Particles and radioactivity

Alpha Decay

radioactive decay in which an atomic nucleus emits an alpha particle (helium-4 nucleus)

Annihilation

conversion of matter into energy, especially the mutual conversion of a particle and an antiparticle into electromagnetic radiation.

Antibaryon

hadron consisting of three antiquarks. Antibaryons have a baryon number of -1.

Antimuon (μ+)

positively-charged antiparticle of the muon.

Antineutron (n̅)

antiparticle of the neutron.

Antiparticle

opposite of a particle, have the same rest mass and, if charged, have opposite charge. They are given the same symbol as the particle but with opposite charge or, if uncharged or a quark, a bar on top: e.g. 𝜈̅ (anti electron-neutrino) and u̅ (anti-up quark).

Atom

smallest constituent of ordinary matter that retains the properties of a chemical element.

Atomic Mass Unit (amu)

unit of atomic mass, equal to one twelfth of the mass of an atom of carbon-12. Its value is given on the data sheet as 1.661 x 10-27 kg.

Baryon

hadron consisting of three quarks. See neutron and proton. Baryons have a baryon number of +1.

Baryon Number (B)

quantum number equal to the number of baryons in a system of subatomic particles minus the number of antibaryons. Quarks and antiquarks respectively have baryon numbers of + 1/3 & - 1/3 .

Beta Decay

radioactive decay in which an atomic nucleus emits a beta particle along with an electron antineutrino or electron neutrino.

Beta Particle

Beta negative particles are fast-moving electrons emitted by unstable, neutron-rich nuclei or the decay of a free neutron. Beta positive particles are positrons, the antiparticle of the electron, emitted by proton-rich nuclei.

Charge, Q

electric charge causes matter to experience the electromagnetic interaction. Positive and negative, an absence of which is referred to as neutral.

Cloud chamber

simple detector revealing the paths of alpha particles in (usually) ethanol-saturated air. The ionisation of the air causes instantaneous formation of condensation trails along the radiation path

Conservation Laws

conservation of energy, momentum, charge, baryon number and lepton number apply to all particle interactions. Conservation of strangeness applies to strong interactions only.

de Broglie Wavelength

effective wavelength of a matter particle

Electron, e-

sub-atomic particle, the lightest lepton.

Electromagnetic Interaction

a force that occurs between electrically charged particles.

Electron Capture

atom inner-shell electron captured by the nucleus, effectively reverse beta decay.

Electron Diffraction

observation that a beam of electrons fired through a suitable target (thin metal or graphite) will behave like waves, diffracting through the gaps between atoms to certain angles only.

Electron Volt (eV)

unit of energy, equal to the amount transferred when one electron is accelerated through a potential difference of 1 V. From the definition of p.d., 1 eV = 1.6 x 10-19 J

Energy Level

given as... h𝑓 = 𝐸1 − 𝐸2

Exchange Particle

virtual particles that interact between particles to give rise to forces of attraction or repulsion

Excitation

when an atom absorbs energy without becoming ionised as a result of an electron transition rom a lower to a higher energy level.

Feynman Diagram

pictorial representations describing the behaviour of subatomic particles, but they are not trajectories of the interacting particles in space and time. Generally, read left to right. Particles are represented by straight lines with arrows, exchange particles are wavy lines, time is usually on the x-axis. Particles are created/annihilated at the vertices between the lines

Geiger counter

instrument for measuring ionising radiation (including alpha, beta and gamma) count or count rate in a Geiger-Müller tube, based on principle discovered in the Cavendish Laboratory in 1908.

Hadron

particles and antiparticles which can interact through the strong nuclear interaction.

Ionisation

creation of ionised atoms (ions) due to the loss (or gain) of one or more electron

Isotope

nuclei of the same element with a different number of neutrons but the same number of protons. Hence they have the same proton number but a different nucleon number

Kaon, K±

short for K meson, they consist of a strange quark or antiquark and another quark or antiquark. They will decay into pions.

Lepton

fundamental particle that does not interact through the strong nuclear interaction. Electrons, muons, neutrinos and associated antiparticles all interact through the weak nuclear interaction and, except for neutrinos/antineutrinos, through the electromagnetic interaction.

Lepton Number, L

assigned to every lepton (+1) and antilepton (-1) such that the total lepton number is always conserved.

