Read more: 鈥�Instant Expert: Neutron science鈥�
Eighty years ago, British physicist James Chadwick the existence of neutrons, which together with protons make up the atomic nucleus. Subsequent discoveries revealed the role that Chadwick鈥檚 particle played in converting one chemical element into another. And by the end of the 1930s, scientists had learned how nuclear reactions and radioactive decay could produce neutrons in large quantity. This led to the discovery of nuclear chain reactions and took physics into the unprecedented territories of nuclear power and nuclear weapons.
But there is another side to the neutron story 鈥� they have also become a powerful research tool that can reveal the structure of matter. Today, neutron science touches everything from the next generation of computers to the structure of viruses. And measuring the neutron鈥檚 unique properties promises to reveal fundamental secrets of the universe.
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Discovery of the neutron
The discovery of isotopes 鈥� atoms of the same element but with slightly different masses 鈥� led to a flurry of theories proposing the existence of a neutral particle within the atom. Among them, Ernest Rutherford conceived the existence of the neutron in 1920. However, the scientific instruments of the time were unable to isolate and identify neutral particles, which were more difficult to manipulate than charged ones.
Things changed in the 1930s, when James Chadwick, a physicist working in Rutherford鈥檚 laboratory at the University of Cambridge, committed himself to proving the neutron鈥檚 existence and assembled the finest equipment of the day.
His breakthrough came following work by French chemist Ir猫ne Joliot-Curie 鈥� the daughter of Marie Curie 鈥� and her husband Fr茅d茅ric. They had bombarded beryllium with fast-moving helium nuclei, known as alpha rays, and detected an astonishing form of radiation capable of penetrating 20 centimetres of lead. To find out more about this strange radiation, the Joliot-Curies surrounded the beryllium with paraffin wax and monitored it for changes. They found that the radiation was capable of knocking high-speed protons from the paraffin (see diagram). This led them to propose that the radiation must be gamma rays with incredible energies.
However, Chadwick quickly found an error in the Joliot-Curies鈥� conclusions. He repeated the experiment in his lab and demonstrated that the gamma-ray hypothesis was wrong. He suggested that the new radiation consisted of uncharged particles with roughly the same mass as protons, and performed a series of experiments to verify his claim. He had found the neutron.
Chadwick鈥檚 discovery gave a more complete picture of the atom and earned him the for physics in 1935. His finding was to underpin all of the nuclear physics that followed and it opened the door to a new, neutron science.
Unique particle
The neutron is an uncharged subatomic particle with a mass 1839 times as large as the electron. It is composed of three fundamental particles known as quarks. Whereas the proton is made of two up quarks and one down, the neutron is made of one up quark and two downs.
Inside the nucleus, neutrons and protons are attracted via the strong nuclear force, one of the four fundamental forces, which does not involve charge. However an alternative idea was put forward in 2007 by Gerald Miller at the University of Washington in Seattle. He suggested that although the neutron is neutral overall, it might be divided into charge, with a negatively charged skin and core sandwiching a positively charged middle. Although still merely a hypothesis, this negative skin might help explain its attraction to protons.
Neutrons are stable when bound in an atomic nucleus, but when they are free particles, they decay after about 15 minutes into a proton, an electron and an electron-antineutrino. Due to their instability, free neutrons are only found when nuclei disintegrate 鈥� in nuclear reactions or in the kind of high-energy reactions produced by cosmic rays or accelerator collisions.
Birth of a new science
Once the existence of the neutron was confirmed, thoughts immediately turned to applications. Neutrons can penetrate deep into materials and their lack of charge means they can go further than their electrically charged cousins, such as the electron or proton.
鈥淣eutrons can penetrate more deeply into materials than their electrically charged cousins, the electron and proton鈥�
As with all quantum particles, neutrons can behave like waves. So when they encounter obstacles comparable in size to their wavelength, they scatter along well-defined angles, much as water waves diffract around a rock. By analysing the scattering patterns, we can work out the structure of materials that neutrons have passed through.
The first neutron diffraction experiments were performed in the 1930s. However, it was not until about 1945, with the advent of nuclear reactors, that it became possible to produce high volumes of neutrons. This opened the door to in-depth investigations of the structure of materials, starting at first in multipurpose nuclear reactors. Neutrons were siphoned off through a beam-pipe and scattered off samples onto specially designed instruments.
The field really took off in the 1960s, when research reactors were optimised for such experiments. This development culminated in a reactor at the Institute (ILL) in Grenoble, France, which has been conducting experiments since 1972. There are now more than 20 active neutron science facilities, which come in two forms. Research reactors such as the ILL and the Isotope Reactor in Tennessee continue to use nuclear fission to produce a steady, reliable source of neutrons. Meanwhile the source at the Rutherford Appleton Laboratory in Didcot, UK, accelerates protons into heavy metal targets, prompting the emission of neutrons.
Facilities such as the ILL can produce neutrons over a vast range of energies, and this translates into a wide range of wavelengths. Thermal neutrons, which whizz around at a couple of kilometres per second, have short wavelengths and can be used to study atomic structures less than a nanometre across. Cold neutrons, which move 10 times more slowly, have long wavelengths and can study molecular structures at the micro scale.
Neutrons are incredibly versatile. Like other quantum particles, they have an intrinsic angular momentum known as spin. This allows them to interact with the electron spins in materials. That makes neutrons ideal for understanding the structure and dynamics of magnetic matter which gets its magnetic properties from the way the spins line up.
Today, the ILL operates the most intense neutron source in the world, feeding beams of neutrons to a suite of 40 high-performance instruments that are continually upgraded. Some 1500 researchers from more than 40 countries visit the lab each year.
