Radioisotope Detection and Dating With Particle Accelerators

Abstract

Abstract Radiocarbon dating is conventionally performed by the accurate measurement of the beta rays emitted during the decay of 14 C. The direct determination of 14 C atoms may also be accomplished by ultrasensitive mass spectrometry using a particle accelerator. Signal to noise ratios in excess of 10 15 : 1 are required for this task, and can only be attained if the background from the isobar 14 N and from interfering molecular ions is eliminated. By using a tandem Van de Graaff accelerator as a molecule disintegrator and accepting only negative ions, this condition may be attained and individual 14 C atoms may thus be counted directly. The higher counting rates inherent in this method, and the ability to use much smaller sample sizes offer significant advantages over the conventional method. In addition, other naturally occurring radioactive isotopes with half lives relevant to Quaternary dating may be measured using the same approach. At the present time the isotopes 10 Be, 14 C, 26 A1, 32 Si, 36 C1 and 129 I have been detected at natural levels without prior isotope enrichment by using negative ions, molecular dissociation with tandem accelerators and atom counting. Ratios of 14 C/ 12 C and 36 C1/ C1 near 10- 15 have been reached during experiments being carried out to develop radiocarbon and 36 C1 dating of milligramme samples. With the ability to use such small samples, valuable geological and archaeological specimens will be datable without significant damage, and the costs involved may be reduced. New facilities dedicated to this type of measurement are being constructed at various centres, and are expected to begin producing routine radiocarbon dates in 1982.