Turning an Accelerator into a Microscope
A transmission electron microscope (TEM) can image a sample with subatomic spatial resolution. The sample, however, must be thinner than about 100 nm to sufficiently transmit the TEM electron beam—an important restriction when studying biological cells and tissues or defects buried inside materials. Since the transmittivity of a sample increases with electron energy, accelerating the electrons can relieve these thickness constraints. Now, Takumi Sannomiya of Tokyo Institute of Technology and co-workers have demonstrated a compact TEM in which the electron energy is boosted by a linear accelerator. Such a TEM could image samples an order of magnitude thicker than those measurable with conventional TEMs.
High-energy TEMs able to study micrometer-thick samples have already been developed. Those machines, however, accelerate the electrons with massive dc voltages, whose shielding requires building-sized facilities. To reduce the device size, the idea of Sannomiya and co-workers is to use a linear accelerator in the form of a radio frequency cavity. In such a cavity, a propagating electromagnetic wave produces an oscillating potential that accelerates a train of electron pulses timed to coincide with the potential peaks.
The team’s device fits in a laboratory room and accelerates electron pulses to energies of 500 keV—about half that achieved in the large-scale facilities. After passing through the sample, the electrons are decelerated by another radio frequency cavity so that they can be measured with a standard TEM detector. Imaging reference samples with known geometries, the team demonstrates that the setup can perform microscopy with high-energy electrons while achieving subnanometer resolution. The use of cavities made of superconductors could produce electrons with even higher energy and make the device smaller and more energy efficient.
This research is published in Physical Review Letters.
Matteo Rini is the Deputy Editor of Physics.