FROM NUCLEAR REACTIONS TO NUCLEAR FISSION

In 1913, Neils Bohr, a brilliant Danish physicist and a post-doctoral student of Rutherford’s, elaborated and modified Rutherford’s model by incorporating quantum mechanical (the study of mechanical processes at the atomic and sub-atomic level) results to describe and delineate the behaviour of the orbiting electrons, which Bohr correctly deduced orbited in circular (and elliptical) orbits like planets orbiting the sun. Bohr also followed up on the work of British physicist Henry Moseley and determined that the central charge was tied to an element’s atomic number, or its place on the periodic chart. The potential of the atom was now clear, as was its complexity. Future research into the mysteries of the atom would occupy specialists in atomic physics, who studied the atom’s structure, while a new field, nuclear physics, emerged to study the powerful core of the atom.

New Zealand-born Ernest Rutherford, later Baron Rutherford of Nelson (1871-1937) immigrated to Britain as a young scientist to join trinity College's Cavendish Laboratory. A brilliant protege of J. J. Thomson, Rutherford expanded on Thomsons work, and between 1898 and 1911 conducted experiments that probed the inner mysteries of the atom. He remained an active researcher even as he mentored and trained many leading scientists (including several Nobel Prize winners]. Rutherford received the Nobel Prize in Chemistry in 1908. This portrait, by 0. H. J. Birley, hangs in the Royal Society, London (Bridgemon Art library).

Between 1919 and 1939, scientists gradually closed in on the secrets of the atom. In 1919, Ernest Rutherford, now Britain’s leading nuclear physicist, published the results from his bombardment of nitrogen atoms by alpha particles. The collision of particles with an atom knocked out a hydrogen nucleus, he determined, and this created a new atom, an isotope of oxygen. This was the world’s first artificially induced nuclear reaction, and the artificial transmutation of an element, the long-sought goal of alchemists since medieval times. When news of the achievement reached the public, it astonished the world. The New York Times, reporting on Rutherford’s work in January 1922, reported that “the dream of charlatans and scientists for nearly a thousand years has been, accomplished.”

The potential of the discovery was greater than elemental transmutation, however. The New York Times quoted British professor O. W. Richardson, who in a late 1921 address to the Mathematics and Physics section of the British Association noted:

But this is only part of the story. It appears that in some eases the kinetic energy of the ejected fragments is greater than that of the bombarding particles. This means that these bombardments are able to release the energy which is stored up in the nuclei of atoms. Now we know from the amount of heat liberated in radioactive disintegration that the amount of energy stored in the nuclei is of a higher order of magnitude, some millions of times greater ... than ... the combustion of coal ... the amounts of energy which have been thus far released ... are themselves small, but they are enormous in comparison with the minute amount of matter effected. If these effects can be sufficiently intensified ... either they will prove uncontrollable, which would presumably spell the end to all things, or ... if they can he both intensified and controlled, then we shall have at our disposal an almost illimitable supply of power which will entirely transcend anything hitherto known... It may be that we are at the beginning of a new age ... the age of sub-atomic power.

Niels Henrik David Bohr (1885-1962). Born in Copenhagen, Bohr studied with Ernest Rutherford after receiving his doctorate in 1911. Bohr proposed a new model for atomic structure in 1913, which in its simple form depicts electrons orbiting the nucleus of an atom. The Bohr model remains one of the most powerful iconic images of the 20th century. As director of the Institute of Theoretical Physics at the University of Copenhagen, Bohr received the Nobel Prize in Physics in 1922 "for his services in the investigation of the structure of atoms and of the radiation emanating from them." (Bridgeman Art Library)

Rutherford named the split-off portion of the nitrogen a “proton”, a major, if not the sole constituent of an atomic nucleus. Within a year, he was thinking that: perhaps there were other parts, and in a lecture in June 1920, speculated that another constituent part might exist. That part was the neutron, which Rutherford’s former student, James Chadwick, discovered at Cambridge in 1932.Three months earlier, Irene Joliot-Curie, daughter of Pierre and Marie, had noted that penetrating particles formed when she bombarded beryllium with alpha rays. Joliot-Curie thought they were gamma rays, but they were actually neutrons, although she did not realize it.

