- Background: A background (radiation) is Natural radiation caused by sun exposure, cosmic rays from space, and radioactive elements found in the earth's crust. Radon, which emanates from the ground, is an example of natural radiation. Cosmic rays include energetic protons, electrons, gamma rays, and x-rays. The primary radioactive elements found in the earth's crust are uranium, thorium, and potassium, and their radioactive derivatives.