The first place to start when talking about radiation is what it is according to the Penguin english dictionary states RADIATION is "the process of emitting radiant energy in the form of waves or particles" as this does not state a source for the radiation you can sensibly assume that it refers to anything that sends out waves. there are many different types of radiation (a list of these is shown in fig 1 below).

Non-ionizing Electromagnetic Radiation:

• Radio
• Microwaves
• Infra red (heat)
• Visible Light (colour)
  • Red
  • Orange
  • Yellow
  • Green
  • Blue
  • Violet
• Ultra-violet

Ionizing Electromagnetic Radiation:

• X-Rays
• Gamma Rays
• Cosmic Rays

Ionizing Atomic Particle Radiation:

• Beta Rays
• Alpha Rays
• Neutrons

Radiation itself is made up of photons small particles with mass' of zero which travel at the speed of light at ll times. These act move in a streamwhich creates a wave (see fig 2).

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As you can see in the above diagram wave (a) has a very long wavelength in comparison to wave (b) this is cause by the amount of energy the photons in the waves posses, the higher the amount of energy a photon posesses the shorter the wavelength of the respective radiation. Below is a copy of the electromagnetic spectrum which shows all the differrent forms of radiation as well as their respective energies and wavelengths.

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The amount of energy a photon posesses is meadsured in electron volts or joules per photon however in the above diagram the energies are represented in electron volts. As you can see from the above diagram there are several different types of radiation however those suitable for use in making a dirty bomb are the gamma rays. Gamma rays are very powerful and high penetrating forms of radiation and unlike alpha and beta radiation gamma radiation does not have a charge (alpha = + and beta = -).

Also as gamma radiation is the most powerful within the electromagnetic spectrum it has a far greater ability to penetrate objects in the path of the waves emitted (see fig 3).

Courtesy of BNFL

However even though gamma radiation is the strongest in that it is able to penetrate much thicker/denser obstructions it is actually alpha and beta radiation that are the most dangerous as gamma radiation is liable to pass through the human body where as alpha and beta particles are trapped it the body where they will the start to cause cell damage.

Below are some details on what alpha and beta radiation are in more detail as well as how it is formed:

Alpha decay:

Alpha particles are positively charge particles, consisting of 2 protons and 2 neutrons. Alpha particles are produced spontaneously in some types of radioactive decay such as the example hown above as well as when helium-4 atoms are completely ionised.

Alpha radiation is not a penetrating radiation such as gamma radiation but is classed as damaging due to the affect it can have on living organisms. Unlike gamma radiation that travels at the speed of light alpha particles only move at approximately 10% the speed of light.

In the case of the example above (equation) the helium that is explelled from the reactionis the alpha particle and is the part that causes the radiactivity.

Beta Decay:

A nucleus with too many neutrons is unstable and will disintegrate to become more stable by emitting a high speed electron or beta particle.

The nucleus produced from the above reaction has 1 more proton and one less neutron therefore the its atomic mass is uncharged.

Beta particles are moderately penetrating.