Physics, as is the case in most disciplines, invariably breaks into two opposing factions. There were the Arabic school spagyrists and the Indian school chrysopoeists;49 the Newtonian astrophysicists and the Einsteinian cosmologists; the bosonic supersymmetric string theorists and the Kuluza-Klein compactified M-theorists.50 The most obvious bifurcation is between the theorists and the experimentalists.
To the theorist, an experimentalist is a mechanic. He is a linear-thinking dullard who has trouble visualizing four dimensions. To the experimentalist, the theorist is a lazy-assed hand waver who couldn’t find the exhaust port on a vacuum pump if his funding depended on it. In the quantum-mechanical sense, both opinions are correct.
In the 1920’s the theorists seemed to have the upper hand. First, Albert Einstein had blown away the physics community with his theories of relativity, reshaping the structure of the universe and putting Isaac Newton away on a shelf. He had also overstepped the boundaries of academic physics, putting his radical theories in the hands of ordinary mortals, non-physicists, and those unwashed by the fire hose of knowledge. The drowning depth of the physics fantasy was not necessarily supposed to be revealed to the general public, but there it was, for all to see. The consequences of this knowledge escape bore watching.
Then came Niels Bohr. His theories of quantum mechanics explained away some mysteries that were too small to be seen, but they required a disturbing leap of faith into darkness. As quantum mechanics spiraled away into unknown territory, physics was in severe need of help from the experimentalists, to nail these theories to the floor of pseudo-reality. Through the decade of the 1920’s, the physical structure of the atomic nucleus remained unknown, as experimental physics sat quietly and thought.
It would seem a long, dry spell, but at the end experimental physics would crash forward at dangerous speed, leaving theory in the dust. The buildup began in the evening of June 3, 1920. Lord Ernest Rutherford, namer of rays alpha and beta, discoverer of the atomic nucleus, modeler of the atom, the crocodile of the Cavendish, gave the Bakerian Lecture at the Royal Society of London. His topic was the successful transmutation of the nitrogen atom using alpha particles. It was a safe topic, and the experiment had predictable results, but Rutherford, completely out of character, began to divert off the topic. By this time it was known that atoms are composed of a fixed and characteristic number of electrons circling around a nucleus that is composed of an identical number of protons, with the negative and positive charges of these particles perfectly canceling each other. The majority of the weight of the atom is in the nucleus, in which the heavy protons are jammed together in a tight wad. It was a workable model, and quantum mechanics would go on to explain how the atom works and combines chemically with other atoms given no more detail than this.
There was, however, a serious problem with this model. The lightest atom, hydrogen, has one electron and one proton canceling its electrical charge. The next lightest atom, helium, has two electrons and two protons. A helium atom should weigh twice what a hydrogen atom weighs, but it in fact weighs four times what a hydrogen atom weighs. How, Rutherford asked, is that possible? If hydrogen has an atomic weight of 1, then helium weighs 4. Nitrogen, farther up the weight-scale, has 7 protons, but has an atomic weight of 17. As the weight goes up, the disparity becomes worse. Barium has 56 protons and weighs 137; uranium has 92 protons and weighs 238.
Rutherford, in his now famous lecture, tossed out a possible solution to this dilemma. Nuclei above hydrogen are heavier than is explicable counting protons, so there must be another particle at work in the nucleus. It obviously has no electrical charge that needs to be canceled, but it has the weight of a proton. He even gave this hypothetical, electrically neutral particle a name: neutron. Think, Rutherford challenged, of the interesting properties of such a particle. It would be free of any electromagnetic influence, and would thus be free to wander in and out of any matter, breezing its way through solid walls, glass tubes, or blocks of lead. Having no electrical charge, it leaves no ionized trail as it flies through the air, and is therefore undetectable using any existing radiation detector. The neutron was a phantom, with its trail never crossed by an experimentalist.
In that quiet period, for twelve years after the Bakerian lecture in 1920, experimentalists including Rutherford combed through matter in ingenious ways, looking for the elusive neutron, and always in the wrong place. An assumption was made, that the neutron was a proton with an electron embedded in its surface, rendering it neutral.51 All they had to do was hit a proton hard enough with an electron; it would stick, and make a neutron. The easiest place to get a free proton was in hydrogen. Hydrogen was tortured mercilessly, in ways too gruesome to recount, but it seemed indestructible. Not a single neutron was produced. Finally, in 1932, James Chadwick, Rutherford’s student and research assistant, would identify and characterize the neutron in a well crafted laboratory experiment.
