The present definition of the SI unit of temperature, the kelvin, is tied to the triple point of water. This dependence of thermometry on a specially prepared state of matter has recently been shown to present both practical and conceptual challenges to the logical development of measurement science into the future. In 2005, the BIPM, the international committee charged with maintaining the SI, suggested that the kelvin be redefined with respect to the Boltzmann constant. This approach would instead tie the kelvin to a stable and universally accessible constant of nature. In preparation for this transition, the BIPM mandated an international effort to redetermine the Boltzmann constant with greater precision than it is currently known. At the completion of this process its value would be fixed.
At that time, the uncertainty in the Boltzmann constant was 1.8 ppm and was derived primarily from a single experiment using an acoustic resonator. It was preferable to garner multiple determinations with comparable accuracy using vastly dierent methods to ensure that no systematic error was overlooked. This thesis explains efforts to pursue one such approach: Doppler Broadened Thermometry (DBT). We have developed a laser spectrometer to accurately record the linear absorption profile of Doppler-broadened transitions in cesium vapour. The Gaussian component of the observed lineshape was extracted using least-squared fitting to a newly-developed model for line profiles in optically-pumped effusive gases, from which the Boltzmann constant may be determined.
We measured the Boltzmann constant with a precision of 28 ppm after 1.5 hrs of signal acquisition. Our achieved accuracy of 129 ppm was limited principally by the uncertainty of the Lorentzian width component of the observed atomic lineshape. This uncertainty is within a factor 6 of the best reported spectroscopic determination of k, and within a factor of 55 of the best proposed spectroscopic determinations of the Boltzmann constant, wherein the expected total systematic uncertainty has been examined but these experiments are still underway. A pathway towards a spectroscopic measurement of the Boltzmann constant at the few-ppm level is presented, contingent only on an improved atomic lifetime determination.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - Mar 2014|