Nuclear Forces and Decay Processes

Overview

This lecture explains what holds atomic nuclei together, the nature of stable and unstable nuclei, and the processes of alpha and beta decay.

Protons repel each other strongly due to their positive charges.
Gravity is too weak to hold nuclei together.
The strong nuclear force attracts protons and neutrons, holding the nucleus together.
The strong nuclear force acts only at short ranges (up to about 3 femtometers).
At distances less than about 1 femtometer, the strong nuclear force becomes repulsive to prevent nucleons from getting too close.
All nuclei have similar density due to the short range of the strong force.

Example: Carbon-14 decays because it has too many neutrons.
In beta-minus decay, a neutron changes into a proton, releasing a beta particle (electron) and an anti-neutrino.
The nucleon number (mass number) stays the same; the atomic number increases by one.
Charge is conserved in the process.
The anti-neutrino is a neutral, nearly undetectable particle produced alongside the electron.
The existence of neutrinos was proposed to account for missing energy in beta decay.

Very large nuclei are unstable and can become more stable by emitting an alpha particle.
An alpha particle is two protons and two neutrons (a helium nucleus).
Emitting an alpha particle reduces the nucleus's mass and atomic number (by four nucleons and two protons).
There are only 92 stable isotopes because larger nuclei become unstable.

Strong nuclear force — the force that holds protons and neutrons together in the nucleus, acting over very short distances.
Nucleon — a particle in the nucleus (either a proton or a neutron).
Beta decay — a type of radioactive decay where a neutron turns into a proton, releasing an electron and an anti-neutrino.
Alpha particle — a cluster of two protons and two neutrons emitted from a nucleus during alpha decay.
Neutrino/Anti-neutrino — neutral, nearly massless particles created in certain types of nuclear decay.