Glossary — Quantum and nuclear physics

Activity — the number of nuclei that decay each second, $A = \dfrac{\Delta N}{\Delta t}$. Its unit is the becquerel.

Alpha particle — a helium nucleus, $^{4}_{2}\text{He}$, emitted by some unstable nuclei. It has 2 protons and 2 neutrons.

Antiparticle — a particle with the same mass as another particle but the opposite charge. The antiparticle of the electron is the positron.

Atomic mass unit — a unit of mass used for nuclei, $1\,\text{u} = 1.66 \times 10^{-27}\,\text{kg} = 931.5\,\text{MeV}$.

Background count rate — the count rate a detector records when no source is present, caused by nearby materials and cosmic rays. It is subtracted from a measured count rate.

Becquerel — the unit of activity, $\text{Bq}$. One becquerel is one decay per second.

Beta-minus decay — decay in which a neutron turns into a proton and the nucleus emits an electron and an antineutrino. The proton number rises by 1.

Beta-plus decay — decay in which a proton turns into a neutron and the nucleus emits a positron and a neutrino. The proton number falls by 1.

Binding energy — the energy needed to separate a nucleus into its separate protons and neutrons, equal to the mass defect multiplied by $c^2$.

Binding energy per nucleon — the binding energy of a nucleus divided by its nucleon number. It is largest near iron-56.

Count rate — the number of particles a detector records each second. It is smaller than the activity, because not every decay is detected.

de Broglie wavelength — the wavelength of a moving particle, $\lambda = \dfrac{h}{p} = \dfrac{h}{mv}$.

Decay constant — the probability that a given nucleus decays in one second, written $\lambda$. Its unit is $\text{s}^{-1}$, and $T_{1/2} = \dfrac{\ln 2}{\lambda}$.

Electron diffraction — the pattern of rings formed when a beam of electrons passes through a thin crystal. It shows that electrons have wave properties.

Energy level — one of the fixed energies an electron can have in an atom. A photon is emitted when an electron moves between two levels, with $hf = E_1 - E_2$.

Gamma radiation — a high-energy photon emitted by an unstable nucleus. The nucleon number and the proton number do not change.

Half-life — the average time for half of the unstable nuclei in a sample to decay, or for the activity to fall to half its starting value.

Isotope — one of two or more atoms of the same element with the same proton number but different nucleon numbers.

Line spectrum — the set of bright lines of particular wavelengths emitted by an element, one for each allowed energy transition.

Mass defect — the difference between the mass of a nucleus and the total mass of its separate protons and neutrons.

Nucleon — a proton or a neutron, the particles that make up a nucleus.

Nucleon number — the total number of protons and neutrons in a nucleus, written $A$.

Nuclide — a particular nucleus with a given proton number and nucleon number, written $^{A}_{Z}\text{X}$.

Photoelectric effect — the release of electrons from a metal surface when light of a high enough frequency falls on it.

Photocurrent — the electric current formed by the photoelectrons released in the photoelectric effect.

Photoelectron — an electron released from a metal surface by the photoelectric effect.

Photon — a packet of electromagnetic energy. Its energy is $E = hf$.

Planck constant — the constant $h = 6.63 \times 10^{-34}\,\text{J}\,\text{s}$ that links a photon's energy to its frequency.

Positron — the antiparticle of the electron. It has the same mass as an electron and a positive charge.

Proton number — the number of protons in a nucleus, written $Z$. It decides which element the atom is.

Radioactive decay — the spontaneous change of an unstable nucleus into a more stable one, with the release of radiation.

Spontaneous — describing a decay that happens without anything outside the nucleus setting it off.

Stopping potential — the potential difference that brings the fastest photoelectrons to rest, $eV_s = E_{k(\text{max})}$.

Threshold frequency — the lowest frequency of light that releases electrons from a metal surface, $f_0 = \dfrac{\phi}{h}$.

Threshold wavelength — the longest wavelength of light that releases electrons from a metal surface, $\lambda_0 = \dfrac{hc}{\phi}$.

Wave-particle duality — the idea that light and matter show both wave properties and particle properties, depending on the experiment.

Work function — the minimum energy an electron needs to escape from a metal surface, written $\phi$.