Radiocarbon Dating

　　Nowadays scientists can answer many questions about the past through a technique called radiocarbon (放射性碳), or carbon-14, dating. One key to understanding how and by something happened is to discover when it happened.

　　Radiocarbon dating was developed in the late 1940s by physicist Willard F. Libby at he University of Chicago. An atom of ordinary carbon, called carbon-12, has six protons(中子) and six neutrons (質(zhì)子) in its nucleus. Carbon-14, or C-14, is a radioactive, unstable form of carbon that has two extra neutrons (原子核). It returns to a more stable form of carbon through a process called decay (衰減). This process involves the loss of he extra neutrons and energy from the nucleus.

　　In Libby's radiocarbon dating technique, the weak radioactive emissions (放射) from his decay process are counted by instruments such as a radiation detector and counter. he decay rate is used to determine the proportion of C-14 atoms in the sample being dated.

　　Carbon-14 is produced in the Earth's atmosphere when nitrogen (氮)-14, or N-14,

　　interacts with cosmic rays (宇宙射線). Scientists believe since the Earth was formed, the mount of nitrogen in the atmosphere has remained constant. Consequently, C-14 formation is thought to occur at a constant rate. Now the ratio of C-14 to other carbon toms in the atmosphere is known. Most scientists agree that this ratio is useful for dating items back to at least 50,000 years.

　　All life on Earth is made of organic molecules (分子) that contain carbon atoms coming from the atmosphere. So all living things have about the same ratio of C-14

　　atoms to other carbon atoms in their tissues (組織). Once an organism (有機(jī)體) dies it tops taking in carbon in any form, and the C-14 already present begins to decay. Over time the amount of C-14 in the material decreases, and the ratio of C-14 to other carbon toms goes down. In terms of radiocarbon dating, the fewer C-14 atoms in a sample, the older that sample is.

　　16　Nowadays many scientists depend on radiocarbon for dating age-old objects

　　A、Right、B　Wrong、C、Not mentioned、

　　17　The radiocarbon dating technique is only about 40 years old

　　A、Right、B　Wrong、C、Not mentioned

　　18　An atom of ordinary carbon has six protons and eight neutrons

　　A、Right、B　Wrong、C、Not mentioned

　　19　Radar is used to determine the characteristics of radiocarbon

　　A、Right、B　Wrong、C、Not mentioned

　　20　Radiocarbon is reliable in dating an object back to at least 50,000 years.

　　A、Right、B　Wrong、C、Not mentioned

　　21　The C-14 in an organism begins to decay when it dies

　　A、Right、B　Wrong、C、Not mentioned

　　22　The half-life of C-14 is about 25,000 years.

　　A、Right、B　Wrong、C、Not mentioned

　　參考答案：

　　16 A　　17 B　　18 B　　19 C　　20 A

　　21 A　　22 C