Apparatus for Methane Synthesis for Radiocarbon Dating

Fairhall, A. W.; Schell, W.R.; Takashima, Yohei

doi:10.1063/1.1717353

Cited by 33 publications

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“…of steel. (Fairhall, Schell and Takashima, 1961 Our working standard for almost all measurements has been a specimen of 275-yr-old Sequoia wood, as determined by ring counting. The age-corrected activity of this specimen, corrected also for C13/C12 differences, is 0.945 + 0.016 of the activity of the NBS oxalic-acid standard.…”

Section: Part I: Apparatus and Methodsmentioning

confidence: 99%

Radiocarbon Dating at the University of Washington I

et al. 1962

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INTRODUCTIONBeginning in 1955 a C14 dating apparatus was constructed at this University for the purpose of measuring the specific activity of tree rings selected from a specimen of Sequoia wood having 3000 growth rings (Dorn, 1958). The original purpose was to obtain a measure of the half life of C14 by essentially the inverse of C14 dating. For various reasons the project was not completed, and for about a year the apparatus stood idle for lack of financial support. This latter deficiency has now been rectified, at least temporarily, and research with the apparatus has been resumed. Fabrication of a second, 1 L counter is nearing completion.In the meantime, others have investigated tree rings and other materials of known age and found interesting fluctuations in the C14 specific activity in past times. Our work on Sequoia tree rings has been resumed and results to date are reported in Part III of this paper.In addition to the investigation of tree rings we have dated samples submitted to us by anthropologists, geologists, glaciologists, etc. These are reported in Part II of this paper.Part IV consists of a short note concerning the mechanism of oxidation of radiogenic C14 in the atmosphere. Our working standard for almost all measurements has been a specimen of 275-yr-old Sequoia wood, as determined by ring counting. The age-corrected activity of this specimen, corrected also for C13/C12 differences, is 0.945 + 0.016 of the activity of the NBS oxalic-acid standard. Therefore the dates reported in Part II would not change significantly if they had been computed on the basis of 0.95 of the activity of the NBS standard as the modern assay of C14. The net counting rate of our Sequoia standard at a pressure of 148.9 cm Hg and 22°C is ca. 101 counts/min and has varied by ± 3% in a non-periodic way over the past two yr, presumably owing to slight changes in the parameters of the electronic system. The standard is measured periodically and is generally reproducible over the short term within 0.5%, i.e. 0.5 counts/min, which is ca. the reproducibility of the background counting rate.The background counting rate of the counter has remained fairly constant over the past two yr at ca. 50 counts/min. The background rate is pressuresensitive, the relation being Background = 40 + 0.066 p counts/min where p is the pressure in cm Hg. Since methane is more sensitive to fast neutrons than the more commonly-used CO2 and no neutron shielding is employed, the pressure-dependent portion of the background may represent the neutron component. The remaining background may well be due to radioactive impurities in the counter and shield, since none of the material was of special purity. A background of 40 counts/min from such a source is not unreasonable in view of the large size of the counter. Background counts are taken once or twice a week as a check on the apparatus. Since the samples are ordinarily counted between 5:00 P.M. and 9:00 A.M. no diurnal variations in background have been looked for or observed in this time interval.Count...

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Section: Part I: Apparatus and Methodsmentioning

confidence: 99%

Radiocarbon Dating at the University of Washington I

et al. 1962

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“…In the reaction, CO (2) as far as synthesis of methane for counting C'4 activity is concerned. Reaction (2) leads to an incomplete conversion of hydrogen and is, therefore, not satisfactory for methane synthesis for H3 measurement.Reaction (2) was successfully used for the first time by Fairhall et al (1961) for radiocarbon measurements. Subsequent modifications were reported by Olson and Nickloff (1965) and Polach and Stipp (1967).…”

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“…Reaction (2) was successfully used for the first time by Fairhall et al (1961) for radiocarbon measurements. Subsequent modifications were reported by Olson and Nickloff (1965) and Polach and Stipp (1967).…”

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Tata Institute Radiocarbon Date List IX

