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7 Natural Abundance of Radioactive Isotopes

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7 Natural Abundance of Radioactive Isotopes 8 NATURAL ABUNDANCE OF RADIOACTIVE ISOTOPES OF C AND H In this chapter the two nuclides, 14C and 3H, will be treated that are the most relevant in the hydrological cycle. Chapter 12 contains a survey of the isotopes with less extensive applications. 8.1 THE RADIOACTIVE CARBON ISOTOPE 8.1.1 ORIGIN OF 14C, DECAY AND HALF-LIFE The natural occurrence of the radioactive carbon isotope, 14C or radiocarbon, was first recognised by W.F. Libby (1946). It is naturally formed in the transitional region between the stratosphere and troposphere about 12 km above the earth's surface through the nuclear reaction: 14N + n J 14C + p (8.1) The thermal neutrons required are produced by reactions between very high-energy primary cosmic ray protons and molecules of the atmosphere. 14C thus formed very soon oxidises to 14 14 CO, and ultimately to CO2 which mixes with the inactive atmospheric CO2 (Fig.8.1). 14 Through exchange with oceanic dissolved carbon (primarily bicarbonate), most CO2 molecules enter the oceans and living marine organisms. Some are also assimilated by land plants, so that all living organisms, vegetable as well as animal, contain 14C in concentrations about equal (Sect.8.1.3) to that of atmospheric CO2. 14C decays according to: 14C J 14N + ß (8.2) with a maximum ß energy of 156 keV and a half-life of 5730 ± 40 years (Godwin, 1962) (Fig.8.2). Originally the half-life was thought to be 5568 years, so that during the first decade or two 14C age determinations were based on the "wrong" half-life. Later, when the "better" half-life became known, so many 14C ages were already published that, in order to avoid confusion, it was decided that the original half-life should continued to be used for reporting 14C ages. Moreover, it was meanwhile known that ages would still be in error, even using the 14 right half-life, because of the natural variations in the C content of atmospheric CO2 during geologic times and deviating from the present. These errors were even larger. Nowadays the 125 Chapter 8 cosmic radiation 14 14 14 14 N + n J C + p J C N + 14 14 C C 14 C 14 12 CO2 + CO2 12 CO2 exchange assimilation fossil nuclear bomb fuel exchange 14 H CO3 14 shells humus CO2 14 LAKE Ca CO3 OCEAN groundwater table ageing 12 14 12 + Ca CO3 + H2O J H CO3 + H CO3 Fig.8.1 Origin and distribution of 14C in nature. The natural 14C concentration by the production by neutrons from cosmic radiation has been influenced by -chronologically- the addition 14 14 to the air of CO2 without C by the combustion of fossil fuels and the production of C by neutrons released by the fission and fusion reactions during nuclear explosions. 14C calibration, based on the known 14C content of tree rings with exactly known age, removes both errors at once (Sect.8.1.4). The production and distribution of 14C in nature occurs through series of chemical and biological processes which has become stationnary throughout much of geologic time. As a consequence, the concentration of 14C in the atmosphere, oceans and biosphere reached a steady-state value which has been almost constant during a geologic period which is long compared to the life span of a 14C nucleus. This natural concentration, 14C/C, is of the order of 10-12, which is equivalent to a specific activity of about 0.25 Bq/gC (disintegrations per second per gram of carbon) (Sect.5.3). 8.1.2 REPORTING 14C VARIATIONS AND THE 14C STANDARD Three modes of reporting 14C activities are in use, in part analogous to the conventions internationally agreed (IAEA) for stable isotopes (Stuiver and Polach, 1977; Mook and Van der Plicht, 1999). 126 Radioactive Isotopes of Hydrogen and Carbon 14 C T1/2 = 5730 a 3 H T1/2 = 12.43 a Emax = 156 keV Emax = 18 keV 3He 14N 3 14 Anatural = 0.6 Bq / liter H2O Anatural = 250 Bq / kg C Fig.8.2 Decay schemes of tritium and radiocarbon. Both nuclides are pure emitters. The present-day specific activities are given as 3A and 14A. The absolute (specific) 14C activity, that is the 14C radioactivity (in Bq or, conventionally, in disintegrations per minute (dpm) per gram of carbon) is given the symbol 14A = number of disintegrations per minute (dpm/gC) (8.3) It is extremely difficult to make an absolute measurement of a 14C activity. Moreover, the absolute 14C content of a sample is generally not relevant. Therefore, the sample activities are compared with the activity of a standard. In reality, the number of 14C registrations ( registrations from 14C decay in radiometric detectors such as proportional counters and liquid scintillation counters, registrations of 14C concentration in AMS systems) are related to the number of registrations from the standard under equal conditions. This results in a 14C activity ratio or 14C concentration ratio: 14 14 14 Asample A a 14 14 A reference A R 14 Cdecayrate in thesample 14 Cconcentration in thesample (8.4) 14 Cdecayrate in the reference 14 Cconcentration in the reference Because in the nominator and denominator of the last two fractions the detection efficiencies are cancelled being equal for sample and reference material, the use of the ratio 14a is adequate for any type of measuring technique. 