Radiation Interactions


Alpha Particles


Alpha particles are produced by the decay of heavy unstable nuclei or in ternary fission. Natural alpha emitters include uranium, thorium and radium. Nuclear reactors also produce transuranic elements including neptunium, plutonium and americium. alpha

The alpha particle is a relatively heavy entity (4 atomic mass units) being composed of two neutrons and two protons. It has a charge of +2e (where e is the electron charge of 1.602 x 10-19 Coulombs). Alpha particles typically have energies between 3 and 7 MeV and travel at speeds of 15,000km·s-1 (5% of the speed of light) which is slow in comparison to beta and gamma radiation. Because of its charge the alpha particle interacts strongly with the medium it is travelling in (mediated by the Coulomb force) causing dense ionisation. Each coulomb interaction costs the alpha particles a small fraction of its energy. Therefore its deflection is almost negligible and it travels in a almost straight paths in matter, losing energy continuously through a large number of collisions. Because it loses a lot of energy over a short path length (in the jargon it has a high linear energy transfer (LET)) it runs out of energy quickly and has a short range in matter. When the alpha's energy is reduced to a low enough level it can capture an electron which reduces its charge and the rate of energy loss. The capture of a second electron neutralises it and turns it into a helium atom.

Coulomb force: There is a force between charges that causes attraction of unlike charges and repulsion of alike charges. This force works at a distance significantly greater than the atomic radius and acts to slow down moving charged particles by transferring some of the kinetic energy of the particle to the atoms they interact with.

Alpha particles are readily stopped due to their charge and mass. This has three main implications:

  • Alpha particles are not generally an external hazard as they are stopped by the dead (epidermal layer) layer of skin, which covers most of the body. The exception is the lens of the eye which has no epidermal layer.
  • The range of alpha particles (which is energy dependent) is typically a few centimetres (3 - 5cm) in air.
  • Internal to the human body alpha particles are more damaging per unit of energy than gamma rays and beta particles as their shorter stopping distance means that they deposit a lot of energy within a smaller volume. This means that they are capable of causing more intense damage which reproducing cells find harder to repair. For a given energy there is more radiological risk associated with an alpha particle than most other types of radiation.

Beta particles


Beta particles are electrons (β or β-) or positrons (β+) which are ejected from the nucleus at relativistic speeds (>75% of the speed of light) and they are often emitted in conjunction with gamma rays. They are relatively light (1/1835 atomic mass units) and have a charge of -e (electron) and +e (positron). They do not interact as strongly as the alpha particle but when they do interact they can be scattered over a wider angle and lose a larger fraction of their energy in each interaction. CO60

Beta particles are produced by a neutron converting to a proton, an electron and a neutrino (beta+ from a neutron converting to a proton, a positron and a neutrino). Particles from a particular decay have a range of energies up to a characteristic maximum energy (the balance of energy being carried of by the ghostly particle called a neutrino). The resulting daughter nucleus following a beta decay is usually in an excited state and emits characteristic gamma radiation in falling to its ground state.

The range of beta particles in air is generally ~15 cm centimetres but for high energy beta particles (2 MeV) it may be up to 2 m. Beta particles are decelerated by electromagnetic interactions and as a result may give off bremsstrahlung (breaking) x-rays. Low atomic number materials (such as water and plastic) are preferred as beta shields because they produce less damaging X-ray radiation than high atomic number (high z) materials.

Beta radiation can penetrate the outer layers of skin (epidermal) and it can cause skin burning (erythema) at high levels of contamination and, like alpha contamination, is more dangerous if the source is taken into the body.

Neutron


The neutron, is simply that, a neutron; one of the sub-atomic particles. Because it has no charge the neutron does not experience the Coulomb force and will only interact with atomic nuclei by direct collision. As it does not interact continuously it can travel some distance without a change in direction or loss of energy. Its range in material is very much greater than that of the charged particles and it deposits energy non-uniformly. For fast neutrons, such as those produced in fission, materials with light atoms (such as water or polythene, which both contain hydrogen) are more effective shields as each collision with a light nucleus takes a larger fraction of the energy from the neutron.

Gamma


Emspec

Gamma radiation is an electromagnetic radiation like light, x-rays and radio waves but is higher in energy. Gamma rays travel at the speed of light and for nuclear decay energies generally range from 100 keV to 10 MeV and are typically ~1 MeV. Gamma radiation has a relatively long range in material and only dense materials such as steel or lead, or a significant path-length in other materials such as water, provide an effective shield. Gamma radiation can deliver a significant dose to internal organs without being taken into the body.

Which nuclei decay?


Decay Summary

The nuclei of atoms are built of neutrons and protons. The number of protons in a nucleus (z) determines which chemical it is. The sum of the number of neutrons (n) and protons is the mass number (a) of the nucleus.

If you plot some nuclear properties of the nuclei on grid of number of neutrons against number of protons you can determine some interesting and useful trends. For example from the NZ decay diagram, which plots decay mode on the NZ grid we see a "valley of stability" (the black boxes denote stable isotopes) that approximately follows the N=Z line for light nuclei but leans towards a higher proportion of neutrons for heavier nuclei, nuclei below this line (relatively neutron rich) tend to beta decay which moves them one step to the left and one step upwards, towards the valley of stability; nuclei above this line (relatively proton rich) tend to beta+ decay, one step down, one step to the right, and very heavy nuclei tend to alpha decay (two steps down, two to the left). These three decay modes are often accompanied with gamma photons. NZ Decay

Decay chains


Decay Chain 1

Not all decays result in a stable nucleus. Some lead to another decay giving us decay chains.