James Chadwick was born in 1891 in Bollington, Cheshire, England. His parents, John Joseph and Anne Knowles Chadwick, started a laundry business in Manchester, and young James attended the Bollington Cross Church of England Primary School, and then the Manchester High School, where he showed aptitude in mathematics. He was accepted at the University of Manchester at the tender age of 16. He planned to major in mathematics, but as he stood in queue for his entrance interview on registration day he realized at the last minute he was in the wrong line. Being too embarrassed to admit his mistake, he wound up majoring in physics.
It was miserable. The physics classes were big and rowdy, and he felt hopelessly lost. On the edge of despair he sat through a visiting lecture by Lord Ernest Rutherford, and his outlook was transformed. He graduated in 1911 with a bachelor’s in physics, gratefully received, and went immediately to earn the Master of Science in physics at the University of Cambridge two years later. He was then awarded a scholarship to study in Germany, at the prestigious University of Berlin, under Hans Geiger. He spent a productive year, working with Geiger on his now famous radiation detection instrument, the Geiger counter, and meeting Albert Einstein.
Fortune took a sudden, downward turn for Chadwick in August 1914, when World War I broke out, and he found that his native England was on the side opposing that of Germany, where he was working. Dawdling with indecision for days, trying to decide whether to run for Holland or Switzerland to escape Germany, he was picked up and arrested, under suspicion of being English, and was rudely interned in a horse stall at the Ruhleben race track, in the Spandau suburb of Berlin. Ruhleben became a POW camp, intended strictly for civilian nationals of the Allied Powers caught in Germany during the war, and Chadwick and several notables would be there for the next four years. The food was terrible, the accommodations were freezing cold and drafty, but the English carried on and kept their wits about them. The 4,000 detainees established their own police force, published a magazine, stocked a lending library, ran a gambling casino, and started a postal service, printing their own unique stamps.52
Although Chadwick never physically escaped Ruhleben, he never allowed his mind to be imprisoned. Taking advantage of the minimum effort put into guarding the prisoners, he started a science club and, being an experimentalist through and through, he and cellmates investigated the ionization of phosphorous and the photo-chemical reaction of carbon monoxide and chlorine using ingeniously devised equipment. At the end of hostilities in 1918 Chadwick was sent home, out of work and with an abused digestive system. Rutherford took him back, grateful to have him, and put him to work.
Chadwick’s eventual assignment was to find the phantom nuclear particle. He started from the beginning, doing what others were doing, hitting various materials with alpha particles to see what happens. The alpha particle is an interesting projectile. It is literally the nucleus of a helium atom. It is heavy, and it can do a lot of damage. Hit a light element, such as boron or aluminum with alpha particles, and the nuclei disintegrate, exploding in bursts of gamma rays and sending proton debris flying. Both effects, the protons and the gamma rays, are easily detected with a 1920’s-vintage radiation counter. Chadwick tried alpha-slapping some beryllium, and the results were odd. The beryllium emitted ten times the gamma-ray burst intensity as anything else hit with alpha particles, and yet there was no proton wreckage from an assumed nuclear destruction. No one knew why beryllium was so different.
For this type of work what Chadwick really needed was a polonium-210 alpha source. Polonium, the element discovered by Marie Curie, was the hottest alpha source available, as it had no accompanying gamma-ray contamination to confuse measurements. Unfortunately, it was a very rare commodity at the time. The polonium industry, tiny as it might seem, was owned by Iréne Joliot-Curie, daughter of the famous Marie Curie, and her husband Frédérick Joliot. They worked in Marie Curie’s Radium Institute at the end of Rue Pierre Curie in the Latin Quarter of Paris, and they collected polonium with a motive in mind. Polonium-210 is a byproduct of the decay of radon-222, a radioactive gas that the Joliot-Curies manufactured for medical use. They would package it in small, glass ampoules for physicians to use for cancer treatment. The radon has a half-life of only four days, and used, played-out ampoules were sent back to the Radium Institute by mail. Iréne and Frédéric developed a chemical separation scheme for processing the decay products from the junked ampoules, and by 1929 they had collected the largest polonium-210 alpha source in the world, ten times bigger than any other.53
In 1931 they turned their big alpha source to the persistent mystery of the odd response of beryllium to alphas. Not only did they have the best alpha source, they also had the best, state-of-the-art radiation detection equipment. The Joliot-Curies had an ionization chamber, which was a metal can, filled with air, with a wire running down the center. With a high voltage applied across the can and the wire, any ionizing event, such as a flying proton, an alpha particle, or a gamma ray, would leave a conductive trail in its wake as it crossed the air space in the detector. The electrical current measured at each crossing was characteristic of the energy of the event, and one could electronically discriminate alpha particles from gamma rays from protons.
