UBC Theses and Dissertations
Development of a cold atom pressure standard Shen, Pinrui
In this thesis, we report the realization of the world's first cold atom based pressure standard for the high- and ultra-high vacuum (UHV) regimes, < 10⁻⁶ Pa (1 Pa=1N/m²). This standard is a fundamentally new approach to vacuum metrology as it is based on a universal law governing quantum diffractive collisions between particles. We show that a measurement of trap loss rate versus trap depth provides the velocity averaged total collision cross-section, <σtotv>, - the only parameter required to quantify the pressure of background particles given a measurement of the collision rate with a sensor atom. This new quantum measurement standard is fully empirical, based on unchanging and fundamental atomic constants, and does not rely on computations of total collision cross-sections based on theoretical interaction potentials. We demonstrate, using a sensor ensemble of ⁸⁷Rb atoms, that this new quantum pressure standard can be applied to gases of both atomic species (He, Ar, and Xe) and molecular species (N₂, CO₂, and H₂), surpassing the scope of existing orifice flow pressure standards. The accuracy of this new standard was also verified by comparing it with an N₂ calibrated ionization gauge traced back to an orifice flow standard. They agree within 0.5%. A complete uncertainty analysis of this cold atom pressure standard is provided here. Moreover, using this standard, we are able to observe and quantify the performance limits of two industry-standard ionization gauges. Using this universal law, we can precisely measure the total collision cross-section <σtotv> for the collision system of interest. As an example, we show a precision measurement of <σtotv> for collisions between Rb and Ar. The experimentally determined value of <σtotv> agrees with the theoretical computation result within 1.0 %. Next, we demonstrate the use of a magneto-optical trap (MOT) as a transfer pressure standard to extend the operational range of the cold atom pressure standard by a factor of 100, from P < 10⁻⁷ Pa to include pressures up to P < 10⁻⁵ Pa. Finally, a proposal for using a MOT as a primary pressure standard is presented.
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