Radioactive Dating

12 Key excerpts on "Radioactive Dating"

• 

  eBook - PDF

  Isotope Geology

  • Claude J. Allègre, Christopher Sutcliffe(Authors)
  • 0(Publication Date)
  • Cambridge University Press

    (Publisher)

  CHAPTER TWO The principles of Radioactive Dating It can never be repeated enough that Radioactive Dating was the greatest revolution in the geological sciences. Geology is an historical science which cannot readily be practised withoutaprecisewayofmeasuringtime.Itissafetosaythatnomoderndiscoveryingeology could have been made without Radioactive Dating: reversals of the magnetic ¢eld, plate tec- tonics, the puzzle ofthe extinction ofthe dinosaurs, lunar exploration, the evolution of life, human ancestry, notto mention the age ofthe Earth orofthe Universe! The ages involved in the earth sciences are very varied.They are measured in years (yr), thousands ofyears (ka), millions ofyears (Ma), and billions ofyears (Ga). Geological clocks mustthereforebevaried too, with mean lives ranging from ayear to abillion years. 2.1 Dating by parent isotopes Imagine we have a radioactive isotope R and N  R is the number of atoms of this isotope. Suppose that geological circumstances (crystallization ofarockor mineral, say) enclose an initial quantityof R, i.e., the numberofatoms of R at time zero, written N  R ð0Þ, in a‘‘box.’’ If thebox has remained closed fromwhen it ¢rst formed until today, the numberofatoms of R remaining is N  R ðtÞ ¼ N  R ð0Þe lt , where t is the time elapsed since the box was closed. If we know the quantity N  R ð0Þ and the decay constant l, by measuring N  R ðtÞ we can calculate theage t at whichtheboxclosed by using the radioactivity formula‘‘upside down’’: t ¼ 1 l ln N  R ð0Þ N  R ðtÞ   : Methods where the initial quantities of radioactive isotopes are well enough known are above all those where the radioactive isotope is produced by irradiation by cosmic rays. This is the case ofcarbon-14 ( 14 C)andberyllium-10( 10 Be). Exercise The half-life of 14 C is 5730 years. The 14 C content of the atmosphere is 13.2 disintegrations per minute and per gram (dpm g 1 ) of carbon (initial activity A 0 ).

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  The Earth Through Time

  • Harold L. Levin, David T. King, Jr.(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  In this case, the atomic number has been increased by 1. A third kind of emission in the radioactive decay process is gamma radiation. Gamma rays are invisible electromagnetic waves (like radio waves) that have extremely high frequencies, higher than X‐rays. Why Radioactivity Lets Us Date Ancient Rocks with Confidence In a nutshell, the rate of decay of radioactive isotopes is uniform and is unaffected by changes in pressure, tempera- ture, or the chemical environment. Even in the most hell- ish environment a thousand kilometers underground, where temperatures and pressures are extreme, and over millions of years, radioactive isotopes continue to decay at exactly the same rate. This is the “multi‐million‐year clock” scientists needed to determine the age of rocks. The mechanism works like this: Once a quantity of radioactive material (such as uranium, which is plenti- ful worldwide) has been incorporated into a growing mineral crystal, that fixed quantity will decay at a steady rate, with a known percentage of the atoms decaying in any given increment of time. Each radioactive isotope has its own mode of decay (alpha, beta, or gamma radi- ation, or a combination) and its own unique decay rate. As time passes, the quantity of the original (parent) iso- tope diminishes, and the number of daughter atoms increases, thereby indicating how much time has elapsed since the clock began its timekeeping. The beginning, or “time zero,” for any mineral con- taining radioactive material is the moment when the radioactive parent atoms became part of that mineral. Encased solidly within the crystal, the daughter elements cannot escape. The retention of daughter ele- ments is critical, because, if any escape, it will throw off the count and give an inaccurate age. “Counting” the quantities of parent and daughter isotopes in a rock sample usually is done with a mass spectrometer. This analytic instrument measures the atomic mass of isotopes.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Principles of Radiometric Dating

