5.2 辐射测量日期

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  辐射测量日期

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  Radioactive Decay
  ::放射性衰减

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  The different ages of these rocks is determined by a process known as radiometric dating. First  described  in 1907 by Bertram Boltwood, this method is now widely used for dating specimens throughout geology and   uses  known properties of atomic physics. All the baryonic matter that we interact with every day is made up of protons, neutrons and electrons. Protons and neutron are made up of     and have three "valence" quarks (  below).   Quarks   are elementary particles and, as such, cannot be broken down any further. They possess intrinsic properties (some of which include charge and mass) and transfer these properties to the hadrons they make up.   Hadrons   is simply the term for something composed of quarks. Hadrons come in two types: mesons, which consist of a quark and an anti-quark, and our familiar   baryons, which consist of three quarks.  Quarks are studied by analyzing the way hadrons interact. As far as we have been able to tell, the electron is an elementary particle itself that cannot be broken down any further.
  ::这些岩石的不同年代是由一个称为辐射测算日期的过程决定的。 1907年Bertram Boltwood首先描述了这个方法,它现在被广泛用于在地质学中对标本进行约会,并使用了已知的原子物理学特性。 我们每天互动的所有交斗物质都由质子、中子和电子组成。质子和中子由三个“valence”夸克组成(下面) 。 夸克是基本粒子, 因而无法再细分。 它们拥有内在特性( 有些包括充电和质量) , 并将这些特性转移到它们构成的粒子上。 哈得伦只是由夸克构成的东西的术语。 哈得伦有两种类型: 由夸克和反夸克组成的米森, 以及由三个“ valence” 夸克组成的我们熟悉的巴因子。 夸克通过分析原子相互作用的方式来研究。 据我们所知, 电是基本粒子本身无法进一步打破的。

  

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  The quarks that make up protons and neutrons. True to their baryon form, protons and neutrons are made of quarks. The outer valence quarks are draw here as blue, red, and green spheres. The u and d stand for up and down respectively, which describe the spin of each quark. Spin is one of the intrinsic properties that describe quarks. What would have to change for a neutron (left) to change into a proton (right)? How about for a neutron to change into a proton?
  ::组成质子和中子的 夸克。 符合质子和中子的质子形式, 质子和中子由 夸克组成。 外值夸克在这里以蓝色、 红色和绿色的面积划线。 u 和 d 分别代表上下, 描述每个质子的旋转。 旋转是夸克的内在特性之一。 要将中子( 左) 改变成质子( 右) , 那么中子变成质子呢 ? 中子变成质子呢 ?

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  Recall that atoms can exist as several different isotopes, which contain different numbers of neutrons in their nucleus. Not all nuclei are stable. Generally heavier isotopes with an unbalanced  number of neutrons relative to protons will undergo radioactive decay.  For example, all carbon atoms have 6 protons, but additional neutrons are possible: carbon-12 and carbon-13 are  stable isotopes, but  carbon-14 is  an unstable isotope. The unstable isotopes (here, carbon-14) are the   parent isotope   and  they spontaneously decay into a different element or isotope, known as the   daughter isotope .
  ::回顾原子可以作为若干不同的同位素存在,这些同位素在核中含有不同数量的中子。并非所有核都是稳定的。一般而言,相对质子而言,中子数量不平衡的较重同位素将经历放射性衰减。例如,所有碳原子有6个质子,但还可能存在额外的中子:碳-12和碳-13是稳定的同位素,但碳-14是一种不稳定的同位素。不稳定的同位素(这里是碳-14)是母同位素,它们自发衰落成一个不同的元素或同位素,称为子同位素。

