radioactivity
Radioactivity is quantified by measuring the number of decay processes per unit time. For example, we can measure radioactivity in terms of counts per minute cpm , where each "count" is a single decay process, such as the emission of an -particle. A sample of one particular isotope may have an activity of 5,000 cpm, while an equal amount of another isotope might result in a radiation level of only 250 cpm. For a given nucleus, the amount of radioactivity gives a rough indication of the amount of the radioisotope present the higher the activity, the more of the radioactive isotope in the sample.