Luminescence is spontaneous emission of light by a substance not resulting from heat. It can be caused by electrical energy, subatomic motions or stress on a crystal; this distinguishes luminescence from incandescence, light emitted by a substance as a result of heating. Radioactivity was thought of as a form of "radio-luminescence", although it is today considered to be separate since it involves more than electromagnetic radiation; the dials, hands and signs of aviation and navigational instruments and markings are coated with luminescent materials in a process known as "luminising". The following are types of luminescence: Chemiluminescence, the emission of light as a result of a chemical reaction Bioluminescence, a result of biochemical reactions in a living organism Electrochemiluminescence, a result of an electrochemical reaction Lyoluminescence, a result of dissolving a solid in a liquid solvent Candoluminescence, is light emitted by certain materials at elevated temperatures, which differs from the blackbody emission expected at the temperature in question.
Crystalloluminescence, produced during crystallization Electroluminescence, a result of an electric current passed through a substance Cathodoluminescence, a result of a luminescent material being struck by electrons Mechanoluminescence, a result of a mechanical action on a solid Triboluminescence, generated when bonds in a material are broken when that material is scratched, crushed, or rubbed Fractoluminescence, generated when bonds in certain crystals are broken by fractures Piezoluminescence, produced by the action of pressure on certain solids Sonoluminescence, a result of imploding bubbles in a liquid when excited by sound Photoluminescence, a result of absorption of photons Fluorescence, photoluminescence as a result of singlet–singlet electronic relaxation Phosphorescence, photoluminescence as a result of triplet–singlet electronic relaxation Raman emission, photoluminescence as a result of inelastic light scattering, Radioluminescence, a result of bombardment by ionizing radiation Thermoluminescence, the re-emission of absorbed energy when a substance is heatedCryoluminescence, the emission of light when an object is cooled Light-emitting diodes emit light via electro-luminescence.
Phosphors, materials that emit light when irradiated by higher-energy electromagnetic radiation or particle radiation Phosphor thermometry, measuring temperature using phosphorescence Thermoluminescence dating Thermoluminescent dosimeter Non-disruptive observation of processes within a cell. Luminescence occurs in some minerals when they are exposed to low-powered sources of ultraviolet or infrared electromagnetic radiation, at atmospheric pressure and atmospheric temperatures; this property of these minerals can be used during the process of mineral identification at rock outcrops in the field, or in the laboratory. List of light sources Fluorophores.org A database of luminescent dyes
In physics, a shock wave, or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a medium but is characterized by an abrupt, nearly discontinuous, change in pressure and density of the medium. For the purpose of comparison, in supersonic flows, additional increased expansion may be achieved through an expansion fan known as a Prandtl–Meyer expansion fan; the accompanying expansion wave may approach and collide and recombine with the shock wave, creating a process of destructive interference. The sonic boom associated with the passage of a supersonic aircraft is a type of sound wave produced by constructive interference. Unlike solitons, the energy and speed of a shock wave alone dissipates quickly with distance; when a shock wave passes through matter, energy is preserved but entropy increases. This change in the matter's properties manifests itself as a decrease in the energy which can be extracted as work, as a drag force on supersonic objects.
Shock waves can be: Normal At 90° to the shock medium's flow direction. Oblique At an angle to the direction of flow. Bow Occurs upstream of the front of a blunt object when the upstream flow velocity exceeds Mach 1; some other terms Shock front: The boundary over which the physical conditions undergo an abrupt change because of a shock wave. Contact front: In a shock wave caused by a driver gas, the boundary between the driver and the driven gases; the Contact Front trails the Shock Front. The abruptness of change in the features of the medium, that characterize shock waves, can be viewed as a phase transition: the pressure-time diagram of a supersonic object propagating shows how the transition induced by a shock wave is analogous to a dynamic phase transition; when an object moves faster than the information can propagate into the surrounding fluid the fluid near the disturbance cannot react or "get out of the way" before the disturbance arrives. In a shock wave the properties of the fluid change instantaneously.