  • Kunchithapadam Gopalan(Author)
  • 2017(Publication Date)
  • Cambridge University Press

    (Publisher)

  5.1 INTRODUCTION Radioactivity is the spontaneous transformation or decay of a potentially unstable nuclide (a specific proton-neutron combination) into a more stable nuclide (Chapter 2). A decaying (radioactive) nuclide and its product (radiogenic) nuclide are conventionally labelled parent (p) and daughter (d), respectively. If d is radioactive, it decays to another nuclide until a stable daughter isotope is produced. As the decay of a radionuclide is a statistical phenomenon (Chapter 2), it is not possible to predict when exactly it will occur. However, it is experimentally found that in a large collection, p of radioactive nuclides at time t, a constant fraction of it will decay in unit time (Rutherford, 1906). Using the symbols p and d to also represent the concentrations of parent and daughter isotopes, respectively, the rate of decay of the parent is expressed mathematically by the equation, dp dt = −lp (5.1) where, l is the constant of proportionality, and considered the probability of decay of a single nuclide in unit time regardless of its previous history and present circumstances. Called the decay constant, Liquids and gases forget, but rocks remember. J A O’Keefe …use of isotope data for geological purposes – measuring of ages, determining the sources of basalts, or understanding the evolution of the mantle – is no different from similar use of other geological, petrological, geochemical, or geophysical data. In all cases, the link between experimental data and geological interpretation must be continually forged. There will be no cookbooks or black boxes which will permit us to grind out generally valid geological conclusions. Wetherill et al. (1981) Radioactivity and Radiometric Dating 5 * A significant part of this chapter has already been published by the author (Gopalan, 2015).

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Introduction to Physical Geology

  • Charles Fletcher, Dan Gibson, Kevin Ansdell(Authors)
  • 2014(Publication Date)
  • Wiley

    (Publisher)

  There is no point in looking for, say, potassium-40 in a rock that is known to contain little potassium. Another important consideration is that only certain radioisotopes will accurately characterize the time frame of the sample being dated. That is, young samples should be dated with short-lived radioisotopes and old samples with long-lived radio- isotopes. If a very young event is being dated, such as a lava flow approximately 500 to 1,000 years old, it is not appropriate to pick a radioisotope with a half-life of a few billion years—there simply will not be enough daughter isotope to be measured accurately because not enough time has passed. Likewise, if a sample to be dated is sev- eral hundred million years old, such as an ancient granite pluton, it is not appropriate to try to date it using a radioisotope with a half-life of only a few thousand years. There will not be enough parent radio- isotope left to be measured accurately because too much time will have passed and most of the parent radioisotope will have decayed. Isotopic Clocks Scientists use an atom-measuring instrument called a mass spec- trometer to measure the abundance of isotopes in a sample. Infor- mation about the ratio of parent radioisotopes to daughter isotopes in a sample is used to calculate an age based on the amount of time necessary for a given ratio to develop at the specified decay rate. Well over 40 isotopic dating techniques are in use around the world, each based on a different radioisotope. In general, useful iso- topes fall into two categories: primary radioisotopes and cosmogenic radioisotopes. Earth inherited the primary radioisotopes when it accreted dur- ing nebular condensation. (See Chapter 2, “Solar System.”) These ra- dioisotopes have been held in rocks for the entire 4.6 billion years of Earth’s history. They are released as a rock melts, only to be incorpo- rated into new rock when mineral crystallization occurs.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Nuclear and Radiochemistry

  Fundamentals and Applications

  • Jens-Volker Kratz(Author)
  • 2021(Publication Date)
  • Wiley-VCH

    (Publisher)

