15.10放射性测年法
Section outline
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Can wood give an age in another way?
::木头能以另一种方式给一个年龄吗?You may have heard of tree ring dating, in which scientists can determine how old a tree is based on the patterns of rings within the wood. This wooden ladder from Mesa Verde National Park in Colorado can also be dated by radiometric dating. Carbon-14 dating is very useful for archaeologists studying ruins that contain wood. If the tree died around the time the ladder was created, then carbon-14 can tell the age of the ladder. If the ladder was built by the people who lived there, then that age of the ladder will tell archaeologists when the site was inhabited.
::你可能听说过树环约会, 科学家们可以在其中根据木质环的形态来决定一棵树有多老。 来自科罗拉多州梅萨维德国家公园的木质梯子也可以通过辐射计数约会来注明日期。 碳-14约会对于考古学家研究含有木材的废墟非常有用。 如果树在梯子创建时死亡, 那么碳-14可以辨别梯子的年代。 如果梯子是由住在那里的人建造的, 那么梯子的年龄会告诉考古学家, 当该地被居住的时候。Radiometric Dating
::辐射测量日期The rate of decay of unstable isotopes can be used to estimate the absolute ages of fossils and . This type of dating is called radiometric dating .
::不稳定同位素的衰减速度可以用来估计化石的绝对年限。这种约会称为辐射计约会。Carbon-14 Dating
::碳-14 碳-14 日期The best-known method of radiometric dating is carbon-14 dating . This method is also called radiocarbon dating. Carbon-14 is formed from cosmic rays in the atmosphere . A living thing takes in carbon-14, along with stable carbon-12, through photosynthesis and the food web . As the carbon-14 decays, it is replaced with more carbon-14. After the organism dies, it stops taking in new carbon. The carbon-14 that is in its body continues to decay into nitrogen-14, which is lost to the atmosphere. So the organism contains less and less carbon-14 as time goes on, while the stable carbon-12 remains in place. We know the ratio of carbon-14 to carbon-12 in a living organism, and we know how fast carbon-14 decays. Therefore, we can use the ratio of remaining carbon-14 to tell how long ago the organism died.
::最著名的辐射计算方法是碳-14 约会。 这个方法也被称为放射性碳约会。 碳-14 由大气中的宇宙射线组成。 一个活物通过光合作用和食物网吸收碳-14, 与稳定的碳-12一起吸收碳-14 。 随着碳-14的衰减, 它被更多的碳-14所取代。 生物体死后, 它不再吸收新的碳。 体内的碳-14 继续腐蚀成氮-14, 大气中失去了这种碳-14。 因此, 生物体随着时间的流逝, 碳-14中含有的碳-14越来越少, 而稳定的碳-12仍然存在。 我们知道一个活体中的碳-14与碳-12的比率, 我们知道碳-14的衰减速度有多快。 因此, 我们可以使用剩余碳-14的比率来判断生物体的死期有多长。Carbon-14 has a relatively short half-life . It decays quickly compared to some other unstable isotopes. So carbon-14 dating is useful for specimens younger than 50,000 years old. That’s a blink of an eye in geologic time. But radiocarbon dating is very useful for more recent events. One important use of radiocarbon is early human sites. Carbon-14 dating is also limited to the remains of once-living things. To date rocks, scientists use other radioactive isotopes.
::碳14的半衰期相对较短。 与其他不稳定同位素相比,碳14的衰减速度很快。 因此,碳14的约会对5万年前的样本很有用。 这是地质学时代的一眨眼。 但放射性碳的约会对于近期的事件非常有用。 早期的人类场所是放射性碳的重要用途之一。碳14的约会也仅限于曾经存在过的残余物。 截止到今天,科学家们使用其他放射性同位素。Other Radioactive Isotopes
::其他放射性同位素The isotopes listed below ( Table ) are used to date . These isotopes have much longer half-lives than carbon-14. Because they decay more slowly, they can be used to date much older specimens. Which of these isotopes could be used to date a rock that formed half a million years ago? How about half a billion years ago?
::以下所列同位素(表)是迄今为止使用的。这些同位素的半衰期比碳14的半衰期长得多。由于这些同位素的衰减速度较慢,它们可以用来进行老得多的标本。这些同位素中的哪一个可以用来进行50万年前形成的岩石的采样?大约5亿年前呢?Unstable Isotope Decays to At a Half-Life of (years) Dates Rocks Aged (years old) Potassium-40 Argon-40 1.3 billion 100 thousand – 1 billion Uranium-235 Lead-207 700 million 1 million – 4.5 billion Uranium-238 Lead-206 4.5 billion 1 million – 4.5 billion Limitations of Radiometric Dating
::辐射测量定时限制Radiometric dating is a very useful tool, but it does have limits:
::辐射测量约会是一个非常有用的工具,但有限度:- The material being dated must have measurable amounts of both the parent and the daughter isotopes.
::注明日期的材料必须具有母同位素和女同位素的可测量量。
- Radiometric dating can only be done on some materials. It is not useful for determining the age of . For this, geologists date a nearby igneous rock . Then they use relative dating techniques to figure out the age of the sedimentary rock . They may not get it exactly, but there will be some idea.
::只能在某些材料上进行辐射测距。 这对于确定 . 年龄没有用处。 为此, 地质学家会与附近一个皮质岩石约会。 然后他们使用相对的约会技术来确定沉积岩的年代。 他们可能不会完全理解, 但是会有一些想法 。
Summary
::摘要- Radiocarbon is useful for relatively young, carbon-based materials. Other longer-lived isotopes are good for older rocks and minerals.
::放射性碳对较年轻的碳基材料有用,其他寿命较长的同位素对老岩石和矿物质也有好处。
- Different isotope pairs are useful for certain materials of certain ages.
::不同的同位素配对对某些年代的某些材料有用。
- Radiometric dating cannot be used if parent or daughter are not measurable.
::如果父母或女儿无法衡量,则无法使用辐射计约会。
Review
::回顾- How would you determine which isotope pair to use to date a certain rock?
::您如何确定使用哪种同位素配对 来与某个岩石约会?
- How does radiocarbon dating work? What materials does it work best on?
::放射性碳的约会如何运作?什么材料最有效?
- Which types of rocks are best for radiometric dating? Which are not good?
::哪种类型的岩石最适合辐射计约会?
- If you dated a mineral from inside an igneous rock, what would that date indicate? How about from a sedimentary rock?
::如果你从一个有色岩石内部 与矿物质约会,那日期会显示什么?从沉积岩中如何?
Explore More
::探索更多Use the resource below to answer the questions that follow.
::利用以下资源回答以下问题。- What do scientists hope to answer with radiometric dating?
::科学家们希望用辐射测量日期回答什么?
- Is the whole rock, in this case graywacke, dated?
::整块石头,在这个案子里,灰色瓦克,是过时的吗?
- How is the rock crushed? How is it powdered?
::岩石是怎么粉碎的?
- Who explained radioactivity? What was his explanation?
::谁解释了辐射?他的解释是什么?
- What is a mass spectrometer? How does it work?
::什么是质量光谱仪?
- What is the spectrometer separating in this rock?
::这块岩石的光谱仪是分什么的?
- How will scientists use this information to date the rock? What else do they need to know before they can get a date?
::科学家们如何利用这一信息找到岩石呢?他们还需要知道什么才能找到日期?
- Why is the dating of rocks important?
::为什么石头的约会很重要?
- The material being dated must have measurable amounts of both the parent and the daughter isotopes.
