The amount of energy, or the speed of a radiation beam is called the intensity. Intensity determines how deeply it will penetrate into something, and how much ionization it will cause. In determining the potential to cause harm to humans and animals, we also need to look at the idea of dose and dose-rate. These factors not only take into consideration the radiation source's intensity, but the rate at which the radiation is being delivered and the amount of time a living organism is exposed to it. Terms such as Rads, REMs, and Roentgens are used by scientists and technicians to measure the amount of radiation exposure, and to help predict potential health effects and plan control measures.
Radioactive materials are unusual.
They gradually become less radioactive over time. This feature is described by the concept of half-life. No, it doesn't rot, like an old apple. It instead follows what mathematicians call a decay curve, which shows how much of the material is left after a given time period. Radon, for example, has a half-life of about four days.