  19 Dating by Nuclear Methods

  19.1 General Aspect

  The laws of radioactive decay are the basis of geo‐ and cosmochronology by nuclear methods. From the variation of the number of atoms with time due to radioactive decay, time differences can be calculated almost exactly. This possibility was realized quite soon after the elucidation of the natural decay series of uranium and thorium. Rutherford was the first to stress the possibility of determining the age of uranium minerals from the amount of helium formed by radioactive decay. Dating by nuclear methods is applied with great success in many fields of science, but mainly in archeology, geology, mineralogy, and cosmochemistry, and various kinds of “chronometers” are available. Two kinds of dating by nuclear methods can be distinguished:

  • dating by measuring the radioactive decay of cosmogenic radionuclides, such as 3 H or 14 C;
  • dating by measuring the daughter nuclides formed by the decay of primordial mother nuclides (various methods, e.g. K/Ar, Rb/Sr, U/Pb, Th/Pb, Pb/Pb).

  All naturally occurring radionuclides can be used for dating. The timescale of applicability depends on the half‐life. With respect to the accuracy of the results, it is most favorable if the age to be determined and the half‐life t1/2 of the radionuclide are on the same order of magnitude. In general, the lower limit is about 0.1 · t1/2 and the upper limit about 10 · t1/2 . Therefore, the long‐lived mother nuclides of the uranium, thorium, and actinium decay series, and other long‐lived naturally occurring radionuclides such as 40 K and 87 Rb, are most important for application in geology and cosmology.

  The attainment of radioactive equilibria in the decay chains depends on the longest‐lived daughter nuclides in the series. These are 234 U (t1/2  = 2.455 · 105  years), 228 Ra (t1/2  = 5.75 years), and 231 Pa (t1/2  = 3.276 · 104  years) in the uranium (4n + 2), thorium (4n), and actinium (4n + 3) series, respectively. After about 10 half‐lives of these radionuclides, equilibrium is practically established (Sections 7.3 and 7.4

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Geochronology, Dating, and Precambrian Time

  The Beginning of the World as We Know It

  • Britannica Educational Publishing, John P Rafferty(Authors)
  • 2010(Publication Date)
  • Britannica Educational Publishing

    (Publisher)

  Different schemes have been developed to deal with the critical assumptions stated above. In uranium-lead dating, minerals virtually free of initial lead can be isolated and corrections made for the trivial amounts present. In whole rock isochron methods that make use of the rubidiumstrontium or samarium-neodymium decay schemes, a series of rocks or minerals are chosen that can be assumed to have the same age and identical abundances of their initial isotopic ratios. The results are then tested for the internal consistency that can validate the assumptions. In all cases, it is the obligation of the investigator making the determinations to include enough tests to indicate that the absolute age quoted is valid within the limits stated. In other words, it is the obligation of geochronologists to try to prove themselves wrong by including a series of crosschecks in their measurements before they publish a result. Such checks include dating a series of ancient units with closely spaced but known relative ages and replicate analysis of different parts of the same rock body with samples collected at widely spaced localities.

  The importance of internal checks as well as interlaboratory comparisons becomes all the more apparent when one realizes that geochronology laboratories are limited in number. Because of the expensive equipment necessary and the combination of geologic, chemical, and laboratory skills required, geochronology is usually carried out by teams of experts. Most geologists must rely on geochronologists for their results. In turn, the geochronologist relies on the geologist for relative ages.

  THE ORIGIN OF RADIOACTIVE ELEMENTS USED IN DATING

  In order for a radioactive parent-daughter pair to be useful for dating, many criteria must be met. This section examines these criteria and explores the ways in which the reliability of the ages measured can be assessed. Because geologic materials are diverse in their origin and chemical content and datable elements are unequally distributed, each method has its strengths and weaknesses.

