This thesis describes a method of using an ultra-cold ensemble of atoms confined in a trap as an atomic primary pressure standard. The development of the standard and its current status are described in detail. This standard uses a 3D MOT to trap ⁸⁷Rb and then transfer them to a quadrupole magnetic trap where the atoms undergoes collisions with a background gas. By measuring the number of atoms left in the magnetic trap as a function of time one extract a loss rate and from this rate determine the background gas density. This loss rate is a product of the density of the background, multiplied by the loss cross section averaged over the velocity distribution of the background gas. By computing the average loss cross section in the magnetic trap and measuring the loss rate, the density of the background gas can be determined. This gives a calibration free measurement of density of a background gas in the UHV range (10-⁶ ‑ 10-⁹) Torr or (10-⁴ - 10-⁷) Pa which allows for it to be used as a standard. In conjunction with this, preparation of the atoms prior to the loss rate measurement is investigated to ensure accuracy and reproducibility of the standard. Finally a comparison between UBC's atomic standard and NIST's (National Institute for Standards and Technology) orifice flow standard is conducted via an ionization gauge which employed as a transfer standard.All measurement are carried out using Argon gas as the background gas of study.
This thesis describes a method of using an ultra-cold ensemble of atoms confined in a trap as an atomic primary pressure standard. The development of the standard and its current status are described in detail. This standard uses a 3D MOT to trap ⁸⁷Rb and then transfer them to a quadrupole magnetic trap where the atoms undergoes collisions with a background gas. By measuring the number of atoms left in the magnetic trap as a function of time one extract a loss rate and from this rate determine the background gas density. This loss rate is a product of the density of the background, multiplied by the loss cross section averaged over the velocity distribution of the background gas. By computing the average loss cross section in the magnetic trap and measuring the loss rate, the density of the background gas can be determined. This gives a calibration free measurement of density of a background gas in the UHV range (10-⁶ ‑ 10-⁹) Torr or (10-⁴ - 10-⁷) Pa which allows for it to be used as a standard. In conjunction with this, preparation of the atoms prior to the loss rate measurement is investigated to ensure accuracy and reproducibility of the standard. Finally a comparison between UBC's atomic standard and NIST's (National Institute for Standards and Technology) orifice flow standard is conducted via an ionization gauge which employed as a transfer standard.All measurement are carried out using Argon gas as the background gas of study.
We report the realization of the first cold-atom primary standard. This standard is based on a universal law governing quantum diffractive collisions between particles that allows an experimental determination of the velocity averaged total collision cross section, the only parameter required to quantify the pressure or flux of particles given a sensor particle collision rate measurement. Using an ensemble of 87Rb sensor atoms, we show that this new quantum pressure standard can be applied to gases of both atomic (He, Ar, and Xe) and molecular species (, , and ), surpassing the scope of existing orifice flow pressure standards. We verify the accuracy of this new standard using an ionization gauge (IG) calibrated for N2 by an orifice flow standard. The gauge calibration factors determined by the cold atom and orifice flow standards differ by less than 0.5% and, thus, agree within their uncertainties of 2% and 2.8% respectively. Using this standard, we evaluate the response of two different IGs to a variety of different gas species and report variations of up to 20% for their measured calibration factors. We also observe a non-linear response of the IG readings for CO2 gas. Finally, we demonstrate the use of a magneto-optical trap (MOT) as a transfer standard to extend the measurement range by a factor of 100 to include pressures up to P ~ 10−5 Pa.
This thesis describes an investigation and application of the loss of laser cooled atoms from a trap induced by background collisions. The loss rate constant depends on the density of background gas and the velocity averaged collisional loss cross section due to collisions. The velocity averaged collisional loss cross section can be calculated and its dependence on trap depth was verified using a magneto-optical trap. This verification involved measurements of the loss rate constant for a quadrupole magnetic and a magneto-optical trap and measurement of the density of Ar background gas using a residual gas analyzer. The second part of the thesis focuses on an application of these measurements of the loss rate constant to measure the pressure of the background gas. The experimental progress to date on the atom pressure sensor is provided.
