万物简史 第217期:把铅撵出去(6)

 　　Up to this time, the oldest reliable dates went back no further than the First Dynasty in Egypt from about 3000B.C. No one could confidently say, for instance, when the last ice sheets had retreated or at what time in the past the Cro-Magnon people had decorated the caves of Lascaux in France.

　　到这个时候，可靠的年代最远只达埃及的第一王朝──公元前3000年左右。例如，谁也没有把握说出，最后一批冰盖是在什么时候退缩的，法国的克罗马农人是在过去什么时候装饰拉斯科山洞的。

　　铅的危害

　　Libby's idea was so useful that he would be awarded a Nobel Prize for it in 1960. It was based on the realization¹ that all living things have within them an isotope² of carbon called carbon-14, which begins to decay at a measurable rate the instant they die. Carbon-14 has a half-life—that is, the time it takes for half of any sample to disappear1—of about 5,600 years, so by working out how much a given sample of carbon had decayed, Libby could get a good fix on the age of an object—though only up to a point. After eight half-lives, only 1/256 of the original radioactive carbon remains³, which is too little to make a reliable measurement, so radiocarbon dating works only for objects up to forty thousand or so years old.

　　利比的方法用途很广，他因此获得了1960年的诺贝尔奖。这种方法基于一种认识：生物内部都有一种碳的同位素──名叫碳－14，生物一死，该同位素马上以可以测定的速度开始衰变。碳－14大约有5600年的半衰期──即任何样品消失一半所需的时间──因此，通过确定某种特定的碳样的衰变程度，利比就可以有效地锁定一个物体的年代──虽然是在一定限度以内。经过八个半衰期以后，原先的放射性碳只剩下0.39%。这个量太小，无法进行可靠的测算，因此碳－14年代测定法只适用于年代不超过4万年左右的物体。

　　Curiously⁴, just as the technique was becoming widespread, certain flaws within it became apparent. To begin with, it was discovered that one of the basic components⁵ of Libby's formula, known as the decay constant, was off by about 3 percent. By this time, however, thousands of measurements had been taken throughout the world.

　　有意思的是，随着这项技术的广泛使用，有些疵点也日渐显露出来。首先，人们发现，利比公式里有个名叫衰变常数的基本成分存在3%的误差。而到了这个时候，全世界已经进行了数千次计算。

1 realization   

n.实现；认识到，深刻了解

参考例句：

• We shall gladly lend every effort in our power toward its realization.我们将乐意为它的实现而竭尽全力。
• He came to the realization that he would never make a good teacher.他逐渐认识到自己永远不会成为好老师。

2 isotope   

n.同位素

参考例句：

• The isotope ratio is directly used for comparing oils or gases.同位素比率直接用于比较各种石油或天然气。
• How to apply a radio isotope?如何运用放射性同位素？

3 remains   

n.剩余物，残留物；遗体，遗迹

参考例句：

• He ate the remains of food hungrily.他狼吞虎咽地吃剩余的食物。
• The remains of the meal were fed to the dog.残羹剩饭喂狗了。

4 curiously   

adv.有求知欲地；好问地；奇特地

参考例句：

• He looked curiously at the people.他好奇地看着那些人。
• He took long stealthy strides. His hands were curiously cold.他迈着悄没声息的大步。他的双手出奇地冷。

5 components   

（机器、设备等的）构成要素，零件，成分； 成分( component的名词复数 )； [物理化学]组分； [数学]分量； （混合物的）组成部分

参考例句：

• the components of a machine 机器部件
• Our chemistry teacher often reduces a compound to its components in lab. 在实验室中化学老师常把化合物分解为各种成分。