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RADIOCARBON DATING BY MASS SPECTROMETRY ; PROGRESS AT OXFORD

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The accelerator method for C dating is based on the principle of the mass spectrometer and counts individual atoms, rather than £ emission from nuclear decay, as is the case in conventional radiocarbon dating. The need to be able to distinguish between large amounts of -^N and much smaller amounts of l^C renders the use of expensive high energy physics equipment necessary.

A general plan of the equipment is given in figure 1. Carbon from the object to be dated is introduced into the system in the form of graphite on a tantalum strip at the ion source. Under vacuum, the graphite is bombanâed by a beam of Cs+ and, among other particles, C~ is sputtered off and accelerated down the first half of the tandem accelerator, the beam of C*~ strikes a carbon foil and is converted to C^ and other species, which are then accelerated down the second half of the accelerator into a series of magnets and velocity filters which completely separate the C-3+ from other particles. The 14C3^ are then directed into a Faraday cup and counted. Several times a second the beam is switched and 12C3+ and 1^C3+ are measured at different Faraday cups.

Graphite is used as the carbon substrate since it has been shown to give high, reproducible beam currents. ^

Because individual atoms rather than ^3 emission are measured, much smaller amounts of carbon are necessary, typically 1-5 mg compared to about 5 g for conventional dating, thus opening up a wide range of possible subjects for study. For this reason it has been decided to build a dedicated facility for radiocarbon dating at Oxford, drawing on the expertise of the Oxford Nuclear Physics department. 1 The building for the new laboratory is almost finished and the accelerator should be installed and running by late 1980. It is hoped to produce the first trial dates by summer 1981 and to start major dating • programs in late 1931.

A high vacuum sample preparation line, based on the work reported at the last conference^, has been built. Figure 2 shows a general view. The sample to be dated is loaded into the combustion train and burnt in a flow of high purity oxygen and nitrogen to carbon dioxide, which after storage is then converted to lithium carbide by passing it into molten lithium at 60OC, in the lithium reaction vessel. Water is then added to convert the lithium carbide into acetylene. This follows conventional procedures, although on a much smaller scale. The novel reaction is then to pyrolyse the acetylene; a resistively heated tantalum wire at about 2CC0 'C cracks the acetylene and carbon is deposited on the wire in the form of graphite, suitable for transport and irrmediate insertion in the ion source.

The acetylene production process gives a yield of 85-90%, the pyrolysis a yield of 60%, so that an overall yield of 50% is achieved for sample preparation. No fractionation has been observed in the acetylene production but pyrolysis gives a fractionation of -1.4%