Study of Radiocarbon Detection By laser spectroscopy

Abstract

<p>This thesis examines the feasibility of using infrared spectroscopy to carry out radiocarbon dating. Conventional radiocarbon dating techniques are limited in sensitivity by the very small fraction of C¹⁴ atoms which disintegrate during the counting period. Our proposed infrared technique does not suffer from this limitation, but we must overcome the problem of interfering infrared spectra caused by stable isotopes, and develop methods for detecting extremely small infrared absorption coefficients.</p> <p>The apparatus employed consists of a tunable diode laser and a multipass optical cell. Radiocarbon is detected in the form of C¹⁴O₂, and it is shown that interference-free detection can be attained provided the gas sample is cooled to dry ice temperature. To achieve sufficient sensitivity we have developed a second harmonic detection scheme. As part of out optimisation procedure, the experimental and theoretical second harmonic lineshapes were compared over a wide range of experimental conditions. Good agreement was found in all cases.</p> <p>With the present apparatus, the minimum detectable infrared absorption coefficient was ~ 1x10ˉ⁷ mˉ¹. This compares with a sensitivity of ~ 1x10ˉ⁹ mˉ¹ which is required for practical radiocarbon dating. The noise mechanisms which limit the present sensitivity were investigated in some detail, and results obtained at short time constants in small optical cells are extrapolated to long optical cells and measurement times of several hours. Based on these extrapolations, radiocarbon dating by infrared spectroscopy appears feasible, and will require much smaller carbon samples than is required by conventional disintegration counting techniques.</p>