Nuclear physics

• 1. Introduction

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  Nuclear physics is the field of science that studies atomic nuclei, their building blocks, and the forces that hold them together or tear them apart. In 1896, Henri Becquerel made a discovery that changed physics forever while investigating phosphorescence in uranium salts: some materials emitted radiation all on their own, without any outside prodding. That single observation cracked open a question that would take decades to answer. What exactly is happening inside an atom? Where does all that energy come from? And how do the rules governing the nucleus connect to the birth of stars, the existence of every element on Earth, and some of the most consequential technologies ever built?

• 2. The Plum Pudding Problem

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  J. J. Thomson discovered the electron just one year after Becquerel's radioactivity work, and that finding signaled that atoms were not indivisible billiard balls. Thomson's own model of the atom, widely accepted at the start of the twentieth century, pictured a positively charged ball with small negatively charged electrons embedded throughout, like fruit suspended in a pudding.

  Marie Curie, whose maiden name was Sklodowska, Pierre Curie, Ernest Rutherford, and others threw themselves into studying radioactivity intensively in the years that followed. By the turn of the century, three distinct types of radiation had been identified and named: alpha, beta, and gamma. Experiments by Otto Hahn in 1911 and by James Chadwick in 1914 revealed that beta decay produced electrons with a continuous range of energies, not the neat discrete packets seen in alpha and gamma decays. That continuous spectrum was a genuine crisis for the field, because it appeared to violate the conservation of energy.

  The 1903 Nobel Prize in Physics went jointly to Becquerel, Marie Curie, and Pierre Curie for their intertwined discoveries. Rutherford received the Nobel Prize in Chemistry in 1908 for his investigations into the disintegration of the elements and the chemistry of radioactive substances. Meanwhile, Albert Einstein published his idea of mass-energy equivalence in 1905, though a full explanation of where radioactive energy comes from would have to wait for a more complete picture of what the nucleus actually contained.

• 3. Rutherford Cracks the Nucleus

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  In 1906, Rutherford published a paper on how alpha particles slow down passing through matter. Hans Geiger extended that work in communication with the Royal Society, and then in 1909 Geiger and an undergraduate named Marsden published results of a striking experiment conducted at the University of Manchester. Under Rutherford's supervision, they fired alpha particles at a thin film of gold foil.

  The plum pudding model predicted that alpha particles would pass through with only slight bending. What Rutherford had instructed the team to check for surprised even him: a small number of particles bounced back at large angles, some completely backwards. Rutherford compared it to firing a bullet at tissue paper and having it bounce off. His analysis, published in May 1911, led to a new picture of the atom. The nucleus was very small and very dense, holding most of the atom's mass, and was surrounded by orbiting electrons at a distance.

  In this early Rutherford model, which is not the modern picture, nitrogen-14 was described as having a nucleus of 14 protons and 7 electrons, totaling 21 particles, with 7 more electrons orbiting outside. In 1911-1912 Rutherford presented this new theory of the atomic nucleus before the Royal Society. Around 1920, Arthur Eddington built on Einstein's equation in his paper The Internal Constitution of the Stars, correctly speculating that the Sun and other stars were powered by the fusion of hydrogen into helium, at a time when no one had yet confirmed that stars are largely composed of hydrogen.

• 4. Finding the Neutron

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  Franco Rasetti carried out studies of nuclear spin at the California Institute of Technology in 1929, and his results troubled the Rutherford model. By 1925 it was known that protons and electrons each had a spin of a specific value. In nitrogen-14 under the Rutherford model, 20 of the 21 nuclear particles should have paired to cancel one another's spin, leaving a net spin from the one leftover particle. Rasetti found instead that nitrogen-14 had a spin of 1, which did not fit.

  The answer arrived in 1932. James Chadwick recognized that radiation observed by Walther Bothe, Herbert Becker, and Irene and Frederic Joliot-Curie was actually the signature of a neutral particle with roughly the same mass as the proton. Chadwick called it the neutron, following a suggestion from Rutherford. That same year, Dmitri Ivanenko proposed that nuclei contained only protons and neutrons, with no electrons hiding inside. The lone unpaired proton and lone unpaired neutron in nitrogen-14, each contributing spin in the same direction, together produced a total spin of 1, exactly what Rasetti had measured.