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  There are two different types of statistically predictable spontaneous decay.  The first  is known as   alpha decay (  below ), so named because the process emits an alpha particle  (two protons and two neutrons). Alpha decay can only occur with very large nuclei. The parent isotope is left with  a reduction of four in atomic mass. The loss of two protons means that the parent isotope has been converted to a lighter element in the Periodic Table.  
  ::在统计上可以预测的自发衰变有两种不同类型。第一类称为阿尔法衰变(下文),其名称是因过程释放了阿尔法粒子(两个质子和两个中子)。阿尔法衰变只能以非常大的核发生。母同位素的原子质量减少4个质子。损失两个质子意味着母同位素在周期表中转换为较轻元素。

   

  

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  A Uranium-238 nucleus undergoing alpha decay. The uranium nucleus loses an alpha particle, or two protons and two neutrons. This is often referred to as a helium nucleus, which typically has those exact proportions. Because both protons and neutrons were loss, the uranium atom changes into a thorium atom after the decay event.
  ::铀-238核正经历α衰变。 铀核会失去一个α粒子, 或两个质子和两个中子。 这通常被称为核, 通常具有这样的精确比例。 因为质子和中子都是流失的, 铀原子会在衰变事件发生后变成原子。

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  A  second type of spontaneous decay is   beta decay . The atomic mass (total number of protons + neutrons) remains the same, but the atomic number (number of protons) changes.  A proton or neutron may change into the other by flipping the  charge of one quark. These changes are possible because protons and neutrons are not elementary particles. With  $\begin{align*}{\beta }^{-}\end{align*}$  decay, a neutron decays into a proton plus an electron (to maintain charge balance) and an electron antineutrino to carry away energy. This  changes  the atom to a heavier element (plus one proton). An example of   $\begin{align*}{\beta }^{-}\end{align*}$  decay  is the conversion of ¹⁴ C (6 protons) to ¹⁴ N (7 protons): 
  ::第二种自发衰变是乙型衰变。 原子质量( 质子总数 + 中子) 保持不变, 但原子数( 质子数量) 变化。 质子或中子可能会通过翻转一个夸克的电荷而变成另一个质子或中子。 这些变化是可能的, 因为质子和中子不是基本粒子。 随着 衰变, 中子会衰变成质子加上电子( 以保持电荷平衡) , 电子反中微子可以携带能量 。 这将改变原子成一个较重元素( 质子数量 ) 。 衰变的例子就是14C ( 6 质子) 转换为 14N ( 7 质子 ) :

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                       $\begin{align*}^{14}_{6}C \rightarrow ^{14}_{7}N + e^- +\nu_-\end{align*}$  
  ::614C714N+e

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  With  $\begin{align*}{\beta }^{+}\end{align*}$  decay, the proton becomes a neutron, absorbing an electron, and the atom is changed to a lighter element (minus one proton) . And example of   $\begin{align*}{\beta }^{+}\end{align*}$  decay is conversion of magnesium (12 protons) to sodium (11 protons): 
  ::随着的衰变,质子变成了中子,吸收了电子,原子变成了较轻元素(减一个质子 ) 。 的衰变实例是镁(12个质子)转化为钠(11个质子 ) :

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                      $\begin{align*}^{23}_{12}Mg \rightarrow ^{23}_{11}Na + e^+ + \nu_e\end{align*}$   
  ::1223Mg1123Na+ee

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  It is impossible to say for sure when a decay event will happen, but we can categorize the rate at which a sample of atoms will decay. If we know the amount of parent and daughter isotope, then knowing the rate of decay allows us to solve for how much time it must have taken for the parent isotope to decay into that much daughter isotope. This rate is  characterized by  the   half-life , or the amount of time it takes for half of the parent isotope to decay into the daughter isotope.  Different elements are useful for  dating different age ranges. The Table below lists information for common isotopes. 
  ::无法肯定何时会发生衰变事件, 但我们可以对原子样本衰变的速度进行分类。 如果我们知道母子同位素的数量, 那么知道衰变的速度可以让我们解决母子同位素衰变到女子同位素中需要多少时间。 这个速度的特征是半衰期, 或母子同位素衰变到女子同位素中需要多少时间。 不同的元素对约会不同年龄范围有用。 下面的表格列出了普通同位素的信息 。

   

  