The Joliot-Curies turned their alpha-source/beryllium combination towards the ionization chamber and watched the gamma rays flood the detector. Then they put barriers in the way of the gammas, to find the penetrating power of these rays. They tried some of everything—lead, steel armor plate, aluminum foil, glass. When they tried some cellophane they were amazed at the response. Blasting through a thin membrane of sandwich wrap, the gamma rays from the beryllium sent a torrent of protons into the detector.54 Iréne immediately presented these interesting findings to the French Academy of Sciences on December 28, 1931, and wrote a paper announcing to the greater physics community, “The emission of protons of high velocity from hydrogenous materials irradiated with very penetrating gamma rays.” Obviously, the gamma rays were knocking protons in the hydrogen in the cellophane, out of the material and into the detector.
Back at the Cavendish, Chadwick read these findings with an interested but skeptical attitude. The observations were credible, but the explanation seemed way off base. They were saying that gamma rays from beryllium, hit with alphas, would knock protons out of a hydrocarbon. It is true that gammas of sufficient energy can push electrons around, but a proton is 1,836 times heavier than an electron. Saying that gammas were tossing protons into the detector was like saying that a dump-lorry could be knocked into the oncoming lane by a well malleted polo ball. He would need to duplicate the experiment and see for himself.
Chadwick, ever the resourceful experimentalist, had an ace up his sleeve. A colleague at the Cavendish named Norman Feather had spent the academic year in Baltimore, Maryland, and became sympathetic with a fellow Brit, a physician at the Kelly Hospital. He was in charge of the radon supply for cancer treatment, and had amassed a big pile of used ampoules.55 The hospital was glad to be rid of them and donate them to a good cause, and Feather shipped them home to Chadwick. Chadwick did the tricky chemical separation himself, resulting in an alpha source as big as any in France.
On a lab table in the Cavendish Chadwick ran the experiment with a fine, orderly setup, aiming his alpha source at a beryllium target, with a paraffin sheet in front, pointed into a new ionization chamber. His results matched perfectly with the Joliot-Curies, but he interpreted differently. He agreed that protons were knocked into the detector from the paraffin, but it was not the result of any gamma ray collision. The only thing that could exchange momentum with a stationary proton and blow it out of the paraffin molecule was a particle of the same weight, coming fast and hitting hard. There were obviously no protons coming out of the beryllium, or they would have showed up in earlier experiments. There was only one explanation. The projectile particles out of the beryllium were neutrons.56 They made no impression on the detection apparatus because they had no electrical charge. That’s why they were called “neutrons.” What he sensed with the ionization chamber was a secondary effect, as the neutral particles made unimpeded collisions with the stationary protons in the hydrocarbon material, sending them reeling into the detector.
Chadwick was woozy from having gotten very little sleep over the previous week, but over the weekend, February 13–14, 1932, he wrote a short letter to the editor of Nature, titled “Possible existence of a neutron.” It would go down in history as his triumphant announcement. The next Wednesday, after having run his experiment over and over to verify the results, he spoke at a closed meeting of the Kapiza Club on the campus. Speaking lucidly and with conviction, Chadwick gave the shortest account of an earth-shaking scientific finding ever heard. In conclusion he, looking ready to faint, said, “Now I want to be chloroformed and put to bed for a fortnight.” He was only half joking. Physics, and the world in which it was studied, would never be the same.
Elsewhere in England, in the lobby of the Imperial Hotel in London, sat Dr. Leó Szilárd, on Tuesday morning, September 12, 1933, reading The Times. Szilárd by any measure was a genius, born to a Jewish family in Budapest, Hungary, in 1898, a time when political strife and storms in the Austro-Hungarian Empire were a normal feature of life. Young Leó was interested in two things: physics and politics. The odd pairing of passions seemed to give him an uncanny ability to predict wars and outcomes. In high school he correctly foresaw the victors and the losers of World War I, and later he was able to predict the rise of the Nazi party in Germany, World War II, and its outcome. He enrolled in the Budapest Technical University in 1916. Trying to be practical, he majored in engineering.
Szilárd survived being drafted into the Hungarian Army in 1917 as an officer candidate, and then in 1919 he vacated Hungary because of rising anti-Semitic policies in the post-war government. Landing in Germany in a frying-pan-to-fire move, he enrolled at the Institute of Technology in Berlin-Charlottenburg. He soon changed his major to physics, and seldom looked back on engineering. Studying under Albert Einstein and Max Planck, he received his doctorate from the Humboldt University in Berlin in 1923.57 In 1927 he became an instructor of physics at the University of Berlin. By 1933 the anti-Semitic crackdowns in Germany had become intolerable for nuclear physics work, and he fled to England. He lived out of two suitcases, always kept packed and ready to run.
