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. Normal matter has no net electrical charge i.e. the positive and negative electrical charges are balanced. The interaction of ionising radiation with matter upsets the natural balance of positive and negative charges. The Sv is the modern unit of dose and has largely replaced an older unit called the Rem. One Sv is equal to one hundred Rems.