Measurements of the thickness of shock waves in air have resulted in values around 200 nm, on the same order of magnitude as the mean free gas molecule path. In reference to the continuum, this implies the shock wave can be treated as either a line or a plane if the flow field is two-dimensional or three-dimensional, respectively. Shock waves are formed when a pressure front moves at supersonic speeds and pushes on the surrounding air. At the region where this occurs, sound waves travelling against the flow reach a point where they cannot travel any further upstream and the pressure progressively builds in that region. Shock waves are not conventional sound waves. Shock waves in air are heard as "snap" noise. Over longer distances, a shock wave can change from a nonlinear wave into a linear wave, degenerating into a conventional sound wave as it heats the air and loses energy; the sound wave is heard as the familiar "thud" or "thump" of a sonic boom created by the supersonic flight of aircraft.
The shock wave is one of several different ways in which a gas in a supersonic flow can be compressed. Some other methods are isentropic compressions, including Prandtl–Meyer compressions; the method of compression of a gas results in different temperatures and densities for a given pressure ratio which can be analytically calculated for a non-reacting gas. A shock wave compression results in a loss of total pressure, meaning that it is a less efficient method of compressing gases for some purposes, for instance in the intake of a scramjet; the appearance of pressure-drag on supersonic aircraft is due to the effect of shock compression on the flow. In elementary fluid mechanics utilizing ideal gases, a shock wave is treated as a discontinuity where entropy increases over a nearly infinitesimal region. Since no fluid flow is discontinuous, a control volume is established around the shock wave, with the control surfaces that bound this volume parallel to the shock wave; the two surfaces are separated by a small depth such that the shock itself is contained between them.
At such control surfaces, mass flux and energy are constant. It is assumed the system is adiabatic and no work is being done; the Rankine–Hugoniot conditions arise from these considerations. Taking into account the established assumptions, in a system where the downstream properties are becoming subsonic: the upstream and downstream flow properties of the fluid are considered isentropic. Since the total amount of energy within the system is constant, the stagnation enthalpy remains constant over both regions. Though, entropy is increasing; when analyzing shock waves in a flow field, which are still attached to the body, the shock wave, deviating at some arbitrary angle from the flow direction is termed oblique shock. These shocks require a component vector analysis of the flow.
Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions. Other forms of nuclear matter are studied. Nuclear physics should not be confused with atomic physics, which studies the atom as a whole, including its electrons. Discoveries in nuclear physics have led to applications in many fields; this includes nuclear power, nuclear weapons, nuclear medicine and magnetic resonance imaging and agricultural isotopes, ion implantation in materials engineering, radiocarbon dating in geology and archaeology. Such applications are studied in the field of nuclear engineering. Particle physics evolved out of nuclear physics and the two fields are taught in close association. Nuclear astrophysics, the application of nuclear physics to astrophysics, is crucial in explaining the inner workings of stars and the origin of the chemical elements; the history of nuclear physics as a discipline distinct from atomic physics starts with the discovery of radioactivity by Henri Becquerel in 1896, while investigating phosphorescence in uranium salts.
The discovery of the electron by J. J. Thomson a year was an indication that the atom had internal structure. At the beginning of the 20th century the accepted model of the atom was J. J. Thomson's "plum pudding" model in which the atom was a positively charged ball with smaller negatively charged electrons embedded inside it. In the years that followed, radioactivity was extensively investigated, notably by Marie and Pierre Curie as well as by Ernest Rutherford and his collaborators. By the turn of the century physicists had discovered three types of radiation emanating from atoms, which they named alpha and gamma radiation. Experiments by Otto Hahn in 1911 and by James Chadwick in 1914 discovered that the beta decay spectrum was continuous rather than discrete; that is, electrons were ejected from the atom with a continuous range of energies, rather than the discrete amounts of energy that were observed in gamma and alpha decays. This was a problem for nuclear physics at the time, because it seemed to indicate that energy was not conserved in these decays.
