Taking the Jiggles Out of One Angstrom Microscopy
December 19, 1997
By Paul Preuss, paul_preuss@lbl.gov
John Turner of the National Center for Electron Microscopy describes the
environment of the new One Ångstrom Microscope (OÅM) and its
neighbors as "the most extensively designed anywhere." The OÅM, based on
a CM300 Field-Effect-Gun microscope built by Philips Electronic Instruments, is
closeted inside an isolated environment planned from the ground up.
Turner visited the site on Christmas Day, 1995, and what he saw astonished him.
By his standards, the vibration was extreme, especially in the crucial
wavelengths of one to five hertz. The readout of a seismic station a quarter of
a mile away confirmed his observations; he called the UC Berkeley Seismological
Laboratory and found that he was observing evidence of a storm track -- the
crashing of breakers against Pacific beaches dozens of miles away.
In order to damp the "micro-seisms" caused by storm winds and distant surf, and
building-borne vibrations too, the OÅM and its neighbors -- a CM200 also
built by Philips, and the custom-built Spin-Polarized Low Energy Electron
Microscope -- rest on massive reinforced concrete slabs which in turn rest on
undisturbed earth. The slabs are a meter thick and more than three by four
meters wide; each weighs 34 tons. To prevent any contact of the slabs with the
surrounding foundation and to aid in damping vibration, the spaces between them
are filled with neoprene, the same material used in wet suits.
On these solid foundations are built rooms that minimize not only mechanical
vibration but acoustic noise -- the walls are sound-proofed, and pumps and other
machinery are in a separate sound-proofed room. Stray electromagnetic fields
are minimized, too -- the main transformer is 20 meters away, outside the
building, and the wiring runs through the second floor ceiling and down to the
microscope equipment rooms. As to variations in temperature, "The temperature
in here changes at most 0.15deg. C," says Turner. "We're aiming to get that
down to 0.1deg.C."
For the success of the isolated-environment project, Turner credits Charles
Allen of the Lab's Facilities Department and Project Manager Greg Raymond, who
sadly died of an unexpected illness during construction.
Not long after the OÅM was installed, researchers confirmed the hoped-for
one-angstrom information limit with a test specimen of microscopic particles of
gold on a thin layer of amorphous tungsten. That goal could not have been
achieved without an almost ideally vibration-free, noise-free environment.
When you're trying to achieve good image resolution at one angstrom -- which is
roughly the distance between atoms -- getting rid of jiggles becomes a major
undertaking.
Before construction could begin, Turner needed to find out what vibrations the
OÅM would undergo when no machinery was running, no trucks were driving
by, and no people were walking the halls. "There are built-in mechanisms to
damp high-frequency vibration," he explains, "but they don't help at very low
frequencies that resonate with the two-meter length of the microscope
itself" -- vibrations, that is, which can cause the microscope column to resonate
as an inaudibly low-pitched organ pipe might do.
Concrete vibration mounts for the microscopes
