Neutron I INTRODUCTION Neutron, electrically neutral elementary particle that is part of the nucleus of the atom.

Neutron I INTRODUCTION Neutron, electrically neutral elementary particle that is part of the nucleus of the atom. Elementary particles are the smallest parts of matter that scientists can isolate. The neutron is about 10-13 cm in diameter and weighs 1.6749 x 10-27 kg. See also Atom Neutrons and protons bind tightly together to create atomic nuclei. The number of protons an atom contains determines which chemical element it is, ranging from 1 proton for hydrogen to 92 for uranium, the largest naturally occurring element. Each atom usually contains about as many neutrons as protons, but different atoms of the same element may have different numbers of neutrons. Atoms that differ only in the number of neutrons are called isotopes. For example, most atoms of the simplest element, hydrogen, have a nucleus containing only a single proton. In natural hydrogen, however, 0.015 percent of the atoms have a neutron in addition to the proton. This isotope is called heavy hydrogen or deuterium. An element usually has several isotopes, all nearly identical in the way they react chemically with other elements and each other. Scientists can distinguish different isotopes of an element by examining properties of the element's nuclei, such as the mass of the nucleus. II CHARACTERISTICS The neutron is slightly heavier than a proton and 1,838 times as heavy as the electron. It is affected by all the four fundamental forces of nature. Because it has mass, it is affected by gravitation, the force of attraction between all objects in the universe. Although the neutron has no electrical charge, it is slightly magnetic, so it is affected by the electromagnetic force, the force of attraction or repulsion between electrically charged or magnetic objects. The neutron is affected by the strong nuclear force, an attraction that binds the neutron to protons and other neutrons in the nucleus. The neutron is also affected by the weak nuclear force, an interaction among the building blocks of the neutron that causes the neutron to decay, or break apart. Isolated from nuclear matter, a free neutron decays into a positively charged proton and a negatively charged electron, releasing energy in the process (see Nuclear Energy). The average lifetime of a free neutron is just under 15 minutes. Scientists discovered neutrons after they first discovered protons in the nuclei of atoms. For a time, physicists thought that protons and neutrons were the smallest particles inside the nucleus. However, after 1947 physicists found other elementary particles, such as the lambda particle and mesons. These particles are not found in the nucleus but can be created by nuclear reactions, changes within the nucleus that release particles. Many of these elementary particles have properties similar to those of the neutron and proton. Physicists reasoned that, since the elementary particles appear to be related to each other, they must all be composed of the same smaller building blocks, which they called quarks. At first, scientists thought that quarks were not actual objects, but in 1967 physicists used high energy beams of electrons to probe deep inside the proton and neutron and revealed evidence for tiny grainlike particles, or quarks. The neutron, like the proton, is made of three quarks. The strong nuclear force is actually a force that attracts quarks to each other to make a proton or neutron. The quarks of a neutron or proton will also attract the quarks of another neutron or proton, thus holding a nucleus together. III NEUTRONS IN ATOMS Isotopes of Hydrogen and Carbon Isotopes of an element differ in the number of neutrons in each atom's nucleus. Hydrogen has two naturally occurring isotopes, normal hydrogen, which contains no neutrons, and heavy hydrogen, which has one neutron in each atom. The third isotope, tritium, has two neutrons. It is radioactive, with a half-life of 12.26 years. Carbon has three naturally occurring isotopes: carbon-12 has six neutrons in each atom; carbon-13 has seven neutrons; and carbon-14 has eight neutrons. Carbon-14 is radioactive with a half-life of 5760 years. © Microsoft Corporation. All Rights Reserved. Even before the discovery of neutrons in 1932, physicists realized that atomic nuclei must have an electrically neutral component. The mass of nearly every hydrogen atom is equal to the sum of one proton and one electron. However, for any other atom, the atomic mass is larger than the sum of the electron and proton masses. The neutrons are responsible for the remaining mass of the atom. Beta Decay Beta decay can occur in two ways. As shown on the left, a neutron turns into a proton by emitting an antineutrino and a negatively charged beta particle. As shown on the right, a proton turns into a neutron by emitting a neutrino and a positively charged beta particle. Positive beta particles are called positrons and negative beta particles are called electrons. After the decay, the nucleus of the atom contains either one less or one more proton. Beta decay changes an atom of one element into an atom of a new element. © Microsoft Corporation. All Rights Reserved. Neutrons play an important role in the stability of a nucleus. In the nucleus, two neighboring protons repel each other with an electrical force that is 100 million times stronger than the electrical attraction that binds the electrons around the positively charged nucleus. Protons and neutrons are bound together by the strong nuclear force. Certain combinations of neutrons and protons bind together especially tightly. An example is the helium nucleus, also called an alpha particle, which contains two protons and two neutrons. Carbon (C-14) Dating All living organisms absorb carbon-14, an unstable form of carbon that has a half-life of about 5,730 years. During its lifetime, an organism continually replenishes its supply of carbon-14 by breathing and eating. After the organism dies and becomes a