This list includes those analyses completed December, 1968, with equipment and procedures previously employed here.

The following list presents results obtained during 1968-69 on a series of samples chosen to investigate temporal variations of C14 concentrations in the atmosphere during the past century. Together with data presented previously (Radiocarbon, 1969, v. 11, p. 45-52) they constitute a study of annual variations of C14 activities at N temperate latitudes.

The present date list covers mainly the datings done from 1964 to 1968. Each sample is measured in one of the two counting units described earlier (Nydal, 1965). The background of the counters has been somewhat reduced during the past few years. Counter 2 with an effective volume of 1.5 L (1.9 L total volume), has a background of 0.9 counts/min, and a recent standard net count of 19.2 counts/min. Counter 3, with an effective volume of 1.1 L (1.3 L total volume), has a background of 2.4 counts/min and a recent standard net count of 14.2 counts/min.

The following date list includes samples processed by the Illinois State Geological Survey Radiocarbon Dating Laboratory from September 1968 through November 1969. Detailed descriptions of sample methods are published elsewhere (Kim and Ruch, 1969; Kim, Ruch, and Kempton, 1969).

The following list includes C14 dates and deals with the results of the methodological investigations carried out at the Geobiochemical Laboratory of the Institute of Zoology and Botany of the Academy of Sciences of the Estonian SSR in 1967–1968.

The laboratory has been operating more than five years, supported entirely from Venezuelan government funds. Approximately 530 samples have been dated to the end of 1969. Of these, 40% are archaeologic specimens, the majority (141 samples) from Venezuela. Materials from most other countries of Latin America have been processed. The rest of the samples are from research programs of the Radiocarbon Laboratory itself. Thirty per cent of the total are measurements on carbonate species extracted from Venezuelan ground water aquifers.

This list reports C14 measurements made in dating projects completed in the year ending December, 1968, and some measurements for projects still in progress. Age calculations are based on C14 half-life of 5568 yr and a modern standard of 95% of NBS oxalic acid. Deviations reported are based on counting statistics of sample, background, and modern, and are ±1σ except that when sample count approaches either modern or background, 2σ limits are reported. The laboratory uses liquid scintillation counting of benzene, with Li2C2 and vanadium activated catalyst in preparation, as described in Texas IV (Radiocarbon, 1966, v. 8, p. 453–466) and earlier lists. Chemical yields average 85%.

Measurements reported in this paper were obtained with the 4.5 L and 3 L CO2 counters, details of which were given earlier (Radiocarbon, v. 11, p. 423). The automatic data recording system built in 1968 (Hänsel, 1968) is now operating for both counting apparatus. For each one the counts of the guard counters ring (A counts), the total counts of the C14 counter (B counts), the coincidences of central and guard ring counter (AB counts), and the anticoincidences (ĀB counts) are tape punched every 100th minute. By an ALGOL program, all counts are checked first for large disturbances. Secondly, equation B = AB + ĀB must hold (as an integral check for proper operation of logical circuitry and the data recording system) and finally statistical compatibility is examined before age and other data for the actual counting apparatus are computed. This detailed check of counting rates by computer has proved to be very efficient to yield reliable long-term measurements.

The C14 measurements reported here were made in this laboratory between October 1968 and October 1969.

The following list covers samples measured since the last list of atmospheric samples (Radiocarbon, 1967, v. 9, p. 471–476) was written, to determine the increase of the C14/C12 ratio due to explosion of nuclear devices.

The C14 dates given below have been obtained by counting CH4 at 3 atm pressure in a 0.6 L stainless steel counter. Details of procedure are given in the previous lists. Dates are reported in terms of the Libby half-life, 5570 ± 30 years; the errors quoted are based on the standard deviations in counting rate of samples and standards.

The dating equipment in the Queen's University Palaeoecology Laboratory was installed to provide data for research projects, initially dealing with the development of agriculture, in the departments of Botany and Archaeology.

