The usual procedure for the preparation of carbon dioxide from the oxalic-acid standard supplied by the National Bureau of Standards is wet oxidation by means of potassium permanganate in acid solution. The procedure is straightforward, but suffers from a certain difficulty in determining the end point of the reaction.

This list includes most of the dates produced from September 1967 to April 1968. The laboratory continues to use the benzene method of the previous date list (Radiocarbon, 1968, v. 10, p. 8–28); however, the counter and chemical treatment equipment were transferred to a field laboratory in order to avoid any possibility of contamination. Two new benzene synthesis lines of our own construction were added to the commercial unit. Their operation is excellent and over-all costs were nominal.

Age calculations in this list include samples dated since January, 1967. Samples are converted to benzene and counted in a liquid scintillation spectrometer. Operations are essentially the same as those described in Radiocarbon, 1966, v. 8, p. 46–53 and Radiocarbon, 1967, v. 9, p. 38–42. Modern reference is 95% the activity of NBS oxalic acid standard, not age-corrected wood as reported in Radiocarbon, 1966, v. 8, p. 46–53.

The Radiocarbon Dating Laboratory of the Geological Survey of Canada routinely operates two proportional counters; one 2 L and one 5 L. The 2 L counter is operated entirely at 2 atm. and the 5 L counter mainly at 1 atm. On occasion the 5 L counter is operated at 4 atm. Detailed descriptions of these two counters have recently been published (Dyck, 1967a). A I L counter has been fabricated and is now undergoing preliminary testing.

Two radiocarbon counting systems have been established in the Chemistry Department, University of Glasgow, since late 1967.

The counting gas is methane, at pressures up to 10 atm, and 2 alternative procedures are followed for methane production, (a) high pressure synthesis in a stainless steel 4.5 1 reactor and (b) low pressure synthesis in an all glass flow-reactor. Both systems employ 0.5% ruthenium on alumina pellets as catalyst (Engelhard Industries Ltd.). Early samples synthesized with Air Products' hydrogen showed evidence of tritium contamination. This gas supply was later replaced with tritium-free hydrogen supplied by Messrs. Griesheim, Düsseldorf, Germany. Both detectors used for routine measurements are 0.5 1 internal gas counters supplied by Beckman Instruments Inc., California. The detectors are surrounded by a concentric-wall multiple anode anticoincidence counter. The entire counter assembly is encased within a 4-in.-thick lead shield manufactured from aged lead by J. Girdler and Co., London. Counter electronics, anticoincidence system and power supply are of Beckman design (Sharp and Ellis, 1965).

The following list presents dates on a portion of the samples measured at ISOTOPES during 1967 and 1968 and measurements made previously for which either complete sample data has been recently received, or, in some cases, deferred due to the editorial load in preparing the definitive list.

The following list comprises selected measurements made during 1967–68. The method is essentially the same as previously described. A 0.6 L proportional gas-counter at 3 atm CH4 pressure is used. Ages are given relative to A.D. 1950 and half-life of 5570 yr has been assumed. The quoted error is the experimental standard deviation and includes the uncertainty on the unknown sample, the modern standard and the background.

The Radiocarbon Dating Laboratory was founded in 1965 by the Department of Geology, University of Lyon, to study the Late Quaternary geology of the Rhone-Alps Region, and to contribute to hydrogeologic and archaeologic studies. It has been installed in the basement of the Nuclear Physics Institute. Preparation began in 1966 and first dates obtained in June 1967.

The following list of dates contains most of the measurements obtained since our last list. Procedures of measurements and calculation are as previously described in Radiocarbon, 1964, v. 6, p. 194–196; 1966, v. 8, p. 286–291. A new 1.1 L counter, all metal and quartz, built in the laboratory has been in use since 1967.

The following list comprises measurements made since those reported in NPL V.

No changes have been made in measurement technique or in the method of calculating results described in NPL III.

The following list of dates is compiled from samples prepared since publication of our last date list (Radiocarbon, 7, v. 9, p 316–332) and includes determinations through June, 1968. Equipment and operating procedures are the same as described in OWU-III.

This date list includes those series of samples completed in this laboratory as of August 1968. 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 min each. Errors quoted are derived from measurement of sample, background, and modern age calibration, but do not include any half-life error. All samples were pretreated with 3N HC1, and some, where noted, were given additional pretreatment with 2% NaOH for the removal of possible humic contaminants.

Included in this list are analyses made during 1967 and the first half of 1968 with equipment and techniques previously employed here. All methane was produced in the bomb reactor.

The following results represent measurements carried out since our 3rd date list was prepared. The entire CO2 purification technique (Radiocarbon, 1962, v. 4, p. 81–83) has been changed to “wet purification.” No measurements of C12/C13 ratios have been made.

This date list is comprised of archaeologic and geophysical samples. The latter are in continuation of our investigations of bomb-produced radiocarbon in atmospheric carbon dioxide reported in Tata V. We continue to count samples in the form of methane; the techniques used have been described elsewhere (Agrawal et al., 1965).

The measurements reported have been carried out during the first half of 1968 in the Isotope Laboratory of the Institute of Geophysics and Planetary Physics as a continuation of the UCLA date lists I through VIII. Samples were analyzed as CO2-gas at close to one atm in a 7.5 L proportional counter with three energy channels described in earlier publication. Radiocarbon ages have been calculated for uniformity on the basis of a 5568 yr half-life in accord with a recommendation by the Sixth International C14 and H3 Dating Conference, June 1965, in Pullman, Washington. The standard for the contemporary biosphere remains as 95% of the count rate of NBS oxalic acid for radiocarbon laboratories. Background determinations have been based on CO2 obtained from marble. The error listed is always at least a one-sigma statistical counting error. In critical cases C13/C12 isotope ratio measurements were made to correct the dates for fractionation. All samples were subjected to accepted NaOH, HCl or other special chemical pretreatments discussed below depending on the individual case to exclude contamination.

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, which is always larger than the one-sigma statistical counting error commonly used, takes into account known uncertainty laboratory factors, but does not include external (field or atmospheric) variations.

The radiocarbon dates obtained since December 1967 are included in this report. The procedures followed have been described previously (Radiocarbon, 1966, v. 8, p. 522–533).