  With the neutron in hand, scientists could for the first time calculate the binding energy of any nucleus by comparing its actual mass to the combined mass of its protons and neutrons. When nuclear reactions were measured in 1934, the results agreed with Einstein's mass-energy equivalence to within one percent.

• 5. Forces That Bind and Break

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  Alexandru Proca developed and reported the first equations for the massive vector boson field, laying groundwork for a theory of nuclear forces. Those equations were known to Wolfgang Pauli, who mentioned them in his Nobel address, and to Hideki Yukawa among others.

  In 1935, Yukawa proposed the first significant theory of the strong nuclear force, the force that stops protons from flying apart despite their mutual electrical repulsion. He proposed that a virtual particle, later called a meson, mediated a force between all nucleons. This mechanism explained both why nuclei hold together and why the attractive strong force operates over a shorter range than the electromagnetic repulsion between protons. The later discovery of the pi meson confirmed it matched the properties of Yukawa's proposed particle.

  Enrico Fermi had explained the weak nuclear force through what became known as Fermi's interaction in 1934, the same year nuclear reaction energies were confirmed against Einstein's predictions. Together, the strong and weak forces drove physicists to collide nuclei and electrons at ever higher energies, a line of inquiry that eventually became particle physics and culminated in the standard model describing the strong, weak, and electromagnetic forces.

• 6. Decay, Fusion, and Fission

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  Eighty elements have at least one stable isotope, accounting for roughly 251 stable nuclides in total. Thousands of other isotopes are unstable, decaying over timescales ranging from fractions of a second to trillions of years. Mapped against atomic and neutron numbers, the binding energies of nuclides trace what physicists call the valley of stability, with stable nuclides along the bottom and progressively less stable ones up the walls.

  Alpha decay, typical of the heaviest nuclei, releases a helium nucleus of 2 protons and 2 neutrons, converting one element into another. In beta decay, a nitrogen-16 atom with 7 protons and 9 neutrons converts to oxygen-16 with 8 protons and 8 neutrons within seconds; a neutron becomes a proton, an electron, and an antineutrino. Gamma decay, by contrast, leaves the element unchanged, simply releasing energy as a high-energy photon when a nucleus drops from an excited state to a lower one.

  Nuclear fusion requires overcoming the electrical repulsion between nuclei so that the strong force can pull them together, which demands very high temperatures or pressures. Stars like the Sun run on the fusion of four protons into a helium nucleus plus two positrons and two neutrinos. The binding energy per nucleon peaks at nickel-62 for fusion and at iron for the crossover to fission, meaning heavy nuclei above that threshold can release energy by splitting apart. The fission chain reaction is the energy source for both nuclear power plants and the fission bombs detonated over Hiroshima and Nagasaki at the close of World War II. In two regions of Oklo, in Gabon, Africa, natural fission reactors were active more than 1.5 billion years ago.

• 7. Origins of the Elements

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  After the Big Bang, as the universe cooled, the first stable particles to form in observable numbers were protons and electrons in equal quantities. Those protons would eventually become hydrogen atoms. Almost all of the neutrons created in the Big Bang were absorbed into helium-4 within the first three minutes, and that process accounts for most of the helium in the universe today.

  Small amounts of lithium, beryllium, and perhaps some boron were also produced in those early minutes as protons and neutrons collided. Every heavier element, starting with carbon at atomic number 6, was forged inside stars through successive fusion stages including the proton-proton chain, the CNO cycle, and the triple-alpha process.

  Because fusion beyond iron requires energy input rather than releasing it, nature turns to neutron capture to build heavier nuclei. The slow neutron capture process, called the s-process, unfolds inside thermally pulsing stars known as asymptotic giant branch stars and takes hundreds to thousands of years to build up to the heaviest stable elements, lead and bismuth. The rapid process, the r-process, is thought to occur in supernova explosions, where extreme temperatures and neutron flux drive successive neutron captures through very neutron-rich species that then beta-decay toward heavier elements. Current research at facilities such as the Joint European Torus and ITER pursues a controlled fusion reaction as an economically viable energy source, working at the same frontier Eddington envisioned when he read the energy of stars in Einstein's equation.