The neutron is a subatomic particle, symbol n or n0, with no net electric charge and a mass slightly larger than that of a proton. Protons and neutrons, each with mass approximately one atomic mass unit, constitute the nucleus of an atom, and they are collectively referred to as "nucleons".[4] Their properties and interactions are described by nuclear physics.
The nucleus consists of a number of protons, or atomic number with symbol Z, and a number of neutrons, or neutron number with symbol N. The atomic number defines the chemical properties of the atom, and the neutron number determines the isotope.[5] The atomic mass number, symbol A, equals Z+N. For example, carbon has atomic number 6, and its abundant carbon-12 isotope has 6 neutrons, whereas its rare carbon-13 isotope has 7 neutrons. Some elements occur in nature with only one stable isotope, such as fluorine (see stable nuclide). Other elements occur as many stable isotopes, such as tin with ten stable isotopes. Even though it is not a chemical element, the neutron is sometimes included in tables of nuclides.[6]
While the neutrons bound in a nucleus can be stable (depending on the nuclide), free neutrons, or individual neutrons free of the nucleus, are unstable. Free neutrons have a mean lifetime of just under 15 minutes (881.5±1.5 s) from a radioactive decay known as beta decay.[7] Within the nucleus, protons and neutrons are bound together through the strong force, and neutrons are required for the stability of nuclei. Neutrons are produced copiously in nuclear fission and fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission, fusion, and neutron capture processes.
The neutron is essential to the production of nuclear power. The neutron was discovered in 1932,[8] and it was realized as early as 1933 that the neutron might mediate a nuclear chain reaction. In the 1930s, neutrons were used to produce many different types of nuclear transmutations. When nuclear fission was discovered in 1938,[9] it became clear that, if a fission event produced neutrons, these neutrons might cause more fission events, etc., in a cascade known as a chain reaction.[5] These events and findings led to the first self-sustaining nuclear reactor (Chicago Pile-1, 1942) and to the first nuclear weapon (Trinity, 1945).
A small natural "neutron background" flux of free neutrons exists on Earth, caused by cosmic ray muons, and by the natural radioactivity of certain fissile elements in the Earth's crust.[10] Free neutrons are effectively a form of ionizing radiation, and as such, are a biological hazard, depending upon dose.[5] Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments.
The story of the discovery of the neutron and its properties is central to the extraordinary developments in atomic physics that occurred in the first half of the 20th century, leading ultimately to the atomic bomb in 1945. The century began with Ernest Rutherford and Thomas Royds proving that alpha radiation is helium ions in 1908[11][12] and Rutherford's model for the atom in 1911,[13] in which atoms have their mass and positive charge concentrated in a very small nucleus.[14] The essential nature of the atomic nucleus was established with the discovery of the neutron in 1932. By mid century, these discoveries and subsequent developments had ushered in the atomic age.
The Rutherford atom
A schematic of the nucleus of an atom indicating β− radiation, the emission of a fast electron from the nucleus (the accompanying antineutrino is omitted). In the Rutherford model for the nucleus, red spheres were protons with positive charge and blue spheres were protons tightly bound to an electron with no net charge.
The inset shows beta decay of a free neutron as it is understood today; an electron and antineutrino are created in this process.
The 1911 Rutherford model was that the atom was made up of a massive central positive charge of small spatial extent surrounded by a larger cloud of negatively charged electrons. This model had been developed from the extraordinary finding that alpha particles were on occasion scattered to high angle when passing through gold foil, indicating the alpha particles were occasionally reflecting from a small, but dense, component of atoms. Rutherford and others noted the disparity between the atomic number of an atom, or number of positive charges, and its mass computed in atomic mass units. The atomic number of an atom is usually about half its atomic mass. In 1920 Rutherford suggested that the disparity could be explained by the existence of a neutrally charged particle within the atomic nucleus.[15] Since at the time no such particle was known to exist, yet the mass of such a particle had to be about equal to that of the proton, Rutherford considered the required particle to be a neutral double consisting of an electron closely orbiting a proton.[15] The mass of protons is about 1800 times greater than that of electrons.