  When the elements in the Earth were first created, many radioactive isotopes were present. Of these, only the radioisotopes with extremely long half-lives remain. It should be mentioned in passing that some of the radioisotopes present early in the history of the solar system and now completely extinct have been recorded in meteorites in the form of the elevated abundances of their daughter isotopes. Analysis of such meteorites makes it possible to estimate the time that elapsed between element creation and meteorite formation. Natural elements that are still radioactive today produce daughter products at a very slow rate. Hence, it is easy to date very old minerals but difficult to obtain the age of those formed in the recent geologic past. This follows from the fact that the amount of daughter isotopes present is so small that it is difficult to measure. The difficulty can be overcome to some degree by achieving lower background contamination, by improving instrument sensitivity, and by finding minerals with abundant parent isotopes. Geologic events of the not-too-distant past are more easily dated by using recently formed radioisotopes with short half-lives that produce more daughter products per unit time. Two sources of such isotopes exist. In one case, intermediate isotopes in the uranium or thorium decay chain can become isolated in certain minerals due to differences in chemical properties and, once fixed, can decay to new isotopes, providing a measure of the time elapsed since they were isolated. To understand this, one needs to know that though uranium-238 (238 U) does indeed decay to lead-206 (206

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Cosmochemistry

  • Harry McSween, Jr, Gary Huss(Authors)
  • 2022(Publication Date)
  • Cambridge University Press

    (Publisher)

  In general, the initial number of radio- nuclide atoms is reduced over time by 2 –n , where n is the number of half-lives, and the number of daughter nuclides approaches the initial abundance of 87 Rb asymptotically 194 Radioisotopes as Chronometers as t ! ∞. For most purposes, a radionuclide can be considered to have decayed away after ~10 half-lives, when N 0 has been reduced by 2 –10 (1/1024). Note that the rate of decay of a radionuclide is not constant, but decreases exponentially with time. What is constant is the proportion of the radionuclides that decay during one half-life. This is counter to our normal experience because the decay of an atom does not become more likely as the atom becomes older. Radioactive decay is a quantum- mechanical phenomenon, and the probability of decay of a radioactive nuclide can be accurately described from its basic quantum-mechanical properties. In order for a radiometric date to provide a valid age, the following criteria must be met: 1. There must have been a specific event that homogen- ized the isotopic compositions of parent and daughter element. The homogenous composition of the parent element assures that additions to the amount of daughter isotope reflect only the amount of parent element and the time that has passed. The homogeneous composition of the daughter element is the background above which the accumulation of a radiogenic daughter isotope can be recognized and measured. 2. The isotopic system (e.g. a rock or mineral) must have remained closed, with no loss or gain of parent or daughter element since the event being dated. 3. The value of the decay constant (λ) must have remained constant over the age of the solar system and the galaxy, and it must be accurately known. As we discussed in Chapter 2, this assumption is well founded for conditions relevant to cosmochemistry.

  Sign up to read

  Learn more about book
• 

  Available until 27 Jan |Learn more

  Geochemistry

  Pathways and Processes

  • Harry Y. McSween, Steven M. Richardson, Maria Uhle(Authors)
  • 2003(Publication Date)
  • Columbia University Press

    (Publisher)

  secular equilibrium. It allows an important simplification of decay series calculations, because the system can be treated as if the original parent decayed directly to the stable daughter without intermediate steps. Secular equilibrium is assumed in uranium prospecting methods that utilize radiation measurements to calculate uranium abundances.

  FIG. 14.3. Representation of the radioactive decay of 238 U to 206 Pb through a series of daughter isotopes. Alpha and beta particles produced at each step are labeled.

  GEOCHRONOLOGY One of the prime uses of radiogenic isotopes is, of course, to determine the ages of rocks. All radiometric dating systems assume that certain conditions are satisfied:

  1. The system, which may be defined as the rock or as an individual mineral, must have remained closed so that neither parent nor daughter atoms were lost or gained except as a result of radioactive decay.
  2. If atoms of the daughter nuclide were present before system closure, it must be possible to assign a value to this initial amount of material.
  3. The value of the decay constant must be known accurately. (This is particularly critical for nuclides with long half-lives, because a small error in the decay constant will translate into a large uncertainty in the age.)
  4. The isotopic compositions of analyzed samples must be representative and must be measured accurately.

