Read each term where it’s used
In Augment’s notes, a marked term shows its definition when you hover or tap it, and each page ends with the terms it introduces.
In Augment’s notes, a marked term shows its definition when you hover or tap it, and each page ends with the terms it introduces.
126 terms from Nuclear and quantum physics, each defined in a sentence or two, with the part of the syllabus where you first meet it. 28 of them are HL only, and marked.
The number of nuclei in a sample that decay per unit time, measured in becquerels (Bq).
A helium-4 nucleus (two protons and two neutrons) emitted by an unstable nucleus in alpha decay.
The meeting of a particle and its antiparticle, which disappear and are replaced by energy; an electron and a positron at rest produce two 0.511 MeV γ photons travelling in opposite directions.
A neutral particle of almost zero mass, the antiparticle of the neutrino, emitted with the electron in β⁻ decay.
A particle with the same mass as a given particle but the opposite charge; the positron is the antiparticle of the electron and the antineutrino that of the neutrino.
A unit of angle equal to 1/3600 of a degree.
Ionising radiation present everywhere from natural and artificial sources (radon, rocks, cosmic rays, food, medical uses), which must be subtracted from measured count rates.
The narrow region of a plot of neutron number against proton number where the stable nuclides lie; it follows for light nuclei and curves to for heavy ones.
The SI unit of activity: one decay per second.
A decay in which a neutron in the nucleus changes into a proton, and an electron (β⁻ particle) and an antineutrino are emitted.
A decay in which a proton in the nucleus changes into a neutron, and a positron (β⁺ particle) and a neutrino are emitted.
The energy needed to separate a nucleus completely into its individual protons and neutrons; equal to the mass defect times .
The binding energy of a nucleus divided by its nucleon number; the larger it is, the more tightly bound the nucleus.
A region of space where matter has collapsed so far that its gravity prevents anything, even light, from escaping; formed from the cores of the most massive stars.
Bohr's model of the hydrogen atom: the electron moves in circular orbits whose angular momentum is quantised, , and emits or absorbs a photon only when it jumps between orbits.
The radius of the smallest () orbit in the Bohr model of hydrogen, about m.
A sequence of fissions in which neutrons released by one fission cause further fissions; it is self-sustaining when, on average, one neutron from each fission causes another.
The largest mass, about 1.4 solar masses, that a white dwarf can have while being supported by electron pressure.
The scattering of an X-ray or gamma-ray photon by a free or loosely bound electron, in which the photon loses energy, so its wavelength increases, and the electron recoils.
The increase in wavelength of a Compton-scattered photon: .
The constant , equal to the Compton shift for a photon scattered through 90° by an electron.
A spectrum containing all wavelengths over a wide range with no gaps, emitted by a hot, dense object such as a lamp filament or a star's interior.
A rod of a strongly neutron-absorbing material, such as boron or cadmium, moved into or out of a reactor core to control the rate of fission.
A fluid pumped through a reactor core to carry away the energy released by fission.
The electrostatic repulsion that two positively charged nuclei must overcome to get close enough for the strong nuclear force to bind them.
The number of decays registered by a detector per unit time; proportional to the activity for a fixed arrangement, but smaller than it.
The smallest mass of fissile material, in a given shape and arrangement, in which a chain reaction is self-sustaining.
The nucleus left after a parent nucleus has decayed.
The wavelength of a particle of momentum : .
The constant in : the fraction of undecayed nuclei that decay per unit time, related to the half-life by .
The energy released by the radioactive decay of fission products, which continues after a reactor's chain reaction has stopped.
A sequence of radioactive decays in which each daughter is itself unstable, ending at a stable nuclide.
A store for long-lived radioactive waste, hundreds of metres underground in stable rock, with several barriers between the waste and groundwater.
A tube of gas at low pressure that glows when a high potential difference is applied across it, giving the emission spectrum of the gas.
The minimum separation of a charged particle from a nucleus in a head-on collision, where all its initial kinetic energy has become electric potential energy: .
: the maximum kinetic energy of a photoelectron equals the energy of the absorbed photon minus the work function of the metal.
The spreading and interference of an electron beam passing through or reflecting off a crystal, which shows that electrons have a wave nature.
A set of bright lines at particular wavelengths, produced by a hot gas at low pressure; each line comes from electrons dropping between two energy levels.
Uranium in which the fraction of uranium-235 has been raised above the natural 0.7%, to about 3–5% for most power reactors.
