Tata Institute Radiocarbon Date List IX

Abstract

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...

Pastoral nomadism in the archaeology of India and Pakistan

“…At Tilwara, the deposit was shallower and there too, microliths and pottery occurred at every depth. Whatever the dates of the earlier deposits, which may be as old as 5,500 years (Agarwal et al 1971), it is only the more recent ones that are of interest here. Iron objects and glass beads place them in the last four centuries B.C.…”

The Quaternary Isotope Laboratory Thermal Diffusion Enrichment System: Description and Performance

“…CO was chosen as the enrichment medium because (i) it is light (ie, the relative mass difference between 14C160 and 12C160 is large); (ii) its intermolecular interaction has a short range, which is favorable for TD separation; (iii) it has a fairly good thermal stability; (iv) it is very easy to make by passing C02, the first step in the preparation of any 14C sample, over hot zinc; (v) it offers the possibility to determine the 14C enrichment by measuring the simultaneous enrichment of molecules of mass 30 (mainly 12C180) mass spectrometrically. The alternative, CH4, was less attractive because the advantage of its smaller mass is offset by a longer interaction range and less thermal stability, while its production, although also a standard procedure in many laboratories (Agrawal, Gupta, and Kusumgar, 1971; Buddemeier et al, 1970), is somewhat more complicated for the large samples used. CH4 also does not readily offer a check on the enrichment.…”