  PROSPECTING AND WELL-LOGGING TECHNIQUES THAT USE RADIOACTIVITY

  Uranium prospecting tools rely principally on detection of gamma radiation, because alpha and beta particles cannot penetrate an overburden cover of even a few cm thickness. The Geiger counter is commonly used for field surveys, although it is a rather inefficient detector. The instrument consists of a sealed glass tube containing a cathode and anode with a voltage applied. The tube is filled with a gas that is normally nonconducting, but when gamma radiation passes through the gas, it is ionized and the ions accelerate toward the electrodes. The resulting current pulses are recorded on a meter or heard as “clicks.”

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  The State of Art of Nuclear Chemistry: Theoretical and Practical Aspects

  • Maria Emilova Velinova(Author)
  • 2020(Publication Date)
  • Arcler Press

    (Publisher)

  SECTION 2: RADIOACTIVE DECAY KINETICS AND RADIOTRACERS Radioactive Decay as A Second-Order Kinetics Transformation Process. Consequences on Radiometric Dating 4 Citation: Giancarlo Cavazzini. “Radioactive Decay as A Second-Order Kinetics Transformation Process. Consequences on Radiometric Dating”. Applied Physics Research; Vol. 12, No. 1; 2020. https://doi.org/10.5539/apr.v12n1p26 Copyright: © for this article is retained by the author(s), with first publication rights granted to the journal. This is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/ licenses/by/4.0/). Giancarlo Cavazzini CNR Istituto di Geoscienze e Georisorse, Padova, Italy ABSTRACT Arguments suggest and recent analysis of experimental work confirm that the current interpretation of the transformation process we call ‘radioactive decay’ should be revised. The characteristics of this process are better accounted for by re-interpreting it in terms of second-order kinetics. Therefore, the atomic systems of nuclides we observe decay are ‘radio-activated’, and not, as hitherto believed, ‘radio-active’. According to this interpretation, the rate of decay of a radioactive nuclide is at any instant proportional to the concentration of the physical species that determines its activation. The analysis of λ of alfa- and beta-emitting nuclides show the dependence of these parameters from solar activity and distance. The State of Art of Nuclear Chemistry: Theoretical and Practical Aspects 44 Therefore, if changes in the emission of energy from the sun occurred over time since the formation of a geological system, changes in the values of λ of the radioactive nuclides would also have occurred, and the calculated radiometric age of the system may differ from the true age. Implications on the science of dating geological samples using parent-daughter decay systematics are investigated.

  Sign up to read

  Learn more about book
• 

  No longer available |Learn more

  Age of Earth, Age of Universe & Role of Radiometric Dating Methods

  • (Author)
  • 2014(Publication Date)
  • Learning Press

    (Publisher)

  It is accompanied by a sister process, in which uranium-235 decays into protactinium-231, which has a half-life of 34,300 years. While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sediments, from which their ratios are measured. The scheme has a range of several hundred thousand years. ____________________ WORLD TECHNOLOGIES ____________________ Radiocarbon dating method Ale's Stones at Kåseberga, around ten kilometres south east of Ystad, Sweden were dated at 600 CE using the carbon-14 method on organic material found at the site. Radiocarbon dating , or carbon dating , is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 ( 14 C) to determine the age of carbonaceous materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years Before Present (BP), Present being defined as 1950 CE. Such raw ages can be calibrated to give calendar dates. One of the most frequent uses of radiocarbon dating is to estimate the age of organic remains from archaeological sites. When plants fix atmospheric carbon dioxide (CO 2 ) into organic material during photosynthesis they incorporate a quantity of 14 C that approximately matches the level of this isotope in the atmosphere (a small difference occurs because of isotope fractionation, but this is corrected after laboratory analysis). After plants die or they are consumed by other organisms (for example, by humans or other animals) the 14 C fraction of this organic material declines at a fixed exponential rate due to the radioactive decay of 14 C. Comparing the remaining 14 C fraction of a sample to that expected from atmospheric 14 C allows the age of the sample to be estimated. The technique of radiocarbon dating was developed by Willard Libby and his colleagues at the University of Chicago in 1949.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Nuclear Physics 2