The dating equipment in the Palaeoecology Laboratory has remained essentially as described in Belfast I (this volume). Rewiring of the counter has increased the detection efficiency slightly. Background count corrected to 1606 mb is now 11.0 counts/min and the net count rate for 95% of the NBS oxalic acid standard is 56.0 counts/min. All charcoal samples have been pretreated in accordance with the schedule given in Belfast I.

The C14 dates given below are a continuation of the work presented in our previous list (Radiocarbon, 1969, v. 11, p. 451-462), and results obtained mainly during 1969 are described. A new 3.3 L copper counter was put into routine operation besides the 2.7 L stainless steel counter employed heretofore, yielding background counting rates of 8.5 and 5.5 cpm, respectively, when filled with dead CO2 at ca. 1.8 atm.

This date list includes those series of samples completed in this laboratory as of November 1969. The b.p. ages are based upon a.d. 1950, and are calculated with a half-life value of 5568 yr. All samples were counted at least twice for periods of not less than 1000 minutes each. Errors quoted are derived from measurement of samples, background, and modern-age calibration, but do not include any half-life error. All samples were pretreated with 3N HCl, and some, where noted, were given additional pretreatment with 2% NaOH for the removal of possible humic contaminants.

A dating system consisting of gas sample counter with internal anticoincidence counter ring, transistorized electronic equipment, and chemical apparatus was developed in the Institute (Felber and Vychytil, 1962; Felber, 1965). A high voltage supply Fluke Model 408B is used. For routine dating, begun in 1965, an improved 2.4 L counter with Teflon insulators is used, shielded on all sides by 20 cm of iron. The counter is run with methane at 760 torr/15°C. Spurious counts are carefully eliminated by a systematic procedure (Felber, 1966). Stability of electronics and counter is checked once a day by taking the topmost part of the peak of Mnα-X-rays following electron capture in Fe-55, radiated through a beryllium window. Checking is done with the same single channel analyzer (switched over to operation with small window) used for energy discrimination (switched over to two discriminator operation) during measurement. If any change should be observed, discriminator settings are corrected. Energy discrimination is not optimized (Felber, 1962) because a neutron generator using (d,t) reaction is run in the same building in which the samples are prepared: the lower discriminator is set above tritium maximum energy at 22 keV, the upper one at 120 keV, the highest possible energy absorption of C14 β particles in the special counter. The background is 1.58 cpm, the net contemporary value (95% of NBS oxalic acid standard activity) is ca. 8.8 cpm.

The University of Cambridge Radiocarbon Dating Laboratory has been completely rebuilt and modernized, incorporating new techniques developed here for the various stages of measurements. Sample radioactivity is measured in gas proportional counters after conversion to highly purified CO2. Four counters are mounted within a single anticoincidence shield consisting of plastic scintillation material. This is surrounded by a graded cosmic ray shield of low radioactive steel, boron loaded wax, and finally, about 16 tons of ancient low radioactive lead blocks. The electronics are all solid state devices except for the stabilized high voltage supplies.

This list contains the results of measurements made during 1967 and 1968. Samples are counted in the form of acetylene gas, as previously, and ages computed on the basis of the Libby half-life, 5568 ± 30 yr. The error listed, always larger than the one-sigma statistical counting error commonly used, takes into account variable laboratory factors, but does not include external (field or atmospheric) variations.

The radiocarbon dates obtained since August, 1968, are reported here. Wood, charcoal, and peat samples are pretreated with dilute NaOH and dilute H3PO4 before conversion to the methane used as counting gas; marls and lake cores are treated with acid only. The reported dates have been calculated using 5568 years as the half-life of C14, 1950 as the reference year. Samples are run at least once in each of two 0.5 liter counters at 3 atm pressure for a minimum total of 15,000 counts. The standard deviation quoted includes only the 1σ of the counting statistics of background, sample, and standard counts.

This list includes age measurements carried out from December 1968 to October 1969. Archaeologic samples are from Italian and Swat (W Pakistan) territories. All geologic samples come from Italian territory. Chemical techniques remain the same (Bella and Cortesi, 1960; Alessio, Bella, and Cortesi, 1964).