Most of us know helium as the
lighter-than-air gas that puts lift into birthday balloons or makes a
baritone's resonant voice sound more like Donald Duck's.
But helium -- a non-toxic element that is abundant in the universe
but a scarce, non-renewable resource on earth -- has many significant
uses in industry, medicine and science.
Researchers at the National Institute of Standards and Technology
(NIST) in Gaithersburg, Maryland, have proposed an improved method for
measuring pressure by using helium rather than mercury. Mercury-based
manometers/barometers are the standard tools of measurement, but
helium-based approach offers significant advantages by avoiding the use
of toxic mercury and making measurement devices more portable and more
precise than those currently in use.
In an article published this week by Physical Review Letters, University of Delaware physics professor
Krzysztof Szalewicz and his collaborators from the University of Warsaw
and the University of Poznan show how they calculated a key property of
the helium atom needed for a precise helium-based barometer -- its
"polarizability" -- at a level of precision 100 times greater than
previously possible either experimentally or computationally.
The helium-based barometer works the following way: A laser beam of
light is sent through two cylinders, one empty, one filled with helium.
Because light moves more slowly in helium than in a vacuum, it appears
to move through a longer cylinder. The difference between the apparent
and true length can be measured very precisely utilizing interference of
light, and this can then be used to determine the speed of light in
helium. Because this velocity depends on helium pressure, the result is
an instrument measuring pressure.
However, the equation connecting the speed of light in helium and
pressure contains a quantity called polarizability. This property of
helium defines how easily it can be squeezed by an electric field, or
how far its electron and nuclear charges separate to form what is called
a "dipole moment."
Szalewicz compares it to trying to squeeze three common items -- a
bowling ball, a football and a balloon. You can't squeeze the bowling
ball. The football could be squeezed a bit, though, and the balloon a
good bit more. So polarizability describes something like the hardness
of an object.
Szalewicz' team developed new theory to calculate helium's
polarizability to within 0.1 parts per million. To do it, they used
quantum electrodynamics (QED) -- an advanced form of quantum mechanics
-- new algorithms and high-performance computers.
In a few years, Szalewicz said, helium-based manometers will replace
mercury-based ones as the standard of pressure, specifically replacing
the 9-foot-high manometer that holds 551 pounds of mercury at NIST with a
suitcase-sized helium-based manometer.
Ultimately, that could provide more accurate instruments for
aviation, where atmospheric pressure is used to gauge altitude, and
other processes requiring precise pressure measurements, such as
manufacture of computer chips.
And the polarizability value his team developed will be part of that advance.
The ongoing work is supported by the National Institute of Standards
and Technology, the National Science Foundation, the Polish Ministry of
Science and the National Science Center.