The 1903 Nobel Prize in Physics was awarded jointly to Becquerel for his discovery and to Marie and Pierre Curie for their subsequent research into radioactivity. Rutherford was awarded the Nobel Prize in Chemistry in 1908 for his "investigations into the disintegration of the elements and the chemistry of radioactive substances". In 1905 Albert Einstein formulated the idea of mass–energy equivalence. While the work on radioactivity by Becquerel and Marie Curie predates this, an explanation of the source of the energy of radioactivity would have to wait for the discovery that the nucleus itself was composed of smaller constituents, the nucleons. In 1906 Ernest Rutherford published "Retardation of the α Particle from Radium in passing through matter." Hans Geiger expanded on this work in a communication to the Royal Society with experiments he and Rutherford had done, passing alpha particles through air, aluminum foil and gold leaf. More work was published in 1909 by Geiger and Ernest Marsden, further expanded work was published in 1910 by Geiger.
In 1911–1912 Rutherford went before the Royal Society to explain the experiments and propound the new theory of the atomic nucleus as we now understand it. The key experiment behind this announcement was performed in 1910 at the University of Manchester: Ernest Rutherford's team performed a remarkable experiment in which Geiger and Marsden under Rutherford's supervision fired alpha particles at a thin film of gold foil; the plum pudding model had predicted that the alpha particles should come out of the foil with their trajectories being at most bent. But Rutherford instructed his team to look for something that shocked him to observe: a few particles were scattered through large angles completely backwards in some cases, he likened it to firing a bullet at tissue paper and having it bounce off. The discovery, with Rutherford's analysis of the data in 1911, led to the Rutherford model of the atom, in which the atom had a small dense nucleus containing most of its mass, consisting of heavy positively charged particles with embedded electrons in order to balance out the charge.
As an example, in this model nitrogen-14 consisted of a nucleus with 14 protons and 7 electrons and the nucleus was surrounded by 7 more orbiting electrons. Around 1920, Arthur Eddington anticipated the discovery and mechanism of nuclear fusion processes in stars, in his paper The Internal Constitution of the Stars. At that time, the source of stellar energy was a complete mystery; this was a remarkable development since at that time fusion and thermonuclear energy, that stars are composed of hydrogen, had not yet been discovered. The Rutherford model worked quite well until studies of nuclear spin were carried out by Franco Rasetti at the California Institute of Technology in 1929. By 1925 it was known that protons and electrons each had a spin of +/-1⁄2. In the Rutherford model of nitrogen-14, 20 of the total 21 nuclear particles should have paired up to cancel each other's spin, the final odd particle should have left the nucleus with a net spin of 1⁄2. Rasetti discovered, that nitrogen-14 had a spin of 1.
In 1932 Chadwick realized that radiation, observed by Walther Bothe, Herbert Becker, Irène and Frédéric Joliot-Curie was due to a neutral particle of about the same mass as the proton, that he called the neutron (following a su
Philipp Eduard Anton von Lenard was a German physicist and the winner of the Nobel Prize for Physics in 1905 for his research on cathode rays and the discovery of many of their properties. Lenard was a anti-Semite. Notably, he labeled Albert Einstein's contributions to science as "Jewish physics". Philipp Lenard was born in Pressburg, on 7 June 1862 in the Kingdom of Hungary; the Lenard family had come from Tyrol in the 17th century, Lenard's parents were German-speakers. His father, Philipp von Lenardis, was a wine-merchant in Pressburg, his mother was Antonie Baumann. The young Lenard studied at the Pozsonyi Királyi Katolikus Főgymnasium, as he writes it in his autobiography, this made a big impression on him. In 1880, he studied chemistry in Vienna and in Budapest. In 1882, Lenard left Budapest and returned to Pressburg, but in 1883, he moved to Heidelberg after his tender for an assistant's position in the University of Budapest was refused. In Heidelberg, he studied under the illustrious Robert Bunsen, interrupted by one semester in Berlin with Hermann von Helmholtz, he obtained a doctoral degree in 1886.
