In this project, we use a high-speed beam chopper to create an electron beam that consists of single electrons, shot at the sample at high repetition rate but exactly only one single electron at a time. These single electrons at precise time points, separated from each other by about one nanosecond, might have a different kind of interaction with the sample, than if the same number and density of electrons would arrive randomly in time at the sample.
We will test, if GHz single electrons cause less beam damage than 10^9/second random-in-time electrons, as was recently reported to have been observed by several other laboratories, who mostly worked on materials sciences samples.
Electron beam damage to biological specimens is the primary limitation of cryo-electron microscopy. The vitrified biological sample is damaged by the bompardement with electrons that are accelerated to half light speed with 300 kV. But how does beam damage actually happen? We only have access to macroscopic observations and statistics. We don't exactly know what happens at the nanoscale and at the level of an individual electron/sample interaction. But our current model is this:
An electron that traverses the sample can interact with the sample in various ways. Elastic scattering results in little or no energy loss of the primary electron, and such electrons can be used for phase-contrast imaging. Inelastic scattering of the primary electron will result in one of several types of secondary effects in the sample, such as the emission of Auger electrons, creation of X-rays or light, scattering of high-energy secondary electrons, or other phenomena. If a primary electron excites a volume plasmon in the frozen sample, then the primary electron has about 27eV energy loss and a plasmon, a wave of free electrons, is created in the sample. That wave will have only a very short life time, things will rapidly come back to normal within the sample.
However, according to a current hypothesis, if during the existence of that plasmon a second primary electron were to hit the sample, then major damage within the sample might occur, such as breakage of covalent bonds, creation of hydrogen gas and radical oxygen, heat, massive phonon creation, etc.. Could this be a mechanism resulting in a strongly non-linear relation between beam damage and the numbers of passing electrons?
In this project, we work with Thermo Fisher Scientific (TFS) amd DrX.works to examine the impact of single electron illumination on the sample. For this, our lab operates a TFS Titan Krios 300kV high-end TEM, equipped with every option needed for highest-resolution performance, while in addition being equipped with a GHz speed resonant cavity between the electron gun and the sample. We will test if the stroboscopic electron illumination, consisting of single electrons that are separated by a guaranteed time window with no electron, allows to perform cryo-EM imaging of biological samples with less beam damage.