Particles in Atoms (P3)

Alpha: Helium nucleus, stopped by paper, slow and heavy.

Beta: Electron, stopped by thin metal, light and fast.

Gamma: Electromagnetic radiation, stopped by thick lead & very thick concrete, no mass and very fast.

Most Ionising ----> Least Ionsing : Alpha, Beta, Gamma.

Most Penetrating ----> Least Penetrating: Gamma, Beta, Alpha.

Beta is an electron and therefore negatively charged, but, there are particles called positrons that are the antiparticle  to electrons - they are the same except they are positively charged.

When an electron and a positron collide they annihilate each other. Since the mass can't just dissapear, this is changed into energy in the form of photons/ Gamma rays. 

The annihilation when a positron and electron collide is used in medicine in PET scanners (Positron Emission Tomography)

This is calculated by E=mc^2. (Energy = Mass * Speed of Light^2)

The speed of light, c, is 3x10^8 so the speed of light squared is 9x10^16. 

Neutrons are more penetrating than alpha or beta and sometimes more penetrating than gamma.

Unlike alpha, beta & gamma, they aren't directly ionising  but can be absorbed by the nuclei of atoms in the substances they pass through.

Absorbing a neutron can make a nucleus radioactive. These radioactive nuclei then emit ionising radiation (alpha, beta or gamma) so neutrons are sometimes called indirectly ionising. 

Neutrons are absorbed best by light nuclei. Hydrogen nuclei are the lightest of all, so Hydrogen-rich materials like water, polythene or concrete are used to make neutron radiation shielding.