So there sat Leó Szilárd that day in 1933, inventive, spontaneous, and eccentric, reading the paper. Up and down the page ran screaming headlines concerning nuclear science. “BREAKING DOWN THE ATOM,” “TRANSFORMATION OF ELEMENTS,” “THE NEUTRON,” and halfway down the second column ran a summary of a stirring talk by Lord Ernest Rutherford, “HOPE OF TRANSFORMING ANY ATOM.” His eyes went there first, reading about Rutherford’s speech:
What, Lord Rutherford asked in conclusion, were the prospects 20 or 30 years ahead?…We might in these processes obtain very much more energy than the protons supplied, but on the average we could not expect to obtain energy this way. It was a very poor and inefficient way of producing energy, and anyone who looked for a source of power in the transformation of the atoms was talking moonshine.
Szilárd was already irritated that there was a scientific meeting going on, involving the top scientists in Great Britain, to which he had not been invited, and here was this blowhard equating energy production from nuclear processes to “moonshine.” He was not exactly sure what moonshine was, but it did not sound encouraging, as if the concept were not worth a second thought. He discarded the paper and stepped out onto the sidewalk, where he could pace and think.
He wandered, mind racing, until he was stopped at a traffic signal on Southampton Row, at Russell Square, across from the British Museum in Bloomsbury. The light turned green, and just as he stepped off the curb to cross the intersection, an idea fell together in his mind. Neutrons, lately discovered by James Chadwick, have no charge, and are not constrained by the shielding effects of the common charged particles, the electrons and the protons. If a stray neutron wandered into a heavy nucleus that was already overburdened by a surfeit of neutrons, it could be absorbed, and that would be more than the nucleus could bear. It would go unstable. It would start to wobble and vibrate, like a fast-spinning wheel having thrown a balancing weight. The nucleus would rip apart, throwing off huge chunks of nuclear matter. As his friend Einstein had pointed out, the fragments if added together would not equal the weight of the original, stable nucleus, and the weight deficit would be counted as pure energy.
Szilárd’s feet reached the middle of the intersection in autopilot mode as he strode on, eyes staring into infinity. Furthermore, suppose the debris of the nuclear destruction included a stray neutron. This neutron would then bounce around until it blundered into another fat nucleus, starting yet another breakdown. But wait, some neutrons would fail to hit a target, and would be wasted. What if two neutrons were included in the nuclear wreckage? There would be neutrons to spare, and the reaction, neutrons causing havoc, would be self perpetuating. It would be a chain-reaction. Energy would be released in quantities millions of times greater than in any chemical reaction, which merely twiddles with weak little electrons, and not involving the nuclear binding forces.
By the time he stepped up on the opposite curb, Leó Szilárd had outlined the process of nuclear power production.58 If there exists an element that will tear apart upon the capture of one extra neutron, and if the destruction results in two free neutrons flung wide, then Lord Rutherford was dead wrong and nuclear power production was possible. Szilárd spent the rest of the day speculating about exploring the solar system with nuclear-powered rockets and building the atomic bomb as detailed in H.G. Wells’ science fiction novel, The World Set Free.59 Now, all that was standing in the way of nuclear power was for some enterprising experimentalist to discover induced nuclear breakdown, a process in which uranium captures a wayward neutron and cracks in half under its influence.
In 1934, before the operative effect was discovered, Szilárd applied for a patent for the nuclear power reactor, which was put under wraps and assigned to the British Admiralty for security reasons. He immigrated to the United States in 1938, scrounging for equipment and funding to experiment and find the element that would come apart, and always looking in the wrong places. He reasoned that beryllium or indium should split, and at the University of Rochester in 1938 he pounded samples with all the neutrons he could find. Nothing happened. Szilárd’s faith in his idea and his enthusiasm started to wane, and in a depressed mood he wrote a letter to the British Admiralty, advising that his patent application for a nuclear reactor should be withdrawn. His letter arrived in late December 1938, just as two Germans, Otto Hahn and Lise Meitner, realized that they had found the power-producing breakdown in the final element on the periodic chart and the last there was to check, uranium.60 They borrowed from cellular biology and named the effect fission.61
Otto Hahn, the man who invented radiochemistry, was born in Frankfurt in 1879, the son of a prosperous glazier and property owner. Young Otto enjoyed a sheltered childhood, and at the age of 15 developed an interest in chemistry. He performed experiments in the laundry room of the family home. His father wanted him to study architecture, but Otto managed to convince him that industrial chemistry was the way to wealth in Germany, and he studied chemistry and mineralogy at the University of Marburg in 1897. He was awarded his doctorate in chemistry in 1901, and after completing a mandatory year of military service, returned to Marburg to work as a research assistant. In 1904 he got a job in Great Britain at the University College London, and there he did his first work in the new field of radiochemistry. He was a natural at working with radioactive elements and compounds, and in 1906 he moved back to Germany with a lock on this interesting specialty. By 1910 he was head of the Radioactivity Department of the new Kaiser Wilhelm Institute of Chemistry in Berlin.
