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...
