Noble gases measurement

FIGURE 1.20 Mobilities of ions in noble gases measurements (circles) and calculations using ab initio interaction potentials (lines). (From Viehland, L.A., Lozeille, J., Soldm, P., Lee, E.P.F., Wright, T.G., J. Chem. Phys., 121, 341, 2004.)... 

Figure 4.8 shows an example of the estimation of paleotemperature based on noble gas measurements in the groundwater of some aquifers in Texas (State et al., 1992). Paleotemperatures estimated from the noble gas concentration shows a large drop (about 5.2°C) at the distance 30km away from the recharge area [Figure 4.8(c)], which corresponds to an age between 12,000 and 17,000 years, close to the last glacial maximum (about 18,000 years ago). 

Excess air in groundwater has been reported by Heaton and Vogel (1981) for cases of special soil conditions observed in South Africa. Hence, periodic noble gas measurements are desired a negative correlation with discharge will establish the karstic nature of a studied spring or well. 

The number of laboratories that measure noble gases in water samples is slowly but steadily increasing. The know-how is actually available in most laboratories that do potassium argon dating of rocks. Because of increased demand from field hydrochemists, more laboratories are willing to enter the domain of noble gas hydrology. Eventually, noble gas measurements will become as routine as <5D, <5lsO, tritium, or 14C measurements. 

Deak, J., Stute, M., Rudolph, J., and Sonntag, C. (1987) Determination of the flow regime of Quaternary and Pliocene layers in the Great Hungarian Plain (Hungary) by D, lsO, 14C and noble gas measurements. In Isotope Techniques in Water Resources Development, IAEA, Vienna, 335-350. 

Noble gas measurements were made on bulk samples of four density fractions (Amari et al., 1995b). In contrast to SiC, a substantial fraction of Ne-E in graphite seems to come from the decay of short-lived (T1/2 = 2.6 yr) Na (Clayton, 1975). This is supported by the low " He/ Ne ratios measured in individual grains (Nichols et al., 2003). Krypton in graphite has two s-process components with apparent different neutron exposures, residing in different density fractions (Amari et al., 1995b). 

By 1975, the CRE ages of hundreds of meteorites had been estimated from noble gas measurements. Histograms of the CRE age distributions pointed to several important observations. 

Lavielle B, Toe S, Gilabert E (1997) Noble Gas Measurements In the L/LL5 Chondrite Kryahinya. Meteoritics Planet Sci 32 97-107. 

The standard for noble gas measurements, and the reference for discussions of data, is the composition of the terrestrial atmosphere. The abundances of the noble gases are in Table 1, along with those of other major and minor gases that are more abundant than Xe. With the exception of Ar, which is dominated by radiogenic " °Ar, the noble gases are present as trace constituents. Isotopic compositions are provided in Table 2. Since air is the reference standard for laboratory analyses, measurements of other media are typically normalized to these values. 

W., Whyte, J. and Meyerhof, D. (2004). Low level noble-gas measurements in the field and laboratory in support of the Comprehensive Nuclear-Test-Ban Treaty, Appl. Radiat. Isotopes, 61, 231-235. 

The choice of tracer gas for the measurements is Kr-85 It has a long half-life so that it can be stored for application when needed. It is a noble gas which is chemically inactive giving a low radio toxicity as it is readily removed in case of accidental contamination. 

The molecular constants that describe the stnicture of a molecule can be measured using many optical teclmiques described in section A3.5.1 as long as the resolution is sufficient to separate the rovibrational states [110. 111 and 112]. Absorption spectroscopy is difficult with ions in the gas phase, hence many ion species have been first studied by matrix isolation methods [113], in which the IR spectrum is observed for ions trapped witliin a frozen noble gas on a liquid-helium cooled surface. The measured frequencies may be shifted as much as 1 % from gas phase values because of the weak interaction witli the matrix. 

In TOF-SARS [9], a low-keV, monoenergetic, mass-selected, pulsed noble gas ion beam is focused onto a sample surface. The velocity distributions of scattered and recoiled particles are measured by standard TOF methods. A chaimel electron multiplier is used to detect fast (>800 eV) neutrals and ions. This type of detector has a small acceptance solid angle. A fixed angle is used between the pulsed ion beam and detector directions with respect to the sample as shown in figure Bl.23.4. The sample has to be rotated to measure ion scattering... 

Typical elements in Groups V. VI and VII would be expected to achieve a noble gas configuration more easily by gaining electrons rather than losing them. Electron affinity is a measure of the energy change when an atom accepts an extra electron. It is difficult to measure directly and this has only been achieved in a few cases more often it is obtained from enthalpy cycle calculations (p. 74). 