In 1887 he worked again in Budapest under Loránd Eötvös as a demonstrator. After posts at Aachen, Breslau and Kiel, he returned to the University of Heidelberg in 1907 as the head of the Philipp Lenard Institute. In 1905, Lenard became a member of the Royal Swedish Academy of Sciences, in 1907, of the Hungarian Academy of Sciences, his early work included the conductivity of flames. As a physicist, Lenard's major contributions were in the study of cathode rays, which he began in 1888. Prior to his work, cathode rays were produced in primitive evacuated glass tubes that had metallic electrodes in them, across which a high voltage could be placed. Cathode rays were difficult to study using this arrangement, because they were inside sealed glass tubes, difficult to access, because the rays were in the presence of air molecules. Lenard overcame these problems by devising a method of making small metallic windows in the glass that were thick enough to be able to withstand the pressure differences, but thin enough to allow passage of the rays.
Having made a window for the rays, he could pass them out into the laboratory, or, into another chamber, evacuated. These windows have come to be known as Lenard windows, he was able to conveniently detect the rays and measure their intensity by means of paper sheets coated with phosphorescent materials. Lenard observed that the absorption of cathode rays was, to first order, proportional to the density of the material they were made to pass through; this appeared to contradict the idea. He showed that the rays could pass through some inches of air of a normal density, appeared to be scattered by it, implying that they must be particles that were smaller than the molecules in air, he confirmed some of J. J. Thomson's work, which arrived at the understanding that cathode rays were streams of negatively charged energetic particles, he called them quanta of electricity or for short quanta, after Helmholtz, while J. J. Thomson proposed the name corpuscles, but electrons became the everyday term. In conjunction with his and other earlier experiments on the absorption of the rays in metals, the general realization that electrons were constituent parts of the atom enabled Lenard to claim that for the most part atoms consist of empty space.
He proposed that every atom consists of empty space and electrically neutral corpuscules called "dynamids", each consisting of an electron and an equal positive charge. As a result of his Crookes tube investigations, he showed that the rays produced by irradiating metals in a vacuum with ultraviolet light were similar in many respects to cathode rays, his most important observations were that the energy of the rays was independent of the light intensity, but was greater for shorter wavelengths of light. These latter observations were explained by Albert Einstein as a quantum effect; this theory predicted that the plot of the cathode ray energy versus the frequency would be a straight line with a slope equal to Planck's constant, h. This was shown to be the case some years later; the photo-electric quantum theory was the work cited when Einstein was awarded the Nobel Prize in Physics. Suspicious of the general adulation of Einstein, Lenard became a prominent skeptic of relativity and of Einstein's theories generally.
Lenard received the 1905 Nobel Prize for Physics in recognition of this work. Lenard was the first person to study what has been termed the Lenard effect in 1892; this is the separation of electric charges accompanying the aerodynamic breakup of water drops. It is known as spray electrification or the waterfall effect, he conducted studies on the size and shape distributions of raindrops and constructed a novel wind tunnel in which water droplets of various sizes could be held stationary for a few seconds. He was the first to recognize that large raindrops are not tear-shaped, but are rather shaped something like a hamburger bun. Lenard is remembered today as a strong German nationalist who despised "English physics", which he considered to have stolen its ideas from Germany, he joined the Nation
J. J. Thomson
Sir Joseph John Thomson was an English physicist and Nobel Laureate in Physics, credited with the discovery and identification of the electron, the first subatomic particle to be discovered. In 1897, Thomson showed that cathode rays were composed of unknown negatively charged particles, which he calculated must have bodies much smaller than atoms and a large charge-to-mass ratio. Thomson is credited with finding the first evidence for isotopes of a stable element in 1913, as part of his exploration into the composition of canal rays, his experiments to determine the nature of positively charged particles, with Francis William Aston, were the first use of mass spectrometry and led to the development of the mass spectrograph. Thomson was awarded the 1906 Nobel Prize in Physics for his work on the conduction of electricity in gases. Joseph John Thomson was born 18 December 1856 in Cheetham Hill, Lancashire, England, his mother, Emma Swindells, came from a local textile family. His father, Joseph James Thomson, ran.
