Neutron

• 1. Introduction

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  In 1932, James Chadwick fired alpha particles at beryllium and watched something pass through that no electric field could bend. The radiation ejected protons from paraffin at enormous speeds. Others had called it gamma rays. Chadwick was not convinced. His experiments revealed an uncharged particle with roughly the mass of a proton. He had found the neutron. For this he won the 1935 Nobel Prize in Physics. The neutron carries no electric charge. Its mass is slightly greater than a proton's. It sits inside the nuclei of atoms, alongside a similar number of protons. Yet this neutral, almost shy particle would reshape the twentieth century within thirteen years. How did a particle nobody could see directly lead to nuclear fission, to a self-sustaining reactor, and to the first atomic bomb? Why does a free neutron survive only about fifteen minutes before falling apart? And how can something with no charge be steered, detected, or used to fight cancer? The answers begin in the contradictions of early atomic physics.

• 2. A Particle Predicted Before Found

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  The word neutron appears in the literature as early as 1899, tied to debates about the nature of the atom. Its name combines the Latin root for neutralis, meaning neuter, with the Greek suffix -on, the same ending used for the electron and proton. In the 1911 Rutherford model, the atom was a small positively charged nucleus wrapped in a much larger cloud of negative electrons. In 1920, Ernest Rutherford proposed that the nucleus held positive protons and neutrally charged particles, which he imagined as a proton and an electron bound together. Physicists assumed electrons lived inside the nucleus, since beta radiation consisted of electrons emitted from it. In 1921, the American chemist W. D. Harkins first gave the hypothetical particle its name. Beginning in 1928, that comfortable picture cracked. An electron confined to a nucleus would, under the Heisenberg uncertainty relation, carry energy exceeding the binding energy of the nucleus. The Klein paradox, found by Oskar Klein that year, said such an electron would simply escape. The spins did not work either. Protons and electrons each carry intrinsic spin, yet no arrangement of a bound electron and proton could produce the fractional spins observed in isotopes.

• 3. Chadwick's Decisive Experiments

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  In 1931, Walther Bothe and Herbert Becker noticed that alpha radiation from polonium striking beryllium, boron, or lithium produced an unusually penetrating radiation. Because no electric field deflected it, they guessed gamma rays. The following year, Irene Joliot-Curie and Frederic Joliot-Curie in Paris showed this radiation knocked high-energy protons out of paraffin and other hydrogen-rich compounds. Neither Rutherford nor James Chadwick at the Cavendish Laboratory in Cambridge accepted the gamma ray explanation. Werner Heisenberg and others quickly built models of a nucleus made of protons and neutrons, which resolved the puzzle of nuclear spins. In 1934, Enrico Fermi explained beta radiation as beta decay, in which a neutron becomes a proton by creating an electron and a then-undiscovered neutrino. In 1935, Chadwick and his doctoral student Maurice Goldhaber reported the first accurate measurement of the neutron's mass. The pieces of a strange new nucleus were falling into place.

• 4. From Discovery To The Bomb

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  By 1934, Fermi was bombarding heavier elements with neutrons to induce radioactivity in elements of high atomic number. In 1938 he received the Nobel Prize in Physics for demonstrating new radioactive elements produced by neutron irradiation, and for nuclear reactions brought about by slow neutrons. In December 1938, Otto Hahn, Lise Meitner, and Fritz Strassmann discovered nuclear fission, the splitting of uranium nuclei into lighter elements under neutron bombardment. In 1945, Hahn received the 1944 Nobel Prize in Chemistry for his discovery of the fission of heavy atomic nuclei. Physicists soon realized that if a fission event released neutrons, each could trigger further fission in a cascade called a nuclear chain reaction. That insight drove Fermi to build Chicago Pile-1 at the University of Chicago in 1942, the first self-sustaining nuclear reactor. Just three years later, the Manhattan Project tested the first atomic bomb in the Trinity nuclear test of July 1945.

