This date list includes those series of samples completed in this laboratory as of November, 1965. The B.P. ages are based upon A.D. 1950, and are calculated with a half-life value of 5568 yr. Errors quoted are those derived from measurements of sample, background, and modern-age calibration, and do not include any half-life error. All samples were pretreated with 3N HCl, and some, where noted, received additional pretreatment with 2% NaOH for the removal of possible humic contaminants.

The measurements reported in the following date list have been made in the Low Level Counting Laboratory of Packard Instrument Company during 1965 and are a continuation of the work reported previously (Packard I). This counting procedure for these dates has remained unchanged — a 5 ml volume of benzene counted in a Packard Tri-Carb® Liquid Scintillation Spectrometer Model 3214. The sample is converted chemically from its original form to benzene in the Packard Tri-Carb Benzene Synthesizer. This instrument was described in a general fashion in our previous publication.

The dates and activity measurements given below have been obtained during 1964 and 1965, and have been made with CO2 at 3 atm pressure in a proportional gas-counter as described in previous contributions from this laboratory. We have continued to concentrate chiefly upon research projects pursued in the University Sub-department of Quaternary Research.

The list includes age measurements carried out from December 1964 to October 1965. As in the previous list (Rome III) the samples dated are almost all of archaeological interest and are drawn from Italian and some European and Asiatic territories.

Most of these analyses were performed during 1965 with equipment and techniques previously employed at this laboratory. A cold (−40°C) charcoal trap stage was added to the CO2 purification to remove radon, which eliminates the lag time between CH4 preparation and counting (Long, 1965b).

The following is a list of dates which has been obtained since the compilation of List I in December 1963. Owing to an irreparable leak in our original counter, it became necessary to replace it with a new counter of similar design using the same shield as before. The new counter has been operated both at 500 mm Hg pressure, where it has a background of 1.7 counts/min and an NBS oxalic count of 11.4 counts/min, and at 1000 mm Hg pressure, where it has a background of 2.5 counts/min and an NBS oxalic count of 22 counts/min.

Preparations for the development of a laboratory for the determination of absolute age by the radiocarbon method were started at the Geobiochemical Laboratory of the Institute of Zoology and Botany of the Academy of Sciences of the Estonian SSR in 1957. Dating of various carbon-containing specimens has been carried out since 1959.

All measurements reported in the previous date lists (Kusumgar et al., 1963a; Agrawal et al., 1964 and 1965a) were based on acetylene counting. Since September 1964 we have changed over to methane counting and all dates presented here are based on this method. Methane counting offers certain advantages over acetylene, e.g., (1) rapid synthesis, (2) the non-explosive nature of gas, (3) the synthesized gas is radon-free. The disadvantage of having only one atom per molecule in methane (CH4), as against two in acetylene (C2H2), is compensated by the fact that methane can be counted at higher pressures with relatively low operating voltages.

This list reports routine measurements made at this laboratory since the preparation of the last list (Texas III). It also includes measurements made on bone samples earlier as part of a study of the suitability of bone for C14 dating (Tamers and Pearson, 1965). In the absence of the laboratory director, M. A. Tamers, who has been on leave at the Caracas (IVIC) laboratory since October, 1963, laboratory operation and the selection and description of geologic samples have been handled by Pearson, and laboratory administration and archaeologic samples by Davis.

The measurements reported have been carried out during 1965 in the Isotope Laboratory of the Institute of Geophysics and Planetary Physics as a continuation of the UCLA date lists I through IV. Samples were analyzed as CO2-gas at one atm in a 7.5 L proportional counter with three energy channels. Dates have been calculated on the basis of a 5568 yr half-life as was recommended by the Sixth International C11 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 made with CO2 obtained from marble. The error listed is always at least a one-sigma statistical counting error.

This date list consists of those measurements made since 1962. The counter is one described previously (Fairhall and Schell, 1963). The results are computed using NBS oxalic acid as the standard and 5568 for the half-life of C14. Standard deviations are computed for each measurement, including the statistical error in the sample count and uncertainties in background and standard. In general, each sample is counted at least twice. The quoted error on the date is the standard deviation. A 2σ criterion is used to establish a lower limit to the age of very old samples with no detectable trace of C14. No correction for isotope fractionation has been made in any of the measurements.

This list contains a selection of results of measurements made since September, 1963. Until the end of 1962, the stability of equipment performance was unsatisfactory and, when the author assumed responsibility for the operation of the laboratory in 1963, several dates that had been published (Focken, 1960, 1962, and private commun.) were withdrawn. The performance and operation of the equipment were re-assessed and improved during 1963 and routine dating was begun towards the end of the year.

This report summarizes the radiocarbon dates obtained at the University of Wisconsin since November 1964; the procedures followed have been described previously (Wisconsin I, 1965).

The counting equipment has been supplemented with a second detector of 500 ml volume for which the nominal background at 1 std atm, 298.2° K, is 2.1 cpm as determined with petroleum methane (Phillips Petroleum Co.). The corresponding standard count for contemporary methane derived from NBS oxalic acid 95% Aox, is nominally 2.8 cpm at 1 std atm, 298.2° K. The additional counting equipment was also obtained from the Sharp Division of Beckman Instrument Co.

The result of a radiocarbon determination is commonly expressed as an age given in radiocarbon years. An error is usually assigned to each value as a measure of the statistical uncertainty of the measurement. Date lists published in this journal use a standard form of reporting dates and their errors (see Editorial Statements in Radiocarbon, v. 3 and v. 4). The conversion of a radiocarbon age, given in radiocarbon years B.P. (i.e., radiocarbon years elapsed since the origin of the sample) to a true calendar year makes necessary certain assumptions with respect to: (1) the half-life of C14, (2) the production rate of C14 by cosmic rays, (3) the size of reservoirs into which C14 is distributed and the exchange rate of this distribution. Libby (1955, p. 10) has shown that as an approximation one may assume that reservoir size and production and distribution rates, and therefore the C14 activity in atmospheric CO2 have been constant. However, the more accurate measurements of recent years have shown that at least one of these quantities must have varied with time. This means that a more complicated relationship exists between radiocarbon age and exact calendar age of a sample than had been assumed by Libby. This relationship cannot be determined theoretically, but can be derived empirically by determination of the radiocarbon contents of samples of known age. The following summarizes our present knowledge regarding differences between radiocarbon ages and true ages and the present status of the empirical calibration of the radiocarbon time scale.