职称英语考试

【单选题】RadiocarbonDating　　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 ofChicago.An atom of ordinary carbon, called carbon-12, has six protons(质子) and six neutrons (中子) in its nucleus(原子核).Carbon-14, orC-14, is a radioactive, unstable form of carbon that has two extra neutrons. It returns to a more stable form of carbon through a process called decay (衰减). 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 this decay process are counted by instruments such as a radiation detector and counter. The decay rate is used to determine the proportion ofC-14 atoms in the sample being dateD、　　Carbon-14 is produced in theEarth’s atmosphere when nitrogen (氮)-14, or N-14,interacts with cosmic rays (宇宙射线). Scientists believe since theEarth 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 ofC-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 onEarth is made of organic molecules (分子) that contain carbon atoms coming from the atmosphere. So all living things have about the same ratio ofC-14 atoms to other carbon atoms in their tissues (组织). Once an organism (有机体) dies it stops taking in carbon in any form, and theC-14 already present begins to decay. Over time the amount ofC-14 in the material decreases, and the ratio ofC-14 to other carbon atoms goes down. In terms of radiocarbon dating, the fewerC-14 atoms in a sample, the older that sample is. When an organism dies, theC-14 in it begins to decay.
A、Right
B．Wrong
C．Not mentioned

网考网参考答案：A
网考网解析：
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