In 1907 Hahn ran into a like-minded physicist, Dr. Lise Meitner, at a University of Berlin physics colloquium. She hailed from Vienna, Austria, and was already published in the field, with papers on alpha and beta radiation. They both needed collaborators, and a radio-chemist teamed with a physicist seemed a natural fit. They shared goals and a love of the chase, and they teamed as friends, working closely together for the next 30 years.
When Chadwick announced the discovery of the neutron in 1932, radiochemistry lit up like radium salt in zinc sulfide. The possibilities of transmuting elements into new and undiscovered isotopes using neutrons were endless, and radiochemists wasted no time jumping onto the neutron craze. Polonium-210 alpha sources for making neutrons were hard to find, much less buy, and researchers scrambled to build substitutes. Anything that could emit alpha particles was powdered and mixed with ground-up beryllium to make a neutron generator, and the results were weak compared with what Joliot-Curie or Chadwick had used.62 The degree of transmutation occurring in a sample could be quite small, involving as few as hundreds of atoms, and the chemistry required to find transmutation products required great skill. Otto Hahn was an old hand and an expert by 1932, and he led the pack in identifying minute contaminations of elements after samples were exposed to neutron sources. He used fractional crystallization, a technique pioneered by Marie Curie. It uses the fact that different substances dissolved in water crystallize out of supersaturated solution at different temperatures.
By 1938 Hahn and Meitner had exposed every element in the periodic table to neutrons, starting with hydrogen, and they were on the last one, uranium. Results looked interesting and peculiar. The political situation in Germany had been getting worse for Meitner, who was Jewish, for many years, but she had always been able to skirt new anti-Semitic laws or policies by finding a loophole or getting a friend to deflect implementations, but now she was beyond help. Things were getting very dangerous, and she was in peril of being shipped off to a relocation camp. She had to drop everything and escape to Holland, quickly, just as the uranium/neutron exposure was looking interesting. Meitner waved goodbye to Hahn and radiochemistry and wound up in Sweden, still communicating with her collaborator by post.
Hahn’s work in radiochemistry was world-class. He had published the definitive work on the subject, in English, Applied Radiochemistry, in 1933, but he desperately needed his physicist to interpret his findings. His repeated chemical analyses of the results of exposing uranium to neutrons were very puzzling. There was obviously a new radioactive isotope being made, but he could not figure out what it was. He guessed it was a new, previously undiscovered isotope of radium. To detect minute quantities of radium he used barium as a carrier for the fractional crystallization. He wrote Meitner, asking her opinion.
Meitner wrote back. She found radium hard to believe, as it would require a double alpha disintegration to decay from uranium down to radium. Hahn kept trying, and found to his amazement that nothing was showing up in his barium carrier. In a burst of insight, Meitner figured it out, and she and Hahn exchanged excited letters on December 21, 1938. Hahn had detected no radium in his barium carrier because there was no radium. The product of his neutron bombardment of uranium was barium.
The implications were stunning. A barium atom is slightly more than half the mass of a uranium atom. When he hit it with neutrons, Hahn had split the uranium roughly in half, leaving him with neutron-heavy, radioactive barium atoms plus chemically undetectable krypton atoms, and the result was sufficiently energetic to detect with his Geiger counter. The next day, December 22, he submitted his discovery paper, cautiously titled, “On the alkaline-earth metals produced by the neutron bombardment of uranium and their behavior.” Nuclear fission was announced to the world. Two months later Hahn wrote a second paper reporting that the fission also seemed to liberate two free neutrons.63
The nuclear physics community, particularly Leó Szilárd, was gripped with frantic excitement at the announcement. Szilárd launched an aggressive telegram at the British Admiralty, asking them to please disregard his last letter. With the Hahn and Meitner’s discovery of fission, nuclear power was not just possible, it was inevitable. The fact that fission occurred in uranium with more than one bonus neutron clinched it, and the progress of nuclear power research suddenly shifted from the fantasy mode to the puzzle mode. The experimentalists had done their jobs with great care and powerful insights, turning the high-flown theories of ten years ago into something approaching reality. Experiments had finally shown that uranium could be used to convert mass directly into energy, just as Einstein had predicted.
But how, exactly?