127 Chapter 8 Henceforth, the symbol 14A may be used for the 14C content (radioactivity or concentration) of a sample, whether the analytical technique applied is radiometric or mass spectrometric (AMS). Under natural circumstances the values of 14a are between 0 and 1. In order to avoid the use of decimals, it is general practice to report these values in %, which is equivalent to the factor 10 2 (see also Sect.11.5.1). It is incorrect to write 14a/102 instead of merely 14a. In several cases the differences in 14C content between the project samples are small. Therefore, the use has been taken over from the stable-isotope field to define relative abundances, in casu the relative 14C content (activity or concentration), 14, defined as the difference between sample and reference 14C content as a fraction of the reference value: 14 A 14A 14 A 14 R 1 14a 1 (P102 %) (8.5) 14 14 A R A R The values of are small numbers and therefore generally given in ‰, which is equivalent to the factor 10 3 (cf. Sect.4.1). In Chapter 7 we have seen that during any process in nature isotope fractionation occurs, not only for the stable isotopes but similarly for the radioactive isotope such as 14C. Neglecting this effect would introduce an error in an age determination, because the original 14C content of the material would be different from what we assume. The degree of fractionation is nicely 13 indicated by the value of the material. An example is the apparent difference in age 13 13 between C3 plants such as trees ( W 25‰) and a C4 plant such as sugar cane ( W 10‰), each assimilating the same atmospheric CO2. Therefore, in defining the standard activity, but also in our treatment of all 14C data, we have to normalise the 14C results to the same 13 value. In Chapter 11 we will go into more detail. Here we suffice by stating that by international agreement all 14C results have to be corrected for a deviation of the 13 value from 25‰, with the exception of Ox1 which has to be corrected to 19‰. The 14C standard activity or concentration was chosen to represent as closely as possible the 14C content of carbon in naturally growing plants. The 14C standard activity does not need to be, in fact it is not, equal to the 14C activity of the standard. The definition of the standard 14C activity is based on 95% of the specific activity of the original NBS oxalic acid (Ox1) in the year AD1950 (Karlén et al., 1966), as will be discussed in more detail later. The 14C standard activity is defined by: 14 0 14 0 M M As tan dard 0.95 AOx1 13.56 0.07 dpm / gC 0.226 0.001 Bq / gC (8.6) 128 Radioactive Isotopes of Hydrogen and Carbon dpm/gC means disintegrations per minute per gram of carbon, while the superscript 0 refers to the fact that the definition is valid for the year 1950 only. Because the original supply of oxalic acid has been exhausted, a new batch of oxalic acid (Ox2) is available for distribution by the NIST (formerly US-Nat. Bureau of Standards). Through careful measurement by a number of laboratories (Mann, 1983), the 14C activity has become related to that of the original Ox1 by: 14 0 M 14 0 AOx2 (1.2736 0.0004) AOx1 (8.7) Both activities refer to AD1950. Consequently the standard activity is: 14 0 14 0 As tan dard 0.7459 AOx2 (8.8) where the 14A0 values for Ox1 and Ox2 refer to the activity of the material in 1950, irrespective of the time of measurement. Details on the measurement and calculation procedures are given in Chapter 11. A survey of available 14C reference samples of different compounds and ages is given in Table 11.5. 8.1.3 SURVEY OF NATURAL 14C VARIATIONS 8.1.3.1 ATMOSPHERIC CO2 Certain fluctuations occur in the stationary state mentioned in Sect.8.1.1. Because of variations in cosmic ray intensity, and in climate affecting the size of the carbon reservoirs, the 14C content of atmospheric carbon has not always been precisely the same as at present.
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