STIM (Scanning Transmission Ion Microscopy)

Scanning transmission ion microscopy (STIM) is a technique in which transmitted ions (e.g., protons or helium ions—alpha particle) are detected; the way the particles loose energy in the sample is exploited to provide information on the structure and density variations within the sample (Fig. 1). STIM is useful for correlating the trace element data with structural features and will be used in this study to facilitate positioning of unstained sections prior to analysis. STIM relies on the peculiarities of the mechanism of energy loss of a high energy ion, caused by ion-electron interactions, as it passes through samples such as a whole cell. Due to the large momentum differences between the ion and the electron, the ion emerges from the ion-electron scattering process at small angles while the probability of large angle scattering from nuclear interactions is negligible (Fig. 2). The ion beam therefore has minimal spreading when it passes through the cell and this makes imaging of fine details possible in relatively thick sections.

The use of high energy ion beams focused to nanodimensions increases the possibility of STIM nano-imaging of tissues and single cells resolving easily the nucleus and several nucleolus, and tissue.

Comparison between proton and electron beam penetration in SiO₂ at 3 MeV and 30 keV, respectively. The p-beam and e-beam images were simulated using SRIM and CASINO software packages, respectively