第2部分：阅读判断(第16～22题，每题1分，共7分)0 }& d9 @2 c8 h) o6 E
下面的短文后列出了7个句子，请根据短文的内容对每个句子做出判断：如果该句提供的是正确信息，请选择A；如果该句提供的是错误信息，请选择B；如果该句的信息文中没有提及，请选择C。
3 \6 i' W+ F; ]( [7 KRadiocarbon Dating
4 O" P/ }1 C+ e, ?( I- T9 O5 H Nowadays scientists can answer many questions about the past through a technique+ @* c4 B, w9 { d7 K, I
called radiocarbon(放射性碳)．or carbon-14，dating One key to understanding how and' j/ w9 S0 P" [- m) }
why something happened is to discover when it happened9 B' B/ Z' Z: _. g$ F9 R
Radiocarbon dating was developedinthelatel940s by physicist Willard F Libby at
/ p7 d2 e9 w( w* E. r6 I7 Ithe University of Chicago An atom of ordinary carbon called carbon-12 has six protons
2 k3 x8 ^4 S0 ^! k9 q8 `/ L0 D (质子)and six neutrons(中子)in its nucleus(原子核)Carbon-14，or C-14，is a radioactive unstable form of carbon that has two extra neutrons .It returns to e more stable form of carbon through a process called decay(衰减)This process involves the loss of the extra neutrons and energy from the nucleus
t3 Y9 o) q0 t& B- u: p 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 of C-14 atoms in the sample being dated
% G) \; O4 O/ O# B; k! M Carbon-14 is produced in the Earth’s atmosphere when nitrogen(氨)-14．or N-14，interacts with cosmic rays(宇宙射线)Scientists believe since the Earth was formed，the amount 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 atoms in the atmosphere is known Most scientists agree that this ratio is useful for dating items back to at least 50．000 years
# L [0 U" i, n5 J( t" f. q 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(组织)Once an organism(有机体)dies it stops 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 atoms goes down In terms of radiocarbon dating．the fewer C-14 atoms in a sample，the older that sample is. |
发表于 2012-8-16 00:07:50
