Figure 1.

Different types of ionizing radiation can be characterized by their depth of penetration. Alpha and beta particles have the lowest penetration, while neutrons and gamma rays are capable of penetrating materials up to and including lead. The machine-generated x-rays, physically identical to gamma rays and thus not included in our discussion, have been added for illustrative purposes. [Modified from Canadian Nuclear Safety Commission Website. Available at: http://nuclearsafety.gc.ca/eng/readingroom/publications/gauges/index.cfm]