Scanning electron microscopy - Lin Yangchen

by Lin Yangchen


The author's "atomic bomb" project, inspired by the nuclear experiments at Los Alamos National Laboratory in 1942. Remote camera footage shows the sample chamber of a scanning electron microscope, where fragments of coconut definitives are being subjected to conditions not too different from interstellar space—a vacuum of 10−9 torr and bombardment by electrons accelerated by an electromotive force of up to 20,000 volts. Electrons are blasted from a tungsten cathode, which withstands the mythical forces with high tensile strength, low expansion coefficient, low vapour pressure and a melting point in excess of 3,000 °C. The electrons focused into a narrow beam by an array of magnetic and electrostatic lenses whose last stage is the conical objective visible in the video feed. Secondary electrons dislodged from the surface of the stamp fragment are captured by the Everhart-Thornley detector positioned beside the objective.

Besides the secondary electrons emitted from the sample, primary electrons originating from the gun are reflected off the sample. These so-called backscattered electrons are collected by a separate silicon-phosphorous-nitrogen-type semiconductor detector.


Radioactive plutonium core being prepared for criticality experiments, as the author likes to make-believe.

Fragments of coconut definitives were affixed to the microscope stage with conductive double-sided carbon tape. The microscope's built-in camera acquired a reference image (above) for targeting the electron beam. The specimens were left uncoated to avoid altering fragile features. Paper has a porous and highly irregular surface that is difficult to sputter-coat evenly (Donaldson 2009). Besides a relatively low accelerating voltage of 3.0 kV, the aperture was kept small to reduce electrical current and minimize charging. This also had the advantage of increasing the depth of field, although it yields a noisier image. Still, observation and image capture had to be done fast before the rapid onset of a whiteout caused by accumulating charge. Tropical coconuts can also get dehydration and heat exhaustion if kept in an electrified vacuum for too long.


Non-destructive whole mount secured by strips of acid-free paper.

Electromagnetic shield in the x, y and z planes improvised from copper wire by physicists at the Science Centre to neutralize external fields through destructive interference.

Passage to the Netherworld, visible to living beings only when the planets are aligned. It begins with a prolonged electron bombardment of a coconut definitive at 20 kilovolts. Unable to escape from the thicket of unconductive fibres, the electrons coalesce into a mirror that reflects the beam directly into the detector and overpowers the secondary electron signal (see Wong et al. 1997). The gates of Hell open during a large drop in the accelerating voltage, when the weakened primary electrons are no longer able to punch through the negatively charged mirror. The stamp image undergoes a rapid transfiguration into an image of the electron gun at the top of the accelerating column, like the warping of the space-time continuum in the immense gravitational field of a black hole. Chinese scientists Xie Lilin and Zhang Xiaona called it “one of the most spectacular observations” in scanning electron microscopy. Beyond the boundary of the mirror is the still-visible but deformed image of the stamp surface. The undistorted left side of the micrograph shows the as-yet unaffected fibres of one of the acid-free paper strips.

In a recreation of the conditions of space, a high-temperature plasma (ionized gas) deposits an ultra-fine mist of gold atoms on a postage stamp paper sample in a partial vacuum.

It’s not just electrons flying around in the microscope. The agitated atoms of the sample also radiate X-rays, which are captured by a silicon-drift detector Peltier-cooled to −30 °C. Each element has a unique atomic structure with a unique X-ray energy spectrum, thereby giving away its identity.

The advantage of the electron microscope’s energy-dispersive X-ray spectroscopy over commoner hand-held X-ray fluorescence devices is its ability to probe a very small and precise area and detect much lighter elements in its vacuum environment. It covers elements from beryllium to uranium, while hand-held devices start from around sodium.

Acknowledgements
I am grateful to Wulf Hofbauer and Li Zhen for stimulating discussions and technical expertise, and Science Centre Singapore for the facilities that made this research possible.

References


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