Studies in the measurement of nuclear radiations of extremely low intensity--carbon-14 dating

Abstract

The technical details of a radiation measuring system based on a quartz torsion fiber electroscope and a pressurized ionization chamber are described. This equipment is designed to measure the radiation intensity from the carbon-14 contained in a sample introduced into the system as carbon dioxide. This system, including the shielding, is shown to be capable of measuring nuclear radiations of extremely low intensities, on the order of 1/200 of the normal laboratory background level, with a probable error of approximately l%. The background radiation intensity observed with the system including the shielding is also on the order of 1/200 of the normal laboratory background level. The nature of background radiation and its relation to the measurement of such low intensities is discussed. It is shown that the main limitation in accuracy, with the equipment described here, is due to cosmic ray bursts. It is also shown that the major source of background radiation is external to the equipment, being thus due to cosmic rays and/or local external gamma ray emitting material contamination. Although the evidence is not conclusive, the data indicates in addition that the cosmic rays are the major contributor to this background radiation. The main conclusion which may be drawn from these results is that the selection of an equipment location is very effective in reducing the level of background radiation, thus permitting the measurement of radiation fields of extremely low intensity. The final application of this equipment was to the well known carbon-14 dating method originated by W. F. Libby. Several samples of interest were dated and the results are given. Of particular interest, are the samples from the Santa Rosa Islands off the California Coast, concerning an Indian culture which existed there before Christ and after.