The neutron, named because it had no electrical charge, was the other constituent of atomic nuclei. More powerful than a proton, a neutron, if tired into nuclei, would split those of even the heaviest elements -something not yet achieved in artificial transmutation. Chadwick’s discovery, building on Rutherford’s, had provided yet another critical step in the ability to take atoms apart - a step closer to nuclear fission.

This fact was realized, in a flash of inspiration, by Hungarian physicist Leo Szilard who was annoyed by an article in the New York Times that quoted Rutherford’s recent address to the British Association. After discussing the latest developments, including the discovery of the neutron, Rutherford answered a question about prospects for the future. Unlike the more effusive Professor Richardson, Rutherford was dismissive. “We might in these processes obtain very much more energy ... but on average we could not expect to obtain energy”, even if high voltage blasts of 30,000 to 70,000 volts were used. Therefore, “anyone who looked for a source of power in the transformation of the atoms was talking moonshine, or nonsense.”

Szilard thought otherwise. He recalled that neutrons, because they did not interact with whatever substance they passed through, would not stop “until they hit a nucleus with which they may react”, that is to say, split. If a chain reaction of neutron collisions split enough nuclei, then an immense release of power would result. Szilard spent time in his laboratory to attempt to generate a chain reaction, but the materials he was using failed to work. Undaunted, he patented the concept, and in 1936, assigned the patent to the British Admiralty to help keep it secret - and out of the hands of the Nazis.

Meanwhile, in the United States, American physicist Ernest O. Lawrence, at the University of California, Berkeley, eager to build the power needed to push harder at and into the atom, designed a new tool, the cyclotron. Lined with electrodes to help give particles recurrent “small pushes” and speed them, Lawrence’s circular instrument was more compact than a linear accelerator, also used to speed particles. Testing his first device in January 1931, Lawrence used 2,000 volts of electricity to spin rapidly 80,000-volt protons in the cyclotron. “Atom smashing” had just taken another step forward.

In France, Irene and Frederic Joliot-Curie, continuing in the tradition of Irene’s famous parents, worked as a team, probing the mysteries of the proton. Bombarding elements and tracking the discharges, the Joliot-Curies made a momentous discovery in 1933. Their bombardment of aluminum foil not only transmuted the aluminum into the isotope of phosphorus. It also transmuted it into radioactive phosphorus, the world’s first artificially induced radioactive material. This meant, as Richard Rhodes would later note, that scientists now could not only chip off pieces of a nucleus, but they could “force it artificially to release some of its energy in radioactive decay.”

Irene Joliot-Curie (1897-1956) was a brilliant scientist who was the daughter of two brilliant scientists, and the wife of another. After serving as a nurse radiographer in World War I, she returned to her studies at the Faculty of Science in Paris, earning her doctorate in 1925. On her own, and working with her husband, she made a number of key discoveries that paved the way to the discovery of nuclear fission. In addition to her scientific work, she also served France as a commissioner of atomic energy, and actively promoted social and intellectual advancement tor women. (Bridgeman Art Library)

In another laboratory, in Rome, another brilliant young physicist, Enrico Fermi, had just completed a theoretical treatise on beta decay (the 15-minute window in which a free neutron decayed to become a proton), and was seeking a new challenge. In January 1934, after reading of the Joliot-Curie discovery, Fermi and his colleagues began a series of experiments, bombarding elements with neutrons. Fermi’s breakthrough, in experiments in May and October, was the discovery that neutron bombardment caused nuclear transformation in nearly every element they subjected to the process. At the same time, the experiments also showed that there were different energy levels in neutrons, and in particular, Fermi identified “slow neutrons”, or neutrons with low energy levels.

In the bombardment of uranium, hitherto impossible without the use of neutrons, Fermi created an isotope, uranium-239, and a new element with a heavier atomic weight than uranium. This was the first roan-made element, the precursor of what the Nobel Prize committee would later recognize as a key step, “the production of elements lying beyond what was until then the Periodic Table.” Fermi’s work was also another important stepping stone in the development of nuclear fission.