He had a brother, Frederick Vernon Thomson, two years younger than he was. J. J. Thomson was a devout Anglican, his early education was in small private schools where he demonstrated outstanding talent and interest in science. In 1870, he was admitted to Owens College in Manchester at the unusually young age of 14, his parents planned to enroll him as an apprentice engineer to Sharp-Stewart & Co, a locomotive manufacturer, but these plans were cut short when his father died in 1873. He moved on to Trinity College, Cambridge, in 1876. In 1880, he obtained his Bachelor of Arts degree in mathematics, he applied for and became a Fellow of Trinity College in 1881. Thomson received his Master of Arts degree in 1883. In 1890, Thomson married Rose Elisabeth Paget, one of his former students, daughter of Sir George Edward Paget, KCB, a physician and Regius Professor of Physic at Cambridge at the church of St. Mary the Less, they had one son, George Paget Thomson, one daughter, Joan Paget Thomson. On 22 December 1884, Thomson was appointed Cavendish Professor of Physics at the University of Cambridge.
The appointment caused considerable surprise, given that candidates such as Osborne Reynolds or Richard Glazebrook were older and more experienced in laboratory work. Thomson was known for his work as a mathematician, he was awarded a Nobel Prize in 1906, "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases." He was knighted in 1908 and appointed to the Order of Merit in 1912. In 1914, he gave the Romanes Lecture in Oxford on "The atomic theory". In 1918, he became Master of Trinity College, where he remained until his death. Joseph John Thomson died on 30 August 1940. One of Thomson's greatest contributions to modern science was in his role as a gifted teacher. One of his students was Ernest Rutherford, who succeeded him as Cavendish Professor of Physics. In addition to Thomson himself, six of his research assistants won Nobel Prizes in physics, two won Nobel prizes in chemistry. In addition, Thomson's son won the 1937 Nobel Prize in physics for proving the wave-like properties of electrons.
Thomson's prize-winning master's work, Treatise on the motion of vortex rings, shows his early interest in atomic structure. In it, Thomson mathematically described the motions of William Thomson's vortex theory of atoms. Thomson published a number of papers addressing both mathematical and experimental issues of electromagnetism, he examined the electromagnetic theory of light of James Clerk Maxwell, introduced the concept of electromagnetic mass of a charged particle, demonstrated that a moving charged body would increase in mass. Much of his work in mathematical modelling of chemical processes can be thought of as early computational chemistry. In further work, published in book form as Applications of dynamics to physics and chemistry, Thomson addressed the transformation of energy in mathematical and theoretical terms, suggesting that all energy might be kinetic, his next book, Notes on recent researches in electricity and magnetism, built upon Maxwell's Treatise upon electricity and magnetism, was sometimes referred to as "the third volume of Maxwell".
In it, Thomson emphasized physical methods and experimentation and included extensive figures and diagrams of apparatus, including a number for the passage of electricity through gases. His third book, Elements of the mathematical theory of electricity and magnetism was a readable introduction to a wide variety of subjects, achieved considerable popularity as a textbook. A series of four lectures, given by Thomson on a visit to Princeton University in 1896, were subsequently published as Discharge of electricity through gases. Thomson presented a series of six lectures at Yale University in 1904. Several scientists, such as William Prout and Norman Lockyer, had suggested that atoms were built up from a more fundamental unit, but they envisioned this unit to be the size of the smallest atom, hydrogen. Thomson in 1897 was the first to suggest that one of the fundamental units was more than 1,000 times smaller than an atom, suggesting th
Wilhelm Conrad Röntgen was a German mechanical engineer and physicist, who, on 8 November 1895, produced and detected electromagnetic radiation in a wavelength range known as X-rays or Röntgen rays, an achievement that earned him the first Nobel Prize in Physics in 1901. In honour of his accomplishments, in 2004 the International Union of Pure and Applied Chemistry named element 111, roentgenium, a radioactive element with multiple unstable isotopes, after him. Born to a German father and a Dutch mother, Röntgen attended high school in Netherlands. In 1865, he was unfairly expelled from high school when one of his teachers intercepted a caricature of one of the teachers, in fact done by someone else. Without a high school diploma, Röntgen could only attend university in the Netherlands as a visitor. In 1865, he tried to attend Utrecht University without having the necessary credentials required for a regular student. Upon hearing that he could enter the Federal Polytechnic Institute in Zurich, he passed its examinations, began studies there as a student of mechanical engineering.