• 5. Inside The Nucleus

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  An atomic nucleus binds together a number of protons, written Z for the atomic number, and a number of neutrons, written N for the neutron number, held by the nuclear force. The atomic number fixes the chemical element, while the neutron number sets the isotope. Atoms that share an atomic number but differ in neutron number are isotopes; those sharing a neutron number but differing in atomic number are isotones. The most common nuclide of lead, 208Pb, carries 82 protons and 126 neutrons. Protons and neutrons behave almost identically under the nuclear force, so they are jointly called nucleons. Heavy nuclei carry a large positive charge, and their protons repel one another electromagnetically more strongly than the nuclear force attracts them. Extra neutrons moderate that repulsion and hold the nucleus together. Curiously, a free neutron is unstable while a free proton is stable, yet inside a nucleus neutrons are often stable and protons sometimes are not. The Pauli exclusion principle can forbid a neutron from decaying when all lower proton states are filled. The carbon isotope carbon-14, with 6 protons and 8 neutrons, has a surplus of neutrons. It decays by beta decay to stable nitrogen-14 with a half-life of about 5730 years.

• 6. Three Quarks And A Decay

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  Within the Standard Model, a neutron is made of two down quarks, each with charge of minus one third, and one up quark with charge of plus two thirds. That makes it a composite hadron, and a baryon, since it holds three valence quarks. Its finite size and its magnetic moment both signal that it is not elementary. The strong force, carried by gluons, binds these quarks; the nuclear force itself is a secondary effect of that deeper strong force. A free neutron decays in a way that conserves baryon number. One of its down quarks changes flavour into a lighter up quark by emitting a W boson through the weak interaction. The neutron becomes a proton, an electron, and an electron antineutrino, with a mean lifetime of about fifteen minutes. The reverse can happen inside a nucleus, where a proton's up quark turns into a down quark, producing a neutron, a positron, and an electron neutrino. A proton can also become a neutron through electron capture. Rarer still, in the heat of stars, neutrons capture positrons.

• 7. Measuring The Unmeasurable

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  The mass of a neutron cannot be read directly by mass spectrometry, because it carries no electric charge. Physicists instead subtract the proton mass from the deuteron mass and add the binding energy of deuterium. That binding energy shows up as a single gamma photon of about 2.224 emitted when a proton captures a neutron. Bell and Elliot first measured this gamma energy by X-ray diffraction in 1948, and the best modern values came from Greene and colleagues in 1986. The neutron's spin stayed ambiguous for years after its discovery. In 1949, Hughes and Burgy reflected neutrons off a ferromagnetic mirror and found the angular pattern matched a spin one-half particle. In 1954, Sherwood, Stephenson, and Bernstein ran neutrons through a Stern-Gerlach experiment and recorded two spin states. Though electrically neutral, the neutron has a magnetic moment, first measured directly by Luis Alvarez and Felix Bloch at Berkeley, California, in 1940. That magnetic moment is negative, pointing opposite to the neutron's spin, and it betrays the quark substructure within. In 1964, Mirza A.B. Beg, Benjamin W. Lee, and Abraham Pais calculated the ratio of proton to neutron magnetic moments as minus three halves, agreeing with experiment to within three percent.

• 8. Harnessing A Neutral Force

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  A neutron's lack of charge makes it nearly impossible to steer, since electric and magnetic fields barely touch it. Researchers control neutrons instead through moderation, reflection, and velocity selection, and they polarize cold neutrons of 6 to 7 angstroms using magnetic mirrors and magnetized filters. To detect a neutron, physicists let it interact with atomic nuclei, often through neutron capture, where the compound nucleus emits an alpha particle, or through elastic scattering off light nuclei like hydrogen or helium. Free neutrons are rare, so they must be made continuously. Californium-252, with a half-life of 2.65 years, undergoes spontaneous fission about 3 percent of the time, releasing 3.7 neutrons per fission. Antimony-124 combined with beryllium gives a source with a half-life of 60.9 days. In nuclear fission, the absorption of a neutron by uranium-235 or plutonium-239 splits the nucleus and releases energy roughly ten million times that of an equal mass of chemical explosive. Neutrons also heal. Fast neutron therapy uses energies above 20 MeV against cancer, and boron neutron capture therapy loads a tumor with boron-10, which captures slow neutrons and decays to lithium-7 and an alpha particle that kills the malignant cell. Shielding such radiation defies intuition. Lead fails, while hydrogen-rich concrete or paraffin scatters and slows neutrons before lithium-6 absorbs them. That same hydrogen in heavy water moderates neutrons inside CANDU-type reactors, a quiet echo of the chain reaction Fermi first sustained in Chicago.