How far the ionising radiation can penetrate a material depends upon its type, and how much energy it has. In nuclear terms, alpha particles are large and slow and can be simply blocked by a sheet of paper, or the layer of dead skin cells which cover our bodies. Beta particles are much smaller travel at higher speeds but can be blocked by a thin sheet of aluminium or Perspex. Gamma radiation and X-rays are electromagnetic radiation, consisting of waves of energy (photons) which travel at high speed and require dense shielding materials (e.g. lead and concrete) to reduce their number to an acceptable level.