Radioactive Nuclides

Nuclear Stability

In a stable nucleus, we can consider that the very short-range attractive strong nuclear forces are balanced by the longer range repulsive electric forces.

A nucleus will be unstable if it has:
- Too many neutrons
- Too many protons
- Too many nucleons (too heavy)
- Too much energy
The stability of a nuclide depends on the ratio of neutrons to protons (N/Z) in its nucleus.
- An unstable atom wants to become stable.

Lighter elements with, fewer protons and neutrons, tend to be much more stable than heavier elements with many protons and neutrons.
For nuclides with Z<20, stable nuclides have N/Z ≈ 1.
- This means they tend to have around the same number of protons has neutrons.
For larger nuclides, N/Z gradually increases to about N/Z ≈ 1.5.
- This means they have more up to 1.5 times more neutrons than protons.
Adding neutrons to larger nuclei with A > about 60 increases the total binding energy, but has little effect on the binding energy per nucleon.

Many radionuclides which emit alpha particles, emit them with only one precise energy.
However, some radionuclides can emit alpha particles with different energies as displayed in an alpha particle spectrum.
- The spectrum of nuclear radiations emitted from an unstable nucleus describes the relative numbers of particles emitted with different energies.

The energy of any emitted alpha particle can only have one of these energies.
Alpha particles with different energies are possible because the nucleus of neptunium-237 can be left in its ground state, or in one of several discrete excited states.
- The energy that isn't left in the neptunium is carried by the alpha particle.

Consider the previous example: after the alpha decays, four excited states of neptunium-237 nuclide can be seen.

When a nucleus changes from an excited state to a lower energy level, a gamma ray photon will be emitted.
- Nuclear energy levels are similar to electron levels.
  - The nucleus cannot emit a continuous range of gamma raysl.
  - The discrete energies of the photons again provide evidence for discrete energy levels within nuclei.
The discrete energy levels of nuclei are the reason why alpha particles and gamma rays are emitted with discrete energies.

The spectrum of beta particles emitted from a particular radionuclide is very different from alpha particles.
- It is a continuous spectrum, unlike alpha particles, which have discrete energies.

The emission of a beta particle involves another particle (an undetected neutrino or antineutrino).
- These particles may travel in random directions, so that a continuous range of beta particle energies is possible.
- The existence of the neutrino was proposed in 1930 by the physicist Wolfgang Pauli and it was confirmed in 1956.
- Neutrinos are incredibly small elementary particles and are neutral so they don't interact with other particles.
  - Many neutrinos can travel in straight lines for millions or even billions of years.

The anti-neutrino is the antiparticle of a neutrino.
- Electron anti-neutrinos are produced during beta-minus (electron) decay.
- Electron neutrinos are produced during beta-plus decay.

The energy released in beta decay is shared between the beta particles (electrons or positrons) and neutrinos (or anti-neutrinos).
The principle of conservation of momentum and energy applies in both alpha and beta emissions.

Sources