There were other motivations for the proton–electron model. As noted by Rutherford at the time, "We have strong reason for believing that the nuclei of atoms contain electrons as well as positively charged bodies...",[15] namely, it was known that beta radiation was electrons emitted from the nucleus.
The nucleus consists of a number of protons, or atomic number with symbol Z, and a number of neutrons, or neutron number with symbol N. The atomic number defines the chemical properties of the atom, and the neutron number determines the isotope.[5] The atomic mass number, symbol A, equals Z+N. For example, carbon has atomic number 6, and its abundant carbon-12 isotope has 6 neutrons, whereas its rare carbon-13 isotope has 7 neutrons. Some elements occur in nature with only one stable isotope, such as fluorine (see stable nuclide). Other elements occur as many stable isotopes, such as tin with ten stable isotopes. Even though it is not a chemical element, the neutron is sometimes included in tables of nuclides.[6]
While the neutrons bound in a nucleus can be stable (depending on the nuclide), free neutrons, or individual neutrons free of the nucleus, are unstable. Free neutrons have a mean lifetime of just under 15 minutes (881.5±1.5 s) from a radioactive decay known as beta decay.[7] Within the nucleus, protons and neutrons are bound together through the strong force, and neutrons are required for the stability of nuclei. Neutrons are produced copiously in nuclear fission and fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission, fusion, and neutron capture processes.
The neutron is essential to the production of nuclear power. The neutron was discovered in 1932,[8] and it was realized as early as 1933 that the neutron might mediate a nuclear chain reaction. In the 1930s, neutrons were used to produce many different types of nuclear transmutations. When nuclear fission was discovered in 1938,[9] it became clear that, if a fission event produced neutrons, these neutrons might cause more fission events, etc., in a cascade known as a chain reaction.[5] These events and findings led to the first self-sustaining nuclear reactor (Chicago Pile-1, 1942) and to the first nuclear weapon (Trinity, 1945).
A small natural "neutron background" flux of free neutrons exists on Earth, caused by cosmic ray muons, and by the natural radioactivity of certain fissile elements in the Earth's crust.[10] Free neutrons are effectively a form of ionizing radiation, and as such, are a biological hazard, depending upon dose.[5] Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments.
The story of the discovery of the neutron and its properties is central to the extraordinary developments in atomic physics that occurred in the first half of the 20th century, leading ultimately to the atomic bomb in 1945. The century began with Ernest Rutherford and Thomas Royds proving that alpha radiation is helium ions in 1908[11][12] and Rutherford's model for the atom in 1911,[13] in which atoms have their mass and positive charge concentrated in a very small nucleus.[14] The essential nature of the atomic nucleus was established with the discovery of the neutron in 1932. By mid century, these discoveries and subsequent developments had ushered in the atomic age.
The Rutherford atom
A schematic of the nucleus of an atom indicating β− radiation, the emission of a fast electron from the nucleus (the accompanying antineutrino is omitted). In the Rutherford model for the nucleus, red spheres were protons with positive charge and blue spheres were protons tightly bound to an electron with no net charge.
The inset shows beta decay of a free neutron as it is understood today; an electron and antineutrino are created in this process.
The 1911 Rutherford model was that the atom was made up of a massive central positive charge of small spatial extent surrounded by a larger cloud of negatively charged electrons. This model had been developed from the extraordinary finding that alpha particles were on occasion scattered to high angle when passing through gold foil, indicating the alpha particles were occasionally reflecting from a small, but dense, component of atoms. Rutherford and others noted the disparity between the atomic number of an atom, or number of positive charges, and its mass computed in atomic mass units. The atomic number of an atom is usually about half its atomic mass. In 1920 Rutherford suggested that the disparity could be explained by the existence of a neutrally charged particle within the atomic nucleus.[15] Since at the time no such particle was known to exist, yet the mass of such a particle had to be about equal to that of the proton, Rutherford considered the required particle to be a neutral double consisting of an electron closely orbiting a proton.[15] The mass of protons is about 1800 times greater than that of electrons.
There were other motivations for the proton–electron model. As noted by Rutherford at the time, "We have strong reason for believing that the nuclei of atoms contain electrons as well as positively charged bodies...",[15] namely, it was known that beta radiation was electrons emitted from the nucleus.
0 comments:
Post a Comment