Helium isotopic abundances

In Inductively Coupled Plasma Mass Spectrometry. Montaser A (ed) Wiley-VCH, New York, p 83-264 Montaser A, Zhang H (1998) Mass spectrometry with mixed-gas and Helium ICPS. In Inductively Coupled Plasma Mass Spectrometry. Montaser A (ed), Wiley-VCH, New York, p 809-890 Nier AO (1940) A mass spectrometer for routine isotope abundances measurement. Rev Sci Instrum 11 212-216... 

In this review we wish to discuss how observations of AGB stars can be used to determine the manner in which heavy elements are created during a thermal pulse, and how these heavy elements and carbon are transported to the stellar surface. In particular we wish to study how the periodic hydrogen and helium shell burning above a degenerate carbon-oxygen (C-0) core forms a neutron capture nucleosynthesis site that may eventually account for the observed abundance enhancements at the surfaces of AGB stars. In section II we discuss the nucleosynthesis provided by stellar evolution models (for a general review see [1]). In section III we discuss the isotopic abundances provided by nucleosynthesis reaction network calculations (see [2, 3]). In section IV we discuss how observations of AGB stars can be used to discriminate between the neutron capture nucleosynthesis sources (see [4]). And in section V we note some of the current uncertainty in this work. 

To set an abundance scale for listing the abundances of the elements, astronomers usually set H = 1012 atoms. Other elemental abundances in stars are then given by their numbers per thousand billion H atoms. In the Sun the ratio H/He = 10. For 3He more reliable information about relative He isotopic abundances comes from the primitive classes of meteorites, which are dominated by silicon. Thus the scale frequently used for geochemistry and for stellar nucleosynthesis takes a sample containing one million Si atoms, so that abundances of the elements are then their numbers per million Si atoms. Since helium in the Sun is observed by astronomers to be 2720 times more abundant than silicon, the He total solar abundance is therefore... 

The determination of isotopic abundance by means of an n,p) reaction induced by thermal neutrons has been described by Coon (16). Variations in the He content of helium gas depending on its source had been recorded and Coon was able to confirm these with gas obtained from wells and from air by means of the reaction He (n,p)H whose cross section for thermal neutrons is approximately 5,000 bams. A search for Si in natural silicon was conducted by Turkevich and Tomkins (WB). Theory had indicated that Si might be a beta-stable isotope and occur in natural silicon in small, undetected amounts. Neutron-irradiated quartz was examined radiochemically for 25-day half-life P , the daughter of Si formed by (ra,y) reactions on the sought isotope. However, only P , probably formed from impurities, was detected and assuming a cross section for the Si (n,y) Si reaction of 0.05 bams an upper limit of 4 x 10 % results for the abundance of Si in natural silicon. Subsequent work has shown Si to be an approximately 300-yr half-life beta emitter. 

The mass spectrometer is used to measure masses of isotopes as well as isotopic abundances, that is, the relative amounts of the isotopes. Helium occurs in nature almost exclusively as He. Its atomic mass can be determined in an experiment such as that illustrated in Figure 5-8. 

Helium isotope measurements in ocean ridge and island basalts provide some of the most basic geochemical information on mantle source reservoirs. More helium isotope analyses have been performed for oceanic volcanic rocks than for other noble gas species, and helium isotopes have played a leading role in the study of mantle heterogeneity. Helium isotope analyses are readily performed by modern mass spectrometers because there is a general absence of atmospheric contamination in samples due to the low concentration of helium in air (5.24 parts per million by volume at standard temperature and pressure). There are 2 naturally occurring isotopes of helium. He is much less abundant than " He for example, the atmospheric He/" He ratio (Ra) is 1.39x10 (Mamyrin et al. 1970 Clarke et al. 1976). Nearly all of the terrestrial " He has been produced as a-particles from the radioactive decay of U, U and Th over... 

In this section, we review noble gas systematics of arc-related volcanism worldwide. Helium isotope studies dominate because most arc products are erupted subaerially, and air contamination is a relatively minor (correctable) problem for helium this is not the case for Ne-Ar-Kr-Xe isotope systematics. Consequently, this section is weighted towards reporting observations of helium isotope variations in arc-related minerals and fluids. However, we summarize also the available database for neon, argon and xenon isotopes (todateKr shows only air-like isotopic compositions). Finally, we consider the limited database of the relative abundances of the noble gases in arc-related products. 

Mahaffy PR, Niemann HB, Alpert A, Atreya SK, Demick J, Donahue TM, Harpold DN, Owen TC (2000) Noble gas abundance and isotope ratios in the atmosphere of Jupiter from the Galileo Probe Mass Spectrometer. J Geophys Res E105 15061-15071 MamyrinBA, TolstikhinIN (1984) Helium Isotopes in Nature. Elsevier, Amsterdam. 

Weiss RF (1971) Solubility of helium and neon in water and seawater. J Chem Eng Data 16 235-241 Wetherill GW (1954) Variations in the isotopic abundances of neon and argon extracted from radioactive minerals. Phys Rev 96 679-683... 

Tellurium is a sort of scattered rare element on the Earth. Its concentration is very low in the Earth s crust, only 1.0 ng/g. However, it is in extremely high abundance in Co-rich crusts, marine polymetallic nodules, deep-sea sediments, and aerolites. The extreme enrichment of tellurium in deep-sea sediments, like helium isotope anomalies, probably results from the input of interplanetary dust particles (IDPs). Similarly, the extreme enrichment of tellurium in marine polymetallic nodules and Co-rich crusts is possibly related to IDPs. 

Except for helium, the abundances of the primordial isotopes change by orders of magnitude when varying the baryon density or 77, respectively. On the other hand, He is very abundant and thus it can be observed with high accuracy. Therefore, it is possible to determine 77 from... 

The possibility that an even larger impact caused the P T extinction received support when Becker and Poreda found that helium and argon atoms were present in the inner cores of some of the fullerenes from the P T boundary sediments (The cover of this book shows a helium atom inside a mol ecule of Ceo) What is special about the fullerene trapped atoms is that the mixtures of both helium and argon isotopes resemble extraterrestrial isotopic mixtures more than earthly ones The He/ He ratio in the P T boundary fullerenes for example is 50 times larger than the natural abundance ratio... 

As can be seen in Fig. 2-1 (abundance of elements), hydrogen and oxygen (along with carbon, magnesium, silicon, sulfur, and iron) are particularly abundant in the solar system, probably because the common isotopic forms of the latter six elements have nuclear masses that are multiples of the helium (He) nucleus. Oxygen is present in the Earth s crust in an abundance that exceeds the amount required to form oxides of silicon, sulfur, and iron in the crust the excess oxygen occurs mostly as the volatiles CO2 and H2O. The CO2 now resides primarily in carbonate rocks whereas the H2O is almost all in the oceans. 

The most abundant isotope of helium has two neutrons and an isotopic molar mass of 4.00260 g/mol. Compute the nuclear binding energy of this nuclide. 

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