The radioisotope carbon-14 is constantly formed from nitrogen-14 (N) in the higher atmosphere by incoming cosmic rays which generate neutrons. These neutrons collide with N to produce C which then combines with oxygen to form CO. This radioactive CO spreads through the lower atmosphere and the oceans where it is absorbed by the plants and the animals that eat the plants. The radioisotope C thus becomes part of the biosphere so that all living organisms contain a certain amount of C. Nuclear testing caused a rapid increase in atmospheric C (see figure), since the explosion of an atomic bomb also creates neutrons which collide again with N and produce C. Since the ban on nuclear testing in 1963, atmospheric C relative concentration is slowly decreasing at a pace of 4% annually. This continuous decrease permits scientists to determine among others the age of deceased people and allows them to study cell activity in tissues. By measuring the amount of C in a population of cells and comparing that to the amount of C in the atmosphere during or after the bomb pulse, scientists can estimate when the cells were created and how often they've turned over since then.