Since Middleton showed the potential for a sputter source working from CO2, we have been building a source which can automatically change samples. The source produces a maximum beam of 25μA of C−, with typical operation between 10 and 20μA. Although beam generation from the source is very reliable, the mechanics of sample changing have given considerable problems. The changing of samples also involves considerable care in gas handling, and a computer control system has been written which ensures the correct sequence of the 16 operations required for sample change.

During the International Radiocarbon Conference in Dubrovnik in June 1988, several laboratories expressed the need for quality assurance. Furthermore, on several occasions 14C laboratories have shown an interest in reference materials of known 14C activity, apart from the recent activity of the NBS oxalic acid.

Groningen, April 3, 1989 W G Mook

We have made the first measurements without pre-enrichment of 41Ca in terrestrial rock and bone samples using accelerator mass spectrometry. Although the results in tufa deposits from Egypt are in good agreement with the saturation value of 8×10-15 predicted by Raisbeck and Yiou (1979), the average 41Ca:40Ca ratio of 2×10-15 (range: 0.6 to 4.2×10-15) that we measure in modern bone is an order of magnitude lower than that obtained previously by Henning, et al (1987) on a cow bone that was measured using AMS following isotope enrichment. The low value and the variability (more than a factor of seven) of the 41Ca:40Ca ratio in modern bone make the possibility of dating bones using 41Ca unlikely.

I recently had the privilege of attending the Conference on Technology-Based Confidence Building: Energy & Environment, hosted by Los Alamos National Laboratory and The University of California, in Santa Fe, New Mexico, July 9–14, 1989.

We made preliminary AMS measurements of 41Ca/Ca ratios in bone and limestone specimens with the Argonne Tandem-Linac Accelerator System (ATLAS). We were able to avoid pre-enrichment of 41Ca used in previous experiments due to a substantial increase in Ca-beam intensity. Most of the measured ratios lie in the 10-14 range, with a few values below 10-14. In general, these values are higher than the ones observed by the AMS group at the University of Pennsylvania. We discuss possible implications of these results. We also present the current status of half-life measurements of 41Ca and discuss 41Ca production processes on earth.

High-intensity ion sources are now capable of producing negative molecular ion beams of Ca of >2μA (Middleton et al, 1989). This and the advanced status of 41Ca AMS at several laboratories (Pennsylvania, Argonne National Laboratory, Rochester, Munich, Weizmann Institute) have revived interest in 41Ca (T1/2 = 1.0×105 yr (Mabuchi et al, 1974)) as a possible dating tool (Yamaguchi, 1963; Raisbeck & Yiou, 1979; Kubik et al, 1986).

Liquid scintillation (LS) 14C spectrometry, using ultra low-level, high-efficiency and resolution α and β particle detection equipment is seen as an excellent tool supporting future 14C radiometry. Modern technology ensures electronic excellence: high-precision, small-sample (100mg) dating ability, flexibility of use for the detection of α and β particle emitting radioisotopes at and below natural abundance levels. Multiple applications to environmental research, supported by elegant software and commercial availability, are its attributes. However, mastering the new technology takes time as lessons learned in gas-proportional 14C radiometry and high-background counting are not directly applicable to ultra low-level 14C LS spectrometry using benzene as the counting medium. That discordant results can be obtained by any technique based on physical measurement is well established. This is demonstrated for 14C age determination by reference to an international cross-check organized by the University of Glasgow. Reasons for aberrations are explored and the question is raised if it is not timely to introduce the type of quality assurance practiced by other analytical disciplines. Better results will not only serve the daters and users but will generally enhance the value of worldwide 14C research.

A Packard low-level liquid scintillation counting system is described which provides superior low-background beta-counting capability for 3H and 14C. The design is based on a novel pulse-discrimination circuit that separates background from valid scintillation pulses. Background discrimination is further enhanced by adding a plastic vial holder that acts as an anticoincidence guard. When excited by background radiation, the scintillation properties of the plastic provide an increased burst of photons to the detection electronics, which discriminate based on the number of component pulses in the burst. Experimental data demonstrate the low-level counting capabilities of this counter.

Important advances have been made in reducing the background counting rate of gas proportional counters for 14C dating through detailed and systematic study of the background components. Until recently, limited work has been reported on the study of the background of liquid scintillation counters (LSC). During the last few years, commercial systems with greatly reduced background have been introduced. It is shown that the best gas proportional counters and LSC have similar backgrounds for the same amount of sample material. Similar results with less effort may be expected with more detailed and fundamental knowledge of the components of the background of LSC. We report the results of a study of one photomultiplier LSC system where we research all parameters of importance: light collection efficiency, absorbed energy per photo-electron, pulse height spectrum and background counting rate.

Results are presented on the comparative characteristics of the Packard 2000 CA/LL liquid scintillation counter with and without the low-level option on line. An initial performance assessment using 14C labeled benzene with butyl-PBD as the scintillant revealed that although the background count rate decreased by approximately a factor of 3 using the low-level option, a substantial decrease in efficiency was also observed. However, subsequent investigations have indicated that, by careful manipulation of both scintillant composition and concentration, this loss in efficiency can largely be overcome with little or no concurrent increase in background count rate. The introduction of an active vial sample holder and a new light-guide system proved to be significant advances on the standard 2000 CA/LL.

The University of Arizona Radiocarbon Laboratory purchased a state-of-the-art LKB Quantulus LSC and placed it into a new underground counting chamber. We have investigated the performance of the Quantulus in this setting comparing different vial types, checking background sources and experimenting with sample size.

A Packard 2260 XL liquid scintillation counter was placed in an underground counting chamber to test performance under immense physical shielding. Results from the Packard 2260 XL are compared with two other counters under the same conditions, the LKB Quantulus, which has operated for two years in this laboratory, and the LKB 1219 SM, in use since January 1988.