Compact TEM beats coherence problem


Rebecca Pool

Tuesday, October 22, 2019 - 19:45
Image: High-voltage TEM is small enough to fit inside a university lab
Japan-based researchers have built an incredibly compact high-voltage transmission electron microscope, using radio frequency cavities to chop and accelerate electrons into coherent beams.
The instrument is small enough to fit into in a university lab and is set to image samples an order of magnitude thicker than those measurable with conventional TEMs.
High-voltage TEMs are already available for studying micrometre-thick samples, but these instruments accelerate the electrons using massive DC voltages and require shielding in building-sized facilities.
Designers have been trying to overcome this size problem by using RF cavity accelerators since the 1970s, but the lack of coherence of the electron beams from such accelerators has made this difficult.
To overcome this issue, Professor Takumi Sannomiya of Tokyo Institute of Technology used a linear accelerator in the form of a series of radio frequency cavities that maintained beam coherence.
According to the researchers, the electron beam is created at 100 kV using a standard TEM accelerator, and then passed through two RF cavity splicers and 'chopped' into synchronised pulses.
Using a 400 kV RF-cavity accelerator, these synchronised electron pulses are then accelerated to energies of 500 kV, about half that achieved in the large-scale facilities, ensuring that coherent pulses are then passed through the sample.
After passing through a sample, the transmitted electrons are decelerated down to 200 kV with a final RF cavity accelerator, so that they can be focused and detected using a conventional TEM set-up.
Using their compact, high voltage-TEM, Sannomiya and colleagues have imaged reference samples to sub-nanometre resolution.
They now hope to use superconductor cavities that will accelerate electrons to even higher voltages, so the TEM can be made even smaller.
Research is published in Physical Review Letters.
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