In 1869, he graduated with a Ph. D. from the University of Zurich. In 1874, Röntgen became a lecturer at the University of Strassburg. In 1875, he became a professor at the Academy of Agriculture at Württemberg, he returned to Strassburg as a professor of physics in 1876, in 1879, he was appointed to the chair of physics at the University of Giessen. In 1888, he obtained the physics chair at the University of Würzburg, in 1900 at the University of Munich, by special request of the Bavarian government. Röntgen planned to emigrate, he accepted an appointment at Columbia University in New York City and bought transatlantic tickets, before the outbreak of World War I changed his plans. He remained in Munich for the rest of his career. During 1895, Röntgen was investigating the external effects from the various types of vacuum tube equipment — apparatuses from Heinrich Hertz, Johann Hittorf, William Crookes, Nikola Tesla and Philipp von Lenard — when an electrical discharge is passed through them. In early November, he was repeating an experiment with one of Lenard's tubes in which a thin aluminium window had been added to permit the cathode rays to exit the tube but a cardboard covering was added to protect the aluminium from damage by the strong electrostatic field that produces the cathode rays.
He knew the cardboard covering prevented light from escaping, yet Röntgen observed that the invisible cathode rays caused a fluorescent effect on a small cardboard screen painted with barium platinocyanide when it was placed close to the aluminium window. It occurred to Röntgen that the Crookes–Hittorf tube, which had a much thicker glass wall than the Lenard tube, might cause this fluorescent effect. In the late afternoon of 8 November 1895, Röntgen was determined to test his idea, he constructed a black cardboard covering similar to the one he had used on the Lenard tube. He covered the Crookes–Hittorf tube with the cardboard and attached electrodes to a Ruhmkorff coil to generate an electrostatic charge. Before setting up the barium platinocyanide screen to test his idea, Röntgen darkened the room to test the opacity of his cardboard cover; as he passed the Ruhmkorff coil charge through the tube, he determined that the cover was light-tight and turned to prepare the next step of the experiment.
It was at this point that Röntgen noticed a faint shimmering from a bench a few feet away from the tube. To be sure, he saw the same shimmering each time. Striking a match, he discovered the shimmering had come from the location of the barium platinocyanide screen he had been intending to use next. Röntgen speculated. 8 November was a Friday, so he took advantage of the weekend to repeat his experiments and made his first notes. In the following weeks he ate and slept in his laboratory as he investigated many properties of the new rays he temporarily termed "X-rays", using the mathematical designation for something unknown; the new rays came to bear his name in many languages as "Röntgen rays". At one point while he was investigating the ability of various materials to stop the rays, Röntgen brought a small piece of lead into position while a discharge was occurring. Röntgen thus saw the first radiographic image, his own flickering ghostly skeleton on the barium platinocyanide screen, he reported that it was at this point that he determined to continue his experiments in secrecy, because he feared for his professional reputation if his observations were in error.
Nearly two weeks after his discovery, he took the first picture using X-rays of his wife Anna Bertha's hand. When she saw her skeleton she exclaimed "I have seen my death!" He made a better picture of his friend Albert von Kölliker's hand at a public lecture. Röntgen's original paper, "On A New Kind Of Rays", was published on 28 December 1895. On 5 January 1896, an Austrian newspaper reported Röntgen's discovery of a new type of radiation. Röntgen was awarded an honorary Doctor of Medicine degree from the University of Würzburg after his discovery, he published a total of three papers on X-rays between 1895 and 1897. Today, Röntgen is considered the father of diagnostic radiology, the medical speciality which uses imaging to diagnose disease. A collection of his papers is held at the National Library of Medicine in Maryland. Röntgen was married to Ann
The neutron is a subatomic particle, symbol n or n0, with no net electric charge and a mass larger than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave within the nucleus, each has a mass of one atomic mass unit, they are both referred to as nucleons, their properties and interactions are described by nuclear physics. The chemical and nuclear properties of the nucleus are determined by the number of protons, called the atomic number, the number of neutrons, called the neutron number; the atomic mass number is the total number of nucleons. For example, carbon has atomic number 6, 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. Other elements occur with many stable isotopes, such as tin with ten stable isotopes. Within the nucleus and neutrons are bound together through the nuclear force. Neutrons are required for the stability of nuclei, with the exception of the single-proton hydrogen atom.
