Residual radiocarbon activity is measured using ten Perkin Elmer 1220 "Quantulus" Liquid Scintillation (LS) spectrometers. The "Quantulus" is the most sophisticated low-level spectrometer available. Its design ensures the optimisation of both counting conditions and data validation. It has extensive shielding designed to reduce the effects of background radiation, enabling both older and smaller samples to be dated more accurately. The instrument also contains twin multi-channel analysers (MCAs) which provide information on the sample and background spectra, necessary for Quality Control.

 

The Waikato laboratory is the only one in the world with ten spectrometers of this type.
More details on Liquid Scintillation Counting and LSC by Quantulus are given in the Radiocarbon WEB-info pages.

Since 1996, the Waikato Radiocarbon Laboratory has been producing CO₂ for Accelerator Mass Spectrometry (AMS) analysis of C¹⁴. This technique allows small samples and, therefore, a tremendous range of new sample types to be dated. At Waikato we have dated a wide range of sample types including rice grains, silk from a painting, individual nuts and seeds, feathers, individual teeth, charred pot residue, frog bone, small bird bone and linen from the wrappings of Egyptian mummys.

 

 

The popularity of AMS has grown from only 4% of our total throughput in 1996, to around 50% in 2004. This resulted in two new AMS labs built at the end of 2001, extension to our existing AMS lines and the development of labview software to control and monitor our graphite production. This has enabled us to double our throughput and reduce our turnaround time for AMS, while increasing our quality control.

 

Organic samples are converted to CO₂ by oxidation at 800^oC using CuO. The CO₂ is purified in the presence of silver wire to absorb any SOx and NOx that is produced. CO₂ is collected from shells by reaction with phosphoric acid. The CO₂ is then reduced to graphite with Zn at 600^oC using an iron catalyst. Graphite is sent to the Rafter Radiocarbon Dating Laboratory for target preparation and analysis.

 

To measure laboratory reproducibility and cross-check our results, we are closely involved in inter-calibration exercises with the radiocarbon community. We have participated in all of the Glasgow University programmes and the International Atomic Energy Agency (IAEA) intercomparison exercise. Waikato was one of five laboratories that provided the standards (A & B standards: swamp kauri ca. 50,000 years old) for the Fourth International Radiocarbon Intercomparison. Alan Hogg was also a member of the international advisory committe for the 2001 University of Glasgow Intercomparison exercise. The results of the 2001 IAEA exercise are presented in Table 1.

The Waikato determinations are statistically indistinguishable from the consensus values at the 95% confidence level and demonstrate the accuracy of the Waikato laboratory.

 

Table 1: Results of the 2001 Fourth International Radiocarbon Intercomparison (FIRI) showing University of Waikato results.

Fire Sample	Firi preliminar consensus values	University of Waikato Results (Standard Radiometric)
A/B (Kauri wood)	0.244 ± 0.002 pmC*	0.216 ± 0.045 pmC
C (Turbidite)	18,173 ± 11 yr BP	18,219 ± 69 yr BP
D/F (Dendro-dated wood)	4508 ± 3 yr BP	4544 ± 26 yr BP
E (Humic acid)	11,778 ± 7 yr BP	11,809 ± 69 yr BP
C/J (Barley mash)	110.69 ± 0.09 pmC	110.77 ± 0.23 pmC
H (Dendro-dated wood)	2232 ± 5 yr BP	2256 ± 30 yr BP
I (Dendro-dated cellulose)	4485 ± 5 yr BP	4438 ± 51 yr BP

*The FIRI results shown for the A/B samples are the mean values for AMS (Accelerator Mass Spectroscopy) and GPC (Gas Proportional Counting) labs only. The mean results for all labs are too young (higher pmC values) because of poor performance by some LSC (Liquid Scintillation Counting) labs.