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  Radioactive half lives of key isotopes. Which of these would be most useful for dating meteorites to learn about the age of the solar system? Which would be most useful for forensic evidence about
  ::关键同位素的放射性半衰期主要同位素的放射性半衰期。

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  Half life
  ::半半衰期

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  Imagine a bag of microwave popcorn kernels. We can think of the popcorn kernels as unstable parent isotopes. The process of popping will symbolically represent spontaneous radioactive decay and the resulting popped popcorn will be  the daughter isotopes. Though it is impossible to say when a specific kernel of popcorn is going to pop, we know how long it takes for most of the kernels to pop. There exists a relationship between number of popped kernels and amount of time passed, known as the  If we know the rate at which an isotope decays, we can calculate  the age of a specimen  given the fraction of parent and daughter isotopes in the sample.
  ::想象一下一袋微波爆米花内核。 我们可以把爆米花内核看作是不稳定的母同位素。 弹出过程将象征性地代表自发的放射性衰减, 由此产生的爆爆爆爆将代表女儿的同位素。 虽然不可能说爆米花内核何时会爆出, 但是我们知道大部分的内核要弹出多久。 爆出内核的数量和经过的时间数量之间存在某种关系, 被称为“ 如果我们知道同位素衰减的速度, 我们可以根据样本中母和女同位素的分数来计算标本的年限 。

  

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  The exponential decay of radioactive isotopes. Time here is represented by number of half-lives. Note how with each half-life, the amount of parent isotope (red) is halved. How much of the parent isotope is left after two half-lives?
  ::放射性同位素的指数衰减。 这里用半衰期数表示时间。 注意每个半衰期, 母同位素( 红) 的数量如何减半 。 母同位素( 红) 在两个半衰期后还剩下多少 ?

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  Rates of radioactive decay can be determined in a laboratory setting. It is known that radioactive decay is an exponential process given by:
  ::放射性衰变速度可在实验室环境中确定。

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  $\begin{align*}N(t)=N(t=0)e^{\frac{-t}{t_{mean}}}\end{align*}$  
  ::N(t) = N(t=0) e- 中文本

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  • where   $\begin{align*}N(t)\end{align*}$  is the amount of the parent isotope that remains,
    ::N(t)是剩余的母同位素含量,
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  • $\begin{align*}N(t=0)\end{align*}$ , or sometime also written   $\begin{align*}N_0\end{align*}$ ,  is the initial amount the parent isotope that a sample started out with,
    ::N(t=0) 或有时也写N0, 是样本开始时的母同位素初始量,
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  • $\begin{align*}t\end{align*}$  is the amount of time that has passed,
    ::t 是过去时间的长度,
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  • and  $\begin{align*}{t}_{mean}\end{align*}$  =   $\begin{align*}\sqrt{2} \times t_{half}\end{align*}$   where  $\begin{align*}{t}_{half}\end{align*}$  is the half-life of the element in question.
    ::值=2×2.5,其中半值为该元素的半衰期。

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  Solving for t, we get an equation for the amount of time that has passed:
  ::解决 t,我们得到一个方程式 时间的长度已经过去:

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  $\begin{align*}t=-ln\left(\frac{N}{N_0}\right)\times \sqrt{2}\times t_{half}\end{align*}$  
  ::tn( NN0) ×2x半

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  As an example, let us imagine a classroom of 150 students who exhibit some very radioactive behavior. At the beginning of class, all 150 students are awake and attentive. Though we certainly hope this is never the case, suppose the students are falling asleep at an exponential rate,  similar to the way that radioactive isotopes decay. After thirty minutes, half of the students have already fallen asleep! How much time has passed when only 30 students, or 20%, remain awake?
  ::举个例子,让我们想象一个有150名学生的教室,他们表现出非常非常的放射性行为。 在课堂开始时,所有150名学生都醒着和专心。 虽然我们当然希望情况并非如此,但假设学生正在以指数速度入睡,这与放射性同位素衰减的方式相似。 30分钟后,一半学生已经睡着了!只有30名学生(20%)保持清醒,时间已经过去了多少?