Abstract We report the realization of the first cold-atom primary standard. This standard is based on a universal law governing quantum diffractive collisions between particles that allows an experimental determination of the velocity averaged total collision cross section, the only parameter required to quantify the pressure or flux of particles given a sensor particle collision rate measurement. Using an ensemble of 87Rb sensor atoms, we show that this new quantum pressure standard can be applied to gases of both atomic (He, Ar, and Xe) and molecular species ( N 2 , C O 2 , and H 2 ), surpassing the scope of existing orifice flow pressure standards. We verify the accuracy of this new standard using an ionization gauge (IG) calibrated for N2 by an orifice flow standard. The gauge calibration factors determined by the cold atom and orifice flow standards differ by less than 0.5% and, thus, agree within their uncertainties of 2% and 2.8% respectively. Using this standard, we evaluate the response of two different IGs to a variety of different gas species and report variations of up to 20% for their measured calibration factors. We also observe a non-linear response of the IG readings for CO2 gas. Finally, we demonstrate the use of a magneto-optical trap (MOT) as a transfer standard to extend the measurement range by a factor of 100 to include pressures up to P ~ 10−5 Pa.
Abstract In our previous work (Shen et al 2020 Metrologia 57 025015; Booth et al 2019 New J. Phys. 21 102001), we have reported the first primary quantum pressure standard based on the loss rate of cold rubidium atoms from a magnetic trap. We have shown that this standard is self-calibrating. That is, the single parameter required to quantify the pressure or particle flux impinging on the sensor atoms, σ t o t v , can be determined experimentally. In this paper, we refine our procedure to extract the trap loss rate coefficient by expressing the measured trap loss rate of the rubidium atoms as a convolution of the universal loss rate expression from Shen et al (2020 Metrologia 57 025015) and Booth et al (2019 New J. Phys. 21 102001) with the energy distribution of the rubidium atoms in the trap. We report improved σ t o t v values for 87Rb–X (X = He, Ar, Xe, H2, N2, and CO2) collision pairs. All are systematically higher than our previously reported values, although the differences are less than 2%. The calibration factor of an ionization gauge for nitrogen obtained using the cold atom standard, i g = 0.950 (19), agrees with the value obtained by NIST, i g = 0.940 (26) calibrated against their orifice flow standard.
We describe the cold-atom vacuum standards (CAVS) under development at the National Institute of Standards and Technology (NIST). The CAVS measures pressure in the ultra-high and extreme-high vacuum regimes by measuring the loss rate of sub-millikelvin sensor atoms from a magnetic trap. Ab initio quantum scattering calculations of cross sections and rate coefficients relate the density of background gas molecules or atoms to the loss rate of ultra-cold sensor atoms. The resulting measurement of pressure through the ideal gas law is traceable to the second and the kelvin, making it a primary realization of the pascal. At NIST, two versions of the CAVS have been constructed: a laboratory standard used to achieve the lowest possible uncertainties and pressures, and a portable version that is a potential replacement for the Bayard-Alpert ionization gauge. Both types of CAVSs are connected to a combined extreme-high vacuum flowmeter and dynamic expansion system to enable sensing of a known pressure of gas. In the near future, we anticipate being able to compare the laboratory scale CAVS, the portable CAVS, and the flowmeter/dynamic expansion system to validate the operation of the CAVS as both a standard and vacuum gauge.
We describe how laser-cooled and trapped alkali-metal atoms have been used as pressure sensors in the high- and ultra-high-vacuum regimes operating with relative uncertainties of just over a percent at pressures from a few nPa and up to a few tens of μPa. These sensors can serve as primary standards of pressure as the determination of pressure requires only knowledge of collision cross sections between atoms and molecules, eliminating the need for calibration with other devices. The collision cross sections can be measured independently or computed with rigorous quantum methods at percent relative accuracies. We argue that this technology has advanced sufficiently to provide an alternative to the existing, expensive pressure standard in the ultra-high vacuum regime, the orifice flow standard. Experiments with trapped atoms can be used to calibrate cheaper pressure sensors (such as devices based on ionization gauges). However, the technology has not yet matured enough to be of use in other research fields because of the complexity and cost of the experiments. We propose a roadmap for this new pressure standard for vacuum metrology.