In recent years, aqueous solutions of Xe03 have been used to oxidize a species in solution, from which A[H°m can be calculated when AH for the oxidation reaction is combined with AH for other reactions. The noble gas oxide Xe03 is used as an oxidant because of its stability and the fact that the final reaction product is Xe(g), which has a zero enthalpy of formation and is easily removed from the reaction mixture. As an example, O Hare4 has reported AfHcm for UCI4. We will not go through the details of his procedure, but the critical step involved measuring A TH for the reaction... 

To accommodate this problem, scientists have come up with several approaches to measuring atomic sizes. A common one is called the covalent radius, which is half the distance between the nuclei of two identical atoms. This technique works well for atoms such as hydrogen or oxygen, both of which readily pair up to form and O2. But how would one determine the covalent radius of a noble gas, which exists only as single atoms ... 

A final note must be made about a common problem that has plagued many kinetic treatments of reactive intermediate chemistry at low temperatures. Most observations of QMT in reactive intermediates have been in solid matrices at cryogenic temperatures. Routinely, reactive intermediates are prepared for spectroscopy by photolyses of precursors imbedded in glassy organic or noble gas (or N2) solids. The low temperatures and inert surroundings generally inhibit inter- and intramolecular reactions sufficiently to allow spectroscopic measurements on conventional and convenient timescales. It is under such conditions, where overbarrier reactions are diminished, that QMT effects become most pronounced. 

The highly fractionated nature of the and Th series nuclides is illustrated by the measured activities in some representative waters in Figure 1. The highest activities are typically observed for Rn, reflecting the lack of reactivity of this noble gas. Groundwater Rn activities are controlled only by rapid in situ decay (Table 1) and supply from host rocks, without the complications of removal by adsorption or precipitation. The actinide U, which is soluble in oxidizing waters, is present in intermediate activities that are moderated by removal onto aquifer rocks. The long-lived... 

As a noble gas, Rn in groundwater does not react with host aquifer surfaces and is present as uncharged single atoms. The radionuclide Rn typically has the highest activities in groundwater (Fig. 1). Krishnaswami et al. (1982) argued that Rn and all of the other isotopes produced by a decay are supplied at similar rates by recoil, so that the differences in concentrations are related to the more reactive nature of the other nuclides. Therefore, the concentration of Rn could be used to calculate the recoil rate for all U-series nuclides produced by a recoil. The only output of Rn is by decay, and with a 3.8 day half-life it is expected to readily reach steady state concentrations at each location. Each measured activity (i.e., the decay or removal rate) can therefore be equated with the input rate. In this case, the fraction released, or emanation efficiency, can be calculated from the bulk rock Ra activity per unit mass ... 

For this purpose, one should measure variation of electric conductivity of one and the same movable sensor in the saturated vapour-gas phase and in a liquid, caused by the presence of any given concentration of oxygen in a carrier gas (hydrogen, nitrogen, noble gas, etc.). From the results of these measurements the Bunsen coefficient P can be found in accordance with the relation (see Chapter 3, Section 4)... 

It is expected that due to the short residence time of Be and Cl in the atmosphere, 10Be and 36C1 measurements on ice cores will directly reveal isotope production variations. Due to dilution in the C02 exchanging system the atmospheric 14C/C-ratio shows a dampened response to 14C production rate variations. In contrast to the noble gas radioisotopes the size of the effective dilution reservoir - atmosphere plus parts of the ocean and biosphere - depends on the characteristic... 

Paleotemperatures derived from noble gas analyses are potentially more meaningful than those from oxygen-deuterium analyses because the noble gas content is a direct measure of the temperature of the water at the time of infiltration rather than a complex function of geographic and meteorological factors as is the case with 2H and 180. Despite this potential superiority, few noble gas studies of water paleotemperatures have been published. Specifically, questions need to be answered relative... 

Janssens et al. [38, 40] used photoemission of adsorbed noble gases to measure the electrostatic surface potential on the potassium-promoted (111) surface of rhodium, to estimate the range that is influenced by the promoter. As explained in Chapter 3, UPS of adsorbed Xe measures the local work function, or, equivalently, the electrostatic potential of adsorption sites. The idea of using Kr and Ar in addition to Xe was that by using probe atoms of different sizes one could vary the distance between the potassium and the noble gas atom. Provided the interpretation in terms of Expression (3-13) is permitted, and this is a point the authors checked [38], one thus obtains information about the variation of the electrostatic potential around potassium promoter atoms. 

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