  The National Institute of Standards and Technology has recently begun a
program to develop a primary pressure standard that is based on ultra-cold
atoms, covering a pressure range of 1 x 10-6 to 1 x 10-10 Pa and possibly
lower. These pressures correspond to the entire ultra-high vacuum range and
extend into the extreme-high vacuum. This cold-atom vacuum standard (CAVS) is
both a primary standard and absolute sensor of vacuum. The CAVS is based on the
loss of cold, sensor atoms (such as the alkali-metal lithium) from a magnetic
trap due to collisions with the background gas (primarily H2) in the vacuum.
The pressure is determined from a thermally-averaged collision cross section,
which is a fundamental atomic property, and the measured loss rate. The CAVS is
primary because it will use collision cross sections determined from ab initio
calculations for the Li + H2 system. Primary traceability is transferred to
other systems of interest using sensitivity coefficients.
