IB Physics Topic E - Nuclear and Quantum Physics Key Definitions

Dark lines in a stellar spectrum at specific wavelengths where photons have been absorbed by atoms in the star's outer atmosphere. Each element produces a unique set of lines, allowing astronomers to identify the chemical composition of a star.

Absorption spectrum is the range of frequencies of electromagnetic radiation readily absorbed by a substance by virtue of its chemical composition.

The activity of a radioactive sample is the number of decays per unit time.

Alpha decay occurs when an unstable nucleus emits an alpha particle, which is essentially a helium nucleus made up of two protons and two neutrons ($^4_2 \text{He}$).

In beta minus decay, a neutron in the nucleus is converted into a proton, emitting an electron ($\beta^-$) and an antineutrino ($\bar{\nu}$):

$$
n \rightarrow p + e^- + \bar{\nu}
$$

In beta plus decay, a proton is converted into a neutron, emitting a positron ($\beta^+$) and a neutrino($\nu$):

$$
p \rightarrow n + e^+ + \nu
$$

Binding energy is the energy required to hold the nucleus together.

A black body is an idealized object that absorbs all radiation falling on it and emits radiation based on its temperature.

A region of spacetime where gravity is so strong that nothing, not even light, can escape from within its event horizon. Formed when the remnant core of a massive star exceeds about 3 solar masses (the Oppenheimer-Volkoff limit).

A self-sustaining sequence of nuclear fission events in which each fission releases neutrons that trigger further fissions in neighbouring fuel nuclei.

The maximum mass of a white dwarf, approximately 1.4 solar masses. Above this limit, electron degeneracy pressure cannot support the star and it collapses further into a neutron star or black hole.

The increase in wavelength of a photon after it scatters off a free or loosely bound electron, caused by the photon transferring energy and momentum to the electron.

Rods made of neutron-absorbing material (typically boron or cadmium) that are inserted into or withdrawn from a reactor core to regulate the rate of the fission chain reaction.

The electrostatic potential energy barrier that two positively charged nuclei must overcome (or tunnel through) in order to come close enough for the strong nuclear force to bind them together.

The wavelength associated with any moving particle, equal to Planck's constant divided by the particle's momentum: $\lambda = h/p$.

The decay constant, $\lambda$, measures how quickly a radioactive substance decays. It is defined as the probability of decay per unit time for a single nucleus.

Diffraction is the spreading of waves as they pass through an aperture or around an obstacle.

The distance of closest approach is the minimum distance an alpha particle can reach during a head-on collision with a nucleus. This distance is determined using energy conservation.

The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an atom or molecule transitioning from a high energy state to a lower energy state.

Gamma decay occurs when an excited nucleus releases excess energy by emitting a gamma ray, a high-energy photon.

A scatter plot of stars showing luminosity on the vertical axis against surface temperature (decreasing to the right) on the horizontal axis, used to classify stars and trace their evolutionary paths.

The state of a star in which the outward radiation pressure from nuclear fusion exactly balances the inward gravitational force, keeping the star stable in size.

Induced fission occurs when a nucleus splits after absorbing a neutron.

Luminosity measures the amount of radiated electromagnetic energy per unit time.