La Jolla natural radiocarbon measurements, [Part] 1.

Abstract

The recently established low-level radiation laboratory of Scripps Institution of Oceanography has been measuring radiocarbon since Aug. 1957. Through 1959 about 160 samples have been measured. In the radiocarbon tests, the La Jolla laboratory staff utilizes the technique, with acetylene as the counting gas, that was developed at the U.S. Geological Survey. Until March 1959 only one set of equipment was in service, with a counter somewhat smaller than either of the 2 used in Washington, having a sensitive volume of approximately 0.6 liter. In March 1959 an Oeschger-Houtermans counter, manufactured at the Physical Institute of the University of Bern, Switzerland, was installed. It operates with a background of less than 2.0 counts/min., decreasing with filling pressure, and a sensitive volume of 1.3 liters. This counter proved to be more stable than the ordinary counters, and it can be operated at pressures as low as 440 mm., at which a background as low as 1.2 counts/min. can be attained. Therefore, this counter proves to be superior also in the measurement of relatively small samples. The installation of one or more additional counters is being considered, with the aim of increasing the annual output of tests. Shell dates reported in this paper are calculated (as in U.S.G.S. I) from 19th century wood as a reference standard, without correction for any supposed fractionation of isotopes. This procedure has been adopted because in general, within the errors of measurement, the activity of 19th century wood corresponds to that of modern shell, despite the differences in the equilibrium distribution of the isotopes as shown by C (super 13) analyses.