The half-life of an element is the time necessary for one half of a given amount of the isotope to decay. Half-lives can range from billions of years to fractions of a second. The rate at which atomic decay occurs in a particular isotope cannot be altered by any outside force such as temperature, pressure, or chemical reaction. The knowledge of the half-life of a particular isotope is essential to the proper handling of the substance for the protection of the medical staff and the patient who is receiving some form of radiation therapy.