  Radiochronometers and Radiopharmaceuticals

  • Ibrahima Sakho(Author)
  • 2024(Publication Date)
  • Wiley-ISTE

    (Publisher)

  The K–Ar dating method exploits the decay property of the father nucleus 40 K to produce the radiogenic son nucleus 40 Ar. This method covers the entire time scale from the origin of the universe to 30,000 years BP [DIO 74]. From a chronological point of view, the exponential decay of potassium-40 was demonstrated in 1948. Radiochronometer Applications in Dating 81 Figure 3.8. Potassium-40 decay energy diagram. Here are its three disintegration paths In general, the K–Ar method is based on the determination of the 40 Ar/ 40 K isotope ratio in a given mass of rock sample. This method is ideally suited to the dating of volcanic rocks. Potassium is one of the eight most abundant elements in the Earth’s crust (Table 3.1). Element O Si Al Fe Ca Na Mg K % 46.6 27.7 8.1 5.0 3.6 2.8 2.3 2.1 Table 3.1. The eight elements that make up more than 98% of the composition of the rocks in the Earth’s crust In magma containing 40 K, some or all of the 40 Ar formed can escape through outgassing. During a volcanic eruption, the magma reaches the surface, then cools and solidifies. The 40 Ar is then trapped in the magmatic rock, where it can no longer escape. This accumulates in the lava crystallized by the disintegration of 40 K. The K–Ar age of a lava thus corresponds to its emplacement age, i.e. the age of the eruption [SAS 15]. Determination of this age by the K–Ar radiochronometer is based on the age equation established in section 3.2.3. 3.2.2. Basic assumptions for the K–Ar radiochronometer The use of the K–Ar clock is based on several assumptions. Knowledge of these assumptions is fundamental, as they play a major role at different levels of sample analysis. This enables us to take a more pertinent, critical look at the results obtained. Let us consider the following basic assumptions [SAS 15]: – At the moment of its formation (i.e. t = 0), the sample is considered to be devoid of any 40 Ar*.

  Sign up to read

  Learn more about book
• 

  No longer available |Learn more

  Advances in Archaeological Science & Techniques

  • (Author)
  • 2014(Publication Date)
  • The English Press

    (Publisher)

  ________________________ WORLD TECHNOLOGIES ________________________ Chapter-3 Dating Techniques Archaeological science has particular value when it can provide absolute dates for archaeological strata and artifacts. Some of the most important dating techniques include: • radiocarbon dating — especially for dating organic materials • dendrochronology — for dating trees; also very important for calibrating radiocarbon dates • thermoluminescence dating — for dating inorganic material (including ceramics) • optically stimulated luminescence (OSL)/optical dating — for absolutely dating and relatively profiling buried land-surfaces in vertical and horizontal stratigraphic sections, most often by measuring photons discharged from grains of quartz within sedimentary bodies (although this technique can also measure feldspars, complications caused by internally-induced dose-rates often favour the use of quartz-based analyzes in archaeological applications) • electron spin resonance, as used (for example) in dating teeth • potassium-argon dating — for dating (for example) fossilized hominid remains Radiocarbon dating Radiocarbon dating (sometimes simply known as carbon dating ) is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 ( 14 C) to estimate the age of carbonaceous materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years Before Present (BP), Present being defined as 1950 CE. Such raw ages can be calibrated to give calendar dates. One of the most frequent uses of radiocarbon dating is to estimate the age of organic remains from archaeological sites. When plants fix atmospheric carbon dioxide (CO 2 ) into organic material during photosynthesis they incorporate a quantity of 14 C that approximately matches the level of this isotope in the atmosphere (a small difference occurs because of isotope fractionation, but this is corrected after laboratory analysis).

  Sign up to read

  Learn more about book