Neutrons are produced copiously in nuclear fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission and neutron capture processes; the neutron is essential to the production of nuclear power. In the decade after the neutron was discovered by James Chadwick in 1932, neutrons were used to induce many different types of nuclear transmutations. With the discovery of nuclear fission in 1938, it was realized that, if a fission event produced neutrons, each of these neutrons might cause further fission events, etc. in a cascade known as a nuclear chain reaction. These events and findings led to the first self-sustaining nuclear reactor and the first nuclear weapon. Free neutrons, while not directly ionizing atoms, cause ionizing radiation; as such they can be a biological hazard, depending upon dose. A small natural "neutron background" flux of free neutrons exists on Earth, caused by cosmic ray showers, by the natural radioactivity of spontaneously fissionable elements in the Earth's crust.
Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments. An atomic nucleus is formed by a number of protons, Z, a number of neutrons, N, bound together by the nuclear force; the atomic number defines the chemical properties of the atom, the neutron number determines the isotope or nuclide. The terms isotope and nuclide are used synonymously, but they refer to chemical and nuclear properties, respectively. Speaking, isotopes are two or more nuclides with the same number of protons; the atomic mass number, symbol A, equals Z+N. Nuclides with the same atomic mass number are called isobars; the nucleus of the most common isotope of the hydrogen atom is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium and tritium contain one proton bound to one and two neutrons, respectively. All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons.
The most common nuclide of the common chemical element lead, 208Pb, has 82 protons and 126 neutrons, for example. The table of nuclides comprises all the known nuclides. Though it is not a chemical element, the neutron is included in this table; the free neutron has 1.674927471 × 10 − 27 kg, or 1.00866491588 u. The neutron has a mean square radius of about 0.8×10−15 m, or 0.8 fm, it is a spin-½ fermion. The neutron has no measurable electric charge. With its positive electric charge, the proton is directly influenced by electric fields, whereas the neutron is unaffected by electric fields; the neutron has a magnetic moment, however. The neutron's magnetic moment has a negative value, because its orientation is opposite to the neutron's spin. A free neutron is unstable, decaying to a proton and antineutrino with a mean lifetime of just under 15 minutes; this radioactive decay, known as beta decay, is possible because the mass of the neutron is greater than the proton. The free proton is stable. Neutrons or protons bound in a nucleus can be stable or unstable, depending on the nuclide.
Beta decay, in which neutrons decay to protons, or vice versa, is governed by the weak force, it requires the emission or absorption of electrons and neutrinos, or their antiparticles. Protons and neutrons behave identically under the influence of the nuclear force within the nucleus; the concept of isospin, in which the proton and neutron are viewed as two quantum states of the same particle, is used to model the interactions of nucleons by the nuclear or weak forces. Because of the strength of the nuclear force at short distances, the binding energy of nucleons is more than seven orders of magnitude larger than the electromagnetic energy binding electrons in atoms. Nuclear reactions therefore have an energy density, more than ten million times that of chemical reactions; because of the mass–energy equivalence, nuclear binding energies reduce the mass of nuclei. The ability of the nuclear force to store energy arising from the electromagnetic repulsion of nuclear components is the basis for most of the energy that makes nuclear reactors or bombs possible.
In nuclear fission, the absorption of a neutron by a heavy nuclide causes the nuclide to become unstable and break into light nuclides and additional neu