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  Using the equation above for time, the number of "surviving" (i.e., awake - no students were harmed in this thought experiment) 30 students is  $\begin{align*}N\end{align*}$ , the initial 150 awake students is   $\begin{align*}N_0\end{align*}$ , and 30 minutes was the half-life,   $\begin{align*}t_{half}\end{align*}$ . Because the half-life is in units of minutes, the answer will also be in minutes.
  ::用上面的方程式来计算时间,“生存”的数量(即醒着的-在这个思考实验中没有学生受到伤害)是N,30名学生是N,最初150名清醒的学生是N0,半衰期是30分钟。由于半衰期以分钟为单位,答案也将是分钟。

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  $\begin{align*}t=-ln\left(\frac{30}{150}\right)\times \sqrt{2}\times 30 \approx 68\text{ minutes} \end{align*}$  
  ::================================================================================================================================68====================================================================================================== =====================================================================================================================================================================================================================================================================================

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  An hour and 8 minutes into the class, only 30 students remain awake in this completely hypothetical classroom.
  ::只有30名学生在这个完全假设的教室里醒着。

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  In the example above was a simple case of misbehaving students. When   dating rocks ,  the use of many  different radioactive isotopes gives even more information about the age of a specimen . The choice of isotopes depends largely on what is present in the rock sample and  a sensible choice, given the relative half-lives of the different isotopes .  Half-lives   can   range  from fractions of a second to billions of billions of years. Elements with longer half-lives are more useful for dating older rocks. Isotopes with half-lives  comparable to the age of the substance being dated are  ideal .
  ::在以上的例子中,学生行为不端是一个简单的例子。当与岩石约会时,许多不同的放射性同位素的使用更能说明标本的年代。同位素的选择在很大程度上取决于岩石样本中的成分和合理选择,考虑到不同同位素的相对半衰期。半衰期可以从秒到数十亿年的分数不等。半衰期较长的元素对于与老岩石约会更为有用。半衰期与该物质的成熟期相似的半衰期的异形是理想的。

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  Atomic elements can also be changed by fission, which splits massive atomic elements into less massive elements. Spontaneous fission releases substantial amounts of energy.  Elements can also be changed by fusion of lighter elements to form heavier elements.  As discussed before, this process takes place in the cores of stars where hydrogen undergoes nuclear fusion to form helium. This process requires the input of a substantial amount of energy. 
  ::原子元素也可以因裂变而改变,裂变将大规模原子元素分裂为较不大规模元素,自发性裂变释放了大量能量,也可以通过将较轻元素聚在一起形成较重元素而改变元素。如前所述,这一过程发生在氢进行核聚变形成氦的恒星核心中。这一过程需要大量能量投入。

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  Statistical uncertainty 
  ::统计不确定性统计不确定性

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  The accuracy of radiometric dating  can be  hard to ensure because the method  depends on knowing both how much of the parent isotope was initially present, and how much of the daughter product is the result of decay. It is possible that the daughter isotope will preferentially escape from a sample, or a contaminating source will add more of either the parent or the daughter isotope. Returning back to the fictitious classroom   example , this would   happen if different students left and entered the room during the class. Then, someone observing the room an hour  after the start of class   would be uncertain about how many students were initially in the room.
  ::辐射计日期的准确性可能很难确保,因为这种方法取决于知道母同位素最初存在多少,以及女儿产物有多少是衰变的结果。 女儿同位素有可能优先从样本中逃脱,或者污染源会增加更多的母同位素或女同位素。 回到虚构的课堂例子,如果不同的学生在课堂上离开并进入教室,就会发生这种情况。 然后,在上课开始后一小时观察房间的人将不确定最初有多少学生在教室里。

