Scientists can determine the age of ancient objects by the method of radiocarbon dating. The bombardment of the upper atmosphere by cosmic rays converts nitrogen to a radioactive isotope of carbon, , with a half-life of about years. Vegetation absorbs carbon dioxide through the atmosphere and animal life assimilates through food chains. When a plant or animal dies, it stops replacing its carbon and the amount of begins to decrease through radioactive decay. Therefore the level of radioactivity must also decay exponentially. A discovery revealed a parchment fragment that had about as much radioactivity as does plant material on the earth today. Estimate the age of the parchment.