1971

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The C14 dates given below are in years B.P. calculated on the basis of Ti/2 = 5568 years. For converting to A.D. JB.C scale, 1950 was used as reference year. Ninety-five per cent activity of NBS oxalic acid was used as a modern standard.Radiocarbon activity was counted after converting sample carbon into methane gas (Agrawal et al., 1965). Up to now, we used the synthesis technique developed by Anand and Lal (1964) in which the sample CO2 and appropriate amounts of zinc and water (tritium-free) are allowed to react in the presence of ruthenium catalyst inside a reaction vessel with temperature zones of 430° and 500°C. In the reaction, CO (2) as far as synthesis of methane for counting C'4 activity is concerned. Reaction (2) leads to an incomplete conversion of hydrogen and is, therefore, not satisfactory for methane synthesis for H3 measurement.Reaction (2) was successfully used for the first time by Fairhall et al. (1961) for radiocarbon measurements. Subsequent modifications were reported by Olson and Nickloff (1965) and Polach and Stipp (1967). Techniques employing reaction (2) are in use in several C14 laboratories. We now use reaction (2) as such for methane synthesis in our laboratory and briefly describe here apparatus and experimental techniques, since our procedures lead to fairly routine, simple, and quick analyses with yields better than 99%. The prime reason for adopting this system was that we occasionally discovered that synthesized methane was contaminated with artificial tritium used in the TIFR laboratories. The technique adopted for reaction (1) involved opening the reaction vessel for every synthesis for replacing consumed zinc and introducing the water ampule; frequently, it also became necessary to remove spilled fragments of pyrex glass (from the fragmented ampules) and grains of zinc oxide. In the system described below, the reaction vessel is not opened and all that is required is introduction of the sample CO2 in the reaction vessel; the CO., is let in through a cold trap (dry ice + acetone) to remove any traces of water.Our reactor vessel which, in some respects, resembles that used by Oeschger (pers. commun.) has a volume of 26 L; it is made of hemispherical stainless steel vessels with flanges on their rims (Fig. 1 Reaction rate can be seen from the pressure vs. time curve (Fig. 2). Maximum pressure in the vessel is reached in 20 minutes. At this point, Trap X (Fig. 1) is cooled with dry ice to remove water that is formed.Drop in pressure is very fast after 35 minutes and reaction is complete in about 3 hours, even with a large sample (10 L CO2).The reaction products are successively let through 3 dry-ice traps (to remove all traces of H20), 2 liquid-N2-cooled traps and a liquid-N2-cooled U-tube, filled with silica gel (12-28 mesh) and here called S. The reactor is fairly quickly emptied by gentle pumping on the leading end of S. No methane is lost from S provided the quantity of silica gel exceeds 7 g per liter of CH4. For purification of methane, pumping with a rotary pump...

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“…Samples are combusted in a quartz tube in a stream of purified oxygen (Libby, 1952) or by the combustion bomb method (Barker, Burleigh, and Meeks, 1969;Switsur, Hall, and West, 1970;Switsur, 1974) followed by trapping and purification of the CO2 and subsequent conversion to CH4 by hydrogenation via Ru catalyst (Fairhall, Schell, and Takashima, 1961). All methane samples are stored for one month in stainless steel containers to permit radon decay.…”

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Lodz Radiocarbon Dates I

Kanwiszer

Trzeciak

1984

Radiocarbon

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The method applied for methane conversion is based on the modified technique of organic combustion analysis. Samples are combusted in a quartz tube in a stream of purified oxygen (Libby, 1952) or by the combustion bomb method (Barker, Burleigh, and Meeks, 1969;Switsur, Hall, and West, 1970;Switsur, 1974) followed by trapping and purification of the CO2 and subsequent conversion to CH4 by hydrogenation via Ru catalyst (Fairhall, Schell, and Takashima, 1961). All methane samples are stored for one month in stainless steel containers to permit radon decay. The basic pretreatment of samples which are generally wood, peat, or charcoal, consists of visual examination for intrusive rootlets followed by boiling in 2% HC1, 2% NaOH, and 2% HC1, successively (Bern et al, 1978). Between each treatment the sample is washed in boiling distilled water and finally is kiln-dried at 100°C in a vacuum.ACKNOWLEDGMENTS

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Apparatus for Methane Synthesis for Radiocarbon Dating

Cited by 33 publications

References 2 publications

Radiocarbon Dating at the University of Washington I

Radiocarbon Dating at the University of Washington I

Tata Institute Radiocarbon Date List IX

Lodz Radiocarbon Dates I

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