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  There are ways to  improve accuracy. For example multiple samples can be analyzed from different locations in  the same rock in case one area suffered contamination. It is also helpful to calculate the age using several different isotopes to  check for consistent  results. This  offers some insurance against potential loss of daughter isotopes since contamination or loss of daughter isotopes should behave differently. Counting accuracy is improved when there is a relatively high concentration of both the parent and daughter isotope. 
  ::有一些方法可以提高准确性。 例如,在同一个岩石的不同地点,如果一个地区受到污染,可以从同一岩石的不同地点分析多个样本。 计算使用几种不同同位素的年限以检查一致结果也是有益的。 这为可能丢失女儿同位素提供了某种保险,因为女儿同位素的污染或损失行为应该不同。 当父母和女儿同位素的相对高度集中时,计算准确性会提高。

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  Even with   the best  laboratory  practices , radiometric dating depends inherently on the type of rock. Rocks are classified into three groups.   Igneous rocks are made from molten magma or lava that solidifies into rock.   Sedimentary rocks are layered rocks formed when sand and silt collect on the surface or in bodies of water and cement together to form new rock.   Metamorphic rocks form when rocks undergoes intense temperature and/or pressure  and transform into a different type of rock altogether. Through various processes, , as depicted in the   below.
  ::即便采用最佳实验室实践,辐射测定日期的确定也必然取决于岩石的类型。岩石被分为三类:岩浆岩浆或熔岩熔化成岩石的熔岩或熔岩,沉积岩是沙子和淤泥在地表或水体中收集时形成的层状岩石,水泥合在一起形成新岩块。岩石经历高温和/或压力并完全转化为不同类型岩石时,形态形形岩石形成形态。如下文所述,通过各种过程,通过以下所述的各种过程,形成层状岩石。

  

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  The rock cycle. Outlined here are the different ways one classification of rock can transform into another. The rocks on this Earth are constantly changing and shifting through these different processes
  ::岩石循环。 这里概括了一种岩石分类可以变换到另一种岩石的不同方式。 地球上的岩石正在不断的变化和变化 通过这些不同的过程

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  Sedimentary rocks and metamorphic rocks are not  good for  radiometric dating. Sedimentary rock is made up of a conglomeration of the particles eroded from different types and ages of rock . M etamorphic rock undergoes too much change. Radiometric dating is only secure for igneous rocks that remain stable. Even so, radiometric dating of igneous rocks  gives only the  time since they last melted. Radiometric data has helped date rocks that are billions of years old going back almost to  almost 4 Gya.  The age of the Earth can be determined by radiometric dating of meteorites, the unprocessed specimens of planet formation. 
  ::沉积岩石和变形岩石对于辐射计程不合适。沉积岩石是由不同类型和不同年代的岩石侵蚀的粒子聚集在一起构成的。变形岩石经历了太多的变化。辐射计程只能对保持稳定的有色岩石安全。即使如此,有色岩石的辐射计程也只提供了上次融化以来的时间。辐射计数数据有助于确定几亿年前几乎可以追溯到近4亿亿年前的岩石的日期。地球的年代可以通过陨石的辐射计程来决定,这些陨石是行星形成的未经处理的标本。

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  Zircon crystals as time  capsules
  ::Zircon晶体作为时空胶囊

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  Zircon crystals offer one of the best ways to clock events that happened millions to billions of years ago. When a zircon crystal forms, the lattice structure admits uranium, but  strongly rejects lead, which is the daughter product of radioactive decay.  Therefore, the ratio of lead to uranium is a nearly perfect clock in a zircon crystal. Zircons are nearly indestructible. Other contaminants in the zircon crystal reveal information about the temperature and presence of water at the time the zircon crystal formed. 
  ::锌晶体是记录数以百万计到数十亿年前发生的事件的最佳方法之一。当锌晶体形成时,铁丝网结构接受铀,但强烈拒绝铅,而铅是放射性衰变的产物。因此,铅与铀的比例是Zircon晶体中几乎完美的时钟。锌结晶几乎是不可摧毁的。锌晶体中的其他污染物揭示了Zircon晶体形成时水的温度和存在情况。