Line Spectra

set of dark or bright spectral lines in an otherwise uniform absence of light or a continuous spectrum. They result from emission (bright) or absorption (dark) of light of a certain photon energy. Line spectra can be used to identify atoms and molecules from their characteristic lines which are associated with transitions between energy levels

Maximum Kinetic Energy

of an emitted electron (sometimes called a photoelectron) from a metal surface via the photoelectric effect

MeV

unit of energy, the mega electron-volt is equal to 106 eV or 1.6 x 10-13J and is commonly used.

Neutron, n

neutral sub-atomic particle, a hadron, quark composition: udd.

Neutron Decay

in an atomic nucleus, a neutron may decay into a proton, leading to the emission of a electron and an anti electron-neutrino. A free neutron (on its own) will also decay (equation should be known: 0n 1 →1p 1 +-1e 0 +𝜈̅𝑒) in this way with a half-life of about 10 minutes.

Neutrino, Ve

sub atomic particles that lack an electric charge and have zero rest energy. The two symbols indicated here are for electron-neutrino and muon-neutrino respectively.

Nucleon Number, A

total number of nucleons in the nucleus of an atom. Sometimes called mass number

Nuclide

type of nucleus with a certain number of protons and neutrons

Pair Production

creation of a particle and its antiparticle

Photoelectric Effect

emission of electrons when light is shone onto a material.

Photoelectron

electrons emitted by the photoelectric effect can be called photoelectrons.

Photon

quantum of electromagnetic radiation and the force carrier for the electromagnetic interaction (even when static via virtual photons). Zero rest mass, they always move at the speed of light. Photons have energy given as... 𝐸 = ℎ𝑓 = ℎ𝑐⁄𝜆 where h is the Planck constant and c is the speed of light.

Pion

they consist of an up or down quark together with an up or down antiquark.

Planck Constant, h

proportionality constant between the energy of a photon and the frequency of electromagnetic radiation. Value: 6.63×10-34 Js

Positron, e+

antiparticle of the electron.

Proton, p

positive sub-atomic particle, a hadron, quark composition: uud. Opposite in charge to an electron. Protons are the only stable baryon, as they are the lightest. All other baryons decay into protons.

Proton Number, Z

the number of protons in an atomic nucleus. Sometimes called atomic number.

Quantum

minimum amount of a physical entity involved in an interaction. Some properties can be 'quantised', such as the energy levels of electrons in atoms, where only certain values are possible.

Quantum Number

one of a number of conserved quantities associated with particle interactions.

Quark

six types exist: up (u), down (d), strange (s), charmed, top and bottom

Rest Energy

energy due to the rest mass

Spark Counter

highly visible (and audible) way of showing and counting ionisation of the air caused by alpha radiation (or a match). It is a useful step towards understanding the Geiger counter.

Specific Charge

ratio of charge to mass for a particle

Strange Particle

subatomic particle classified as having a non-zero value for strangeness. Produced through the strong interaction they decay through the weak interaction. E.g. kaons.

Strangeness, S

assigned to every particle and antiparticle, strangeness is always conserved in the strong nuclear interaction, but not in a weak nuclear interaction (can change by 0, 1 or -1), or a decay involving a strange quark or antiquark.

Stopping Potential

minimum p.d. that needs to be applied to a metal plate to attract emitted photoelectrons back to the surface.

Strong Nuclear Force (Strong Nuclear Interaction)

attractive force between nucleons that holds the nucleus together, an attraction solely between two hadrons.

Threshold Frequency

minimum frequency of radiation incident on a metal surface which will cause electrons to be emitted from that metal in the photoelectric effect.

Transition

change from one state to another, for electrons in atoms from one energy level to another. Associated with the absorption or emission of a photon.

Virtual Photon

carrier of the electromagnetic interaction (force), a photon exchanged between two charged particles when they interact.

Wave-Particle Duality

matter particles have a wave-like nature (given by the de Broglie wavelength) as well as particle-like. Similarly, photons have particle-like nature as well as wave-like

Weak Nuclear Interaction

between two leptons or a lepton and hadron. Responsible for beta decay.

Work Function

(energy) of a metal is the minimum energy required by an electron to escape (called a photoelectron) from the surface of the metal. Commonly expressed in either eV or J.