Any energy level of an atom above the ground state; an atom in an excited state soon drops to a lower level and emits a photon.
Describes a nuclide, such as uranium-235 or plutonium-239, that can undergo fission after absorbing a slow (thermal) neutron.
A nucleus formed when a heavy nucleus splits; fission products are neutron-rich and so are usually radioactive β⁻ emitters.
The dark absorption lines in the spectrum of the Sun, produced by elements in its cooler outer layers.
A sealed metal tube filled with pellets of nuclear fuel (usually uranium dioxide), which keeps the fission products inside.
A detector of ionising radiation: each particle or photon that ionises the gas inside produces a pulse of current, which a counter records.
The lowest energy level of an atom, the state in which it is most stable.
The time taken for the number of undecayed nuclei of a nuclide, and so the activity of a sample, to fall to half its initial value.
A device that transfers energy from the hot primary coolant to water in a separate loop, turning it into steam, without the two fluids mixing.
A graph of stellar luminosity (vertical, log scale) against surface temperature (horizontal, log scale, decreasing to the right), on which stars fall into groups such as the main sequence, giants and white dwarfs.
The perpendicular distance between the line along which a particle approaches and a parallel line through the centre of the nucleus; the smaller it is, the larger the deflection.
A narrow, nearly vertical band of the HR diagram, at surface temperatures of roughly 5000–8000 K, in which evolved stars pulsate, regularly changing in size and luminosity.
The minimum energy needed to remove an electron from an atom in its ground state; 13.6 eV for hydrogen.
Radiation with enough energy to remove electrons from atoms it passes, leaving ions; α, β and γ radiation are ionising.
One of two or more nuclides of the same element: the same proton number but different numbers of neutrons, and so different nucleon numbers.
The distance light travels in a vacuum in one year, ; in light-years and years, .
A straight line on an HR diagram joining stars of equal radius; since , it slopes down from top left to bottom right.
The band on the HR diagram, from hot luminous stars to cool dim ones, of stars fusing hydrogen into helium in their cores; stars spend most of their lives on it.
The difference between the total mass of the separated nucleons of a nucleus and the mass of the nucleus itself.
The principle that mass and energy are equivalent, : any change in the energy of a system changes its mass by .
The wave associated with a moving particle, whose wavelength is the de Broglie wavelength .
A material of light nuclei, such as water or graphite, that slows fast neutrons to thermal speeds by elastic collisions so that they are more likely to cause fission.
A huge cloud of gas, mostly hydrogen and helium, and dust in space, in which stars form.
A neutral particle of almost zero mass that interacts extremely weakly with matter; emitted with the positron in β⁺ decay.
The number of neutrons in a nucleus, .
The extremely dense remnant core of a massive star after a supernova, about 20 km across, made mostly of neutrons and supported by the pressure of the closely packed neutrons.
Fission that happens after a heavy nucleus, such as uranium-235, absorbs a neutron and becomes highly unstable.
The ratio of the numbers of neutrons and protons in a nucleus; stable nuclei have values from about 1 (light) to about 1.5 (heavy).
The mass of a nucleus divided by its volume; about , the same for all nuclei.
One of the discrete energies a nucleus can have; transitions between them give γ photons and α particles of definite energies.
The splitting of a heavy nucleus into two smaller nuclei of comparable mass, usually with the release of two or three neutrons and energy.
The joining of two light nuclei to form a heavier nucleus, which releases energy when the product has a higher binding energy per nucleon.
Rutherford's model of the atom (1911): a tiny, massive, positive nucleus surrounded by electrons in mostly empty space.
The symbol for a nuclide, where X is the chemical symbol, the proton number and the nucleon number.
The radius of a nucleus, given by with m.
A device in which a controlled, self-sustaining fission chain reaction releases energy at a steady rate.
A particle in the nucleus: a proton or a neutron.
The total number of protons and neutrons in a nucleus.
The formation of chemical elements by nuclear reactions, mainly fusion in stars and reactions in supernovae.
The small, dense, positively charged centre of an atom, made of protons and neutrons, which holds nearly all of the atom's mass in a region about to m across.
A particular kind of nucleus, with a definite number of protons and neutrons, written .
The original nucleus that undergoes radioactive decay.
The distance at which 1 AU subtends an angle of one arc-second; .
How far a radiation travels into matter before it is absorbed, measured by the thickness of material needed to absorb it.
An evacuated tube containing a photosensitive cathode and an anode, used to measure the current and energy of photoelectrons.
