These emissions are called ionising radiation. As the radiation is emitted, the atoms change their nature from one element to a 'daughter' element. This may also be radioactive leading to a second generation radioactive daughter and so the process will continue until eventually a stable atom is reached. This process is known as a radioactive decay series. Half-life can vary from fractions of a second to thousands of millions of years.
Radioactivity is measured in terms of the number of disintegrations that occur per second.
The universally recognised symbol for radioactivity and ionising radiation is the trefoil symbol. The main ionising radiations are alpha particles, beta particles, gamma rays, X-rays and neutrons. Alpha particles have very little penetration power and can be stopped by a sheet of paper. Beta particles also have poor penetration and, on average, can travel only a few millimeters through biological tissue. Gamma and X-rays and neutrons are very penetrating and can readily pass through the body.