fossil, C-14 continues to decay without being replaced. To measure the amount of radiocarbon left in a fossil, scientists burn a small piece to convert it into carbon dioxide gas. Radiation counters are used to detect the electrons given off by decaying C-14 as it turns into nitrogen. The amount of C-14 is compared to the amount of C-12, the stable form of carbon, to determine how much radiocarbon has decayed and to date the fossil. © Microsoft Corporation. All Rights Reserved. Embedded in a nucleus, a neutron is usually stable--that is, it will not decay into a proton and an electron. The nucleus itself is then stable. However, if the nuclear conditions are not optimal--for example, if the nucleus has too many neutrons--one or more of the neutrons may decay. Scientists describe unstable nuclei as being radioactive and describe their changes as nuclear reactions. An example of a radioactive nucleus occurs in the element carbon. Carbon mainly consists of carbon-12 (with six protons and six neutrons) and a small amount of carbon-14 (with six protons and eight neutrons). Carbon-14 is radioactive--its combination of protons and neutrons is unstable and a neutron in its nucleus can decay. When an unstable carbon-14 neutron decays, it splits into a proton and an electron. With seven protons and seven neutrons, the atom is now nitrogen. This natural decay is used in a process called carbon dating to determine the age of anything that was once living matter, such as fossils, wood, and natural fabrics (see Dating Methods: Carbon-14 Method). The carbon content of living matter is continually renewed, so the proportion of carbon-12 to carbon-14 remains the same. Once the organism dies, the carbon is no longer renewed. Because scientists know how much carbon-14 was present in the beginning and how long it takes for the carbon-14 to decay, they can determine the age of a relic by measuring the residual amount of carbon-14 in the object. In another form of radioactivity, nuclear fission, the unstable nucleus of a large atom splits into two roughly equal smaller nuclei, losing several spare neutrons in the process and releasing energy. The fast-moving free neutrons usually pass through matter, but each one can be captured more easily by another nucleus after the neutron loses some energy and slows down. If a free neutron hits a large nucleus, such as that of uranium, the nucleus can capture it and become unstable. Each new unstable nucleus splits into two roughly equal smaller nuclei and creates more spare neutrons and more energy. Those spare neutrons can then strike more large nuclei to repeat the process in a chain reaction. See also Nuclear Chemistry and See also Nuclear Energy: Nuclear Energy from Fission IV HISTORY AND CURRENT RESEARCH Ernest Rutherford British physicist Ernest Rutherford, winner of the 1908 Nobel Prize in chemistry, pioneered the field of nuclear physics with his research and development of the nuclear theory of atomic structure. Rutherford stated that an atom consists largely of empty space, with an electrically positive nucleus in the center and electrically negative electrons orbiting the nucleus. By bombarding nitrogen gas with alpha particles (nuclear particles emitted through radioactivity), Rutherford engineered the transformation of an atom of nitrogen into both an atom of oxygen and an atom of hydrogen. This experiment was an early stimulus to the development of nuclear energy, a form of energy in which nuclear transformation and disintegration release extraordinary power. Archive Photos Following his discovery of the proton in 1919, British physicist Ernest Rutherford suggested that a third particle, in addition to the proton and the electron, existed inside the atom. In 1930 the German physicists Walther Bothe and Herbert Becker bombarded beryllium with alpha particles and produced a radiation that passed through ten centimeters of lead. In 1932 French physicists Irène and Frédéric Joliot-Curie found that this radiation could knock protons out of hydrogen atoms. In the same year, British physicist James Chadwick measured the energy of the protons emerging from the hydrogen atoms and showed that they had been knocked out by a particle of about the same mass, but electrically neutral. This new particle was therefore named the neutron. Sir James Chadwick British physicist Sir James Chadwick won the Nobel Prize in physics in 1935. Chadwick's discovery of the neutron led to the development of nuclear fission and the atomic bomb. © The Nobel Foundation By studying the physics of the neutron, scientists can better understand what happens inside neutron stars, stars that are made up entirely of neutrons. Neutron stars form when a star contains so much matter that the gravitational attraction between all of its atoms is powerful enough to crush them. The outer electrons are forced into the nucleus and combine with protons, thus creating a neutron star. One cubic centimeter of a neutron star weighs 100 million tons. Knowledge of neutron physics also aids in the design of nuclear reactors and nuclear weapons, and it furthers the study of molecular structure. Nuclear reactions release a tremendous amount of energy. This energy can be used in nuclear weapons or, when the reactions are carefully controlled in nuclear reactors, as a source of electricity (See also Nuclear Energy: Nuclear Power Reactors). Physics researchers use beams of neutrons to study the inner structure of materials. They create the neutron beams from reactors, or by accelerating protons with magnetic fields in a particle accelerator, then slamming these protons into large nuclei such as uranium. These neutron beams can be directed at a sample material. When the neutrons pass through the sample, they behave like waves traveling around barriers and their paths bend to form a pattern called a diffraction pattern. This pattern reveals information about the internal structure of the sample. Contributed By: Gordon Fraser Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.