A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product. The rate of decay is proportional to the number of parent atoms present. The proportion of parent to daughter atoms tells us the number of half-lives, which we can use to find the age in years. For example, if there are equal amounts of parent and daughter, then one half-life has passed. If there is three times as much daughter as parent, then two half-lives have passed.
Radioactive decay occurs by releasing particles and energy.
Uranium decays producing subatomic particles, energy, and lead. Highly penetrating electromagnetic radiation. No charge or mass. Can penetrate concrete. Lead shield can be used.