Radiocarbon Dating at the University of Washington I

Abstract

INTRODUCTIONBeginning in 1955 a C14 dating apparatus was constructed at this University for the purpose of measuring the specific activity of tree rings selected from a specimen of Sequoia wood having 3000 growth rings (Dorn, 1958). The original purpose was to obtain a measure of the half life of C14 by essentially the inverse of C14 dating. For various reasons the project was not completed, and for about a year the apparatus stood idle for lack of financial support. This latter deficiency has now been rectified, at least temporarily, and research with the apparatus has been resumed. Fabrication of a second, 1 L counter is nearing completion.In the meantime, others have investigated tree rings and other materials of known age and found interesting fluctuations in the C14 specific activity in past times. Our work on Sequoia tree rings has been resumed and results to date are reported in Part III of this paper.In addition to the investigation of tree rings we have dated samples submitted to us by anthropologists, geologists, glaciologists, etc. These are reported in Part II of this paper.Part IV consists of a short note concerning the mechanism of oxidation of radiogenic C14 in the atmosphere. Our working standard for almost all measurements has been a specimen of 275-yr-old Sequoia wood, as determined by ring counting. The age-corrected activity of this specimen, corrected also for C13/C12 differences, is 0.945 + 0.016 of the activity of the NBS oxalic-acid standard. Therefore the dates reported in Part II would not change significantly if they had been computed on the basis of 0.95 of the activity of the NBS standard as the modern assay of C14. The net counting rate of our Sequoia standard at a pressure of 148.9 cm Hg and 22°C is ca. 101 counts/min and has varied by ± 3% in a non-periodic way over the past two yr, presumably owing to slight changes in the parameters of the electronic system. The standard is measured periodically and is generally reproducible over the short term within 0.5%, i.e. 0.5 counts/min, which is ca. the reproducibility of the background counting rate.The background counting rate of the counter has remained fairly constant over the past two yr at ca. 50 counts/min. The background rate is pressuresensitive, the relation being Background = 40 + 0.066 p counts/min where p is the pressure in cm Hg. Since methane is more sensitive to fast neutrons than the more commonly-used CO2 and no neutron shielding is employed, the pressure-dependent portion of the background may represent the neutron component. The remaining background may well be due to radioactive impurities in the counter and shield, since none of the material was of special purity. A background of 40 counts/min from such a source is not unreasonable in view of the large size of the counter. Background counts are taken once or twice a week as a check on the apparatus. Since the samples are ordinarily counted between 5:00 P.M. and 9:00 A.M. no diurnal variations in background have been looked for or observed in this time interval.Count...

A list and index of published radiocarbon dates for Alaska with a bibliography

“…TA-£ Stipp and others, 1962 Tx-157 Tx-158-a Pewe, 1975bTx-158-b Pewe, 1975bTx-159 Pewe, 1975b7x-160 Pewe, 1975bTx-220 Brown, 1965Pearson and others, 1965;Carter, 1984 TA-4047 Hopkins, 1982 UCLA -University of California, Los Angeles Lab Robinson, 1981 USGS-883 Hopkins andHopkins and Robinson, 1981;Nelson, 1982USGS-923 Molnina, 1986USGS-924 Molnina, 1986USGS-927 Molnina, 1986USGS-985 Hamilton and others, 1983USGS-986 Hamilton and others, 1983USGS-987 Hamilton and others, 1983USGS-988 Hamilton and others, 1983USGS-1029Carter, 1983USGS-1034 Williams Derksen, 1976;Dorn ando'thers, 1962 Derksen, 1976;Dorn and others, 1962Dorn and others, 1962Dorn and others, 1962Derksen, 1976Dorn andothers, 1962 Fairhall andPorter, 1967 Fairhall andPorter, 1967 Fairhall andPorter, 1967 Fairhall andPorter, 1967 Hamilton Hamilton and Porter, 1975Fairhall and others, 1976Fairhall and others, 1976Fairhall and others, 1976 W- Detter lan and Reed, 1973;…”

Pleistocene tephrostratigraphy and paleogeography of southern Puget Sound near Olympia, Washington

“…Landslides and colluvium disrupt and obscure the continuity of exposures so that pre-Vashon geologic history is not easily deciphered. In the south Puget Lowland (south of Tacoma), all fi nite radiocarbon ages reported before 1966 are suspect due to laboratory contamination (Dorn et al, 1962;Fairhall et al, 1966). Stratigraphic assignments based on these invalid radiocarbon ages are now questionable and need to be re-evaluated.…”

Dendrochronology and Radiocarbon Dating: The Laboratory of Tree-Ring Research Connection

“…Suess (1965) later verified de Vries' original observations of 14 C wiggles (the "de Vries effect") using dendrodated giant sequoia and archaeological Douglas-fir and ponderosa pine (Pinus ponderosa) samples provided from LTRR by B Bannister and M A Stokes. In 1955, Dorn et al (1962) had begun 14 C analysis of 2 dendrodated giant sequoia specimens of 3000 and 2000 yr in length, as provided from LTRR by Smiley, to examine the stability of 14 C production and to estimate the half-life of 14 C decay. Their coarse-resolution measurements indicated some of these wiggles, which they thought "may limit the ultimate usefulness of the C14-dating method for obtaining absolute ages," but they could not reasonably estimate half-life as originally intended (Dorn et al 1962).…”

“…In 1955, Dorn et al (1962) had begun 14 C analysis of 2 dendrodated giant sequoia specimens of 3000 and 2000 yr in length, as provided from LTRR by Smiley, to examine the stability of 14 C production and to estimate the half-life of 14 C decay. Their coarse-resolution measurements indicated some of these wiggles, which they thought "may limit the ultimate usefulness of the C14-dating method for obtaining absolute ages," but they could not reasonably estimate half-life as originally intended (Dorn et al 1962). Stuiver (1965) independently verified the de Vries wiggles with high-resolution 17th and 18th century LTRR dendrodated Douglas-fir wood from the Hitchcock tree near Tucson, provided by Stokes and Smiley.…”