AVS2017 Session VT-MoA: Material Outgassing, Adsorption/Desorption and XHV

In 2018, after over 30 years of research by national measurement laboratories around the globe, the unit Kilogram is expected to be redefined in terms of a fundamental constant of nature; Planck’s constant. The present definition has not significantly changed in over 120 years and relates to a single cylinder made of exactly one kilogram of platinum-iridium alloy that is stored in a vault in Sèvres, France. In order to connect the present Kilogram measured in air to a redefined Kilogram measured in vacuum, new tools and methodologies have been developed to understand and quantitatively determine the change in mass that occurs when metals are placed in vacuum. While initially concerned with platinum, work has extended to stainless steel and other surfaces. With resolutions on the order of a 100 parts per trillion possible, sorption of less than 0.01 monolayers of water is observable.

In this talk we will present measurement techniques and tools used to quantitatively and traceably determine the weight of water and hydrocarbons desorbed from a surface as it is exposed to vacuum. The techniques have been used to study factors such as pressure, surface roughness and contamination, which influence the quantity and dynamics of desorbed mass. The investigations will be presented in context of efforts at the National Research Council of Canada to make the world’s most accurate measurement of Planck’s constant.

The TDS system, designed for measuring outgassing rate of a diameter of 10 mm sample, with two pumping paths; one is ordinary throughput path with an orifice, the other a UHV path, has been newly developed to measure any molecules on samples with a sophisticated DAQ system.

Two paths are directly connected to the main chamber equipped with devices including a rod-guided halogen heater allowing the sample temperature up to 1200 ^oC .

The throughput path utilizes the UHV equipment to measure the outgassing rate quantitatively and qualitatively. In the case of quantitative hydrogen measurement, the throughput path does not have enough pumping speed since a small orifice diameter has the conductance limitation.

The experimentally acquired system calibration factor for the throughput method is 16.0, which is defined as the ratio of background outgassing rates with the gate valve closed and open using the throughput method.

In order to verify the measurement reliability a NIST Standard Reference Materials (SRM) was introduced to the TDS system. Most of hydrogen was desorbed during the course of heating process up to 800 ^oC for 90 minutes. The area under the H2 peak is proportional to the nominal value of 126.8 wt ppm (uncertainty of 2 %). The hydrogen calibration factor of 4.38686E23 was realized. The system claims the hydrogen measurement resolution of 1.3E-6 wt ppm.

In this presentation we briefly introduce the accurate UHV outgassing measurement system for both qualitative and quantitative analyses, which has an 18 % uncertainty of total outgassing rate with Inficon BPG400 HV gauges and a unique self-calibration function.

Acknowledgements: Results are partially attributed to two national projects sponsored by the Korean Ministry of Trade, Industry & Energy, and the KRISS main project (Contract Nos. 10048806, and 170111649).

NIST has a long history of laser cooling and trapping of neutral atoms, largely motivated by building better time standards or clocks, and has recently begun a program to extend the metrological capabilities of cold trapped atoms to measurement of vacuum. This will align vacuum metrology to the emergent NIST “Quantum SI” paradigm, in which a measurement has intrinsic traceability and the line between sensor and standard is blurred. Since the earliest days of neutral atom trapping it has been known that the background gas in the vacuum limits the lifetime of atoms in the trap. We are inverting this problem to create a quantum-based standard and sensor. Indeed, because the measured loss-rate of ultra-cold atoms from the trap depends on a fundamental atomic property (the loss-rate coefficient or thermalized cross section) such atoms can be used as an absolute sensor and primary vacuum standard. Researchers have often observed that the relationship between the trap lifetime and background gas can be an indication of the vacuum level, but a true absolute sensor of vacuum has not yet been realized. This is because there are many technical challenges that must be overcome to create a device that’s truly absolute and primary. The NIST program addresses these challenges both theoretically and experimentally: we have begun ab initio calculations of collision cross sections between the trapped cold atoms and the background gas and, on the experimental side, we are thoroughly investigating the systematic uncertainties associated with using an atom trap to determine vacuum level, particularly those associated with loss mechanisms (in a non-ideal trap) other than due to background collisions. We are designing and building the apparatus to measure relevant cross sections, and building our first prototype vacuum sensing apparatus. In this presentation, we will discuss our theory progress, and present our newest measurements, as well as discuss how the Cold Atom Vacuum Standard fits into the broader picture of the NIST dissemination of the Quantum SI.