The electric current carried by photoelectrons that travel from the cathode to the anode of a photocell.
The emission of electrons from a metal surface when electromagnetic radiation above a threshold frequency falls on it.
An electron released from a surface by the photoelectric effect.
The number of photons arriving per second; at a fixed frequency it is proportional to the intensity of the beam.
The constant that links the energy of a photon to its frequency () and the momentum of any particle to its wavelength ().
A glowing shell of gas thrown off by a low-mass star at the end of its red-giant stage, leaving a white dwarf at its centre.
A gas so hot that its atoms are fully or largely ionised, made of free nuclei (or ions) and free electrons.
Thomson's model of the atom (1904): a sphere of evenly spread positive charge with electrons embedded in it. Alpha-particle scattering showed it to be wrong.
The antiparticle of the electron, with the same mass and a charge of ; emitted in β⁺ decay.
The commonest type of power reactor, in which water under high pressure acts as both moderator and coolant and transfers energy to a separate steam loop.
The positive whole number that labels an energy level; in the Bohr model the hydrogen energy is eV.
The number of protons in a nucleus; it identifies the element.
The sequence of fusion reactions by which the Sun converts hydrogen into helium-4, releasing 26.7 MeV per helium nucleus.
A collapsing, heating ball of gas that has not yet started hydrogen fusion in its core.
The restriction of a quantity, such as the energy of an electron in an atom, to certain discrete values rather than any value.
The physics of systems in which energy and other quantities are exchanged in discrete amounts (quanta), and in which radiation and matter both show wave and particle behaviour.
The pressure exerted by electromagnetic radiation on matter that absorbs or reflects it, because photons carry momentum.
The random and spontaneous change of an unstable nucleus into a more stable one, with the emission of radiation (α, β or γ).
Unwanted radioactive material, such as spent fuel, that must be isolated from people and the environment until its activity has fallen to safe levels.
Finding the time since an organism died from the activity of carbon-14 per gram of carbon in its remains, which falls with a half-life of 5730 years once intake of carbon stops.
A large, luminous star with a cool surface, formed when a low-mass star has used up the hydrogen in its core; its main energy source is helium fusion in the core.
The chemical separation of spent fuel into reusable uranium and plutonium and the remaining high-level waste.
The deflection of alpha particles by the electric repulsion of nuclei, first measured by Geiger and Marsden by firing alpha particles at a thin metal foil; it showed that atoms have a nucleus.
The largest photocurrent from a photocell, reached when every photoelectron emitted is collected by the anode; it is proportional to the photon flux.
Material such as steel, water and concrete around a reactor that absorbs neutrons and gamma radiation to protect people.
The measurement of the wavelengths and intensities in a spectrum, used to identify the elements in a source of light or in a gas the light has passed through.
The range of wavelengths present in light (or other radiation) when they are spread out and separated, for example by a prism or a diffraction grating.
Fuel removed from a reactor after use, containing unused uranium, highly radioactive fission products and plutonium.
Fission of an unstable heavy nucleus that happens on its own, randomly, without being triggered by an incoming particle.
The state of a stable star in which, at every depth, the outward force from gas and radiation pressure balances the inward gravitational force.
The apparent shift of a nearby star against distant stars as Earth orbits the Sun; the parallax angle is the angle subtended at the star by 1 AU.
The minimum reverse potential difference between anode and cathode that reduces the photocurrent to zero; .
The short-range (about ) attractive force between nucleons that holds the nucleus together against the electric repulsion of the protons.
A very large and very luminous star, at the top of the HR diagram, formed when a massive star leaves the main sequence.
The explosion of a massive star at the end of its life, when its iron core collapses, leaving a neutron star or a black hole.
A neutron slowed until its kinetic energy is similar to that of the surrounding particles, about 0.025 eV; it is far more likely than a fast neutron to cause fission of uranium-235.
The minimum frequency of electromagnetic radiation that releases electrons from a given metal surface: .
The longest wavelength of electromagnetic radiation that releases electrons from a given metal surface: .
One-twelfth of the mass of a carbon-12 atom: .
The behaviour of both radiation and matter as waves in some experiments (diffraction, interference) and as particles in others (photoelectric effect, Compton scattering, detection at a point).
The hot, dense, Earth-sized remnant core of a low-mass star, in which no fusion takes place; it is supported by the pressure of closely packed electrons and slowly cools.
The minimum energy needed to remove an electron from the surface of a metal, symbol ; usually given in eV.