were both convinced by Curie’s results—her experiment was just the kind of elegant and simple setup that Rutherford so admired—but her explanation of what was going on was another matter entirely. Curie had set out to study gamma rays from beryllium and so had never considered that the radiation she was observing might be something other than gamma rays. Chadwick, on the other hand, who had spent eleven years on a futile search for the neutron, immediately saw the significance of the Paris result. Beryllium wasn’t emitting gamma rays at all; it was emitting neutrons.

  Chadwick realized that if you assumed the radiation given off by beryllium was made of neutrons instead of gamma rays, then the energy problem disappeared. A gamma ray has no mass, and so to knock a massive proton out of paraffin wax it has to have a very high energy. Imagine firing a ping-pong ball at a bowling ball—that ping-pong ball is going to have to be moving incredibly quickly to get the much heavier bowling ball to budge an inch.

  The neutron, on the other hand, should have a similar mass to the proton,*5 which meant hitting a proton with a neutron was more like hitting a bowling ball with another bowling ball. Chadwick calculated that while gamma rays needed to have energies of 50 MeV, a neutron only needed an energy of 4.5 MeV, less than the 5.3 MeV carried by the alpha particles absorbed by the beryllium nucleus. Suddenly, it all made a lot more sense. But still, he needed proof.

  Chadwick knew he was in a race against time. Surely it wouldn’t be long before Curie or the Berlin group realized the significance of the beryllium results. Locking himself away in the lab with a polonium source scavenged from a hospital in Baltimore, he worked like a man possessed, allowing himself only around three hours’ sleep each night for fear that his competitors might be on the same trail. After a decade of failure and frustration he was damned if he was going to be nosed out at the last. A fortnight later he emerged, gray faced with exhaustion but triumphant.

  In February, Chadwick attended a meeting of the Kapitza Club—a deliberately informal gathering of physicists organized by the exuberant Russian Pyotr Kapitza in his private rooms at Trinity College. Loosened up by a good dinner and a few glasses of wine, the usually reserved Chadwick gave an uncharacteristically confident presentation, using only a piece of chalk and a blackboard. Fielding frequent interruptions from Kapitza and the rapt gathering, he took his audi
ence from the original clue provided by Curie and Joliot to his final conclusion. After weeks of bombarding paraffin wax and a whole host of other materials, Chadwick had conclusively demolished the idea that the mysterious particles emitted by beryllium were gamma rays. For that to be true, the sacred law of the conservation of energy would have to be broken. Curie and Joliot’s results, and all his own observations, pointed unambiguously to a neutral particle with a mass close to that of the proton. The rumors that had been swirling around the Cavendish for the past few weeks were true. After more than a decade of fruitless struggle, Chadwick had discovered the final and most elusive building block of the atom, the neutron.

  After such a long drought of new discoveries, Rutherford and the Cavendish Laboratory as a whole basked in the glow of Chadwick’s triumph. Shortly after Chadwick submitted an account to the journal Nature, Rutherford publicized the discovery during a lecture at the Royal Institution in London, just as his old boss J.J. had done when he first caught a whiff of the electron back in 1897. The discovery of the neutron was particularly sweet for Rutherford—after all, he was the one who had first predicted its existence a dozen years earlier, back in 1920.

  However, it was not a total victory. Chadwick had attempted to measure the mass of the neutron, finding that it weighed slightly less than a proton. Rather counterintuitively, this actually supported Rutherford’s idea that a neutron was made of a proton and an electron, as for the neutron to be stable, some energy must be released when the proton and the electron fuse. This “binding energy” has the effect of actually making the combination weigh less than the sum of its parts.