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STIM tomography of microsamples
Dipl.-Phys Tobias Andrea
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The technique of scanning transmission ion micro-tomography (STIM-T) has been successfully employed in recent years at LIPSION and has proved to be a valuable method for the three-dimensional visualization of the density distribution of micro-samples with sub-micron resolution [1]. In STIM tomography a series of density images of a rotatable sample are obtained by scanning a focused MeV ion beam in low-current mode over the sample and measuring the energy loss of each individual ion. These projections, covering an angular range of 180°, correspond to the Radon transform of the samples density distribution. Tomographic reconstruction is achieved using the backprojection of filtered projections algorithm.
The three-dimensional density reconstruction of the head of a rust mite, 40 µm wide, revealed not only the mites surface structure but also dense grains inside the creatures body [2].
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Figure 1: Reconstruction of a rust mite showing three dense grains
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STIM tomography is an ideal tool for the characterization of proton beam sculpted 3D microstructures. This 90 µm high model of the Eiffel tower was written in PMMA by structured proton irradiation from two angles.
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Figure 2: Reconstruction of PMMA microsculpture of Eiffel tower
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Limited-Angle STIM and PIXE Tomography of Cells
Dipl.-Phys Tobias Andrea, Dipl.-Phys Martin Rothermel
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Cells that have been grown and fixated on a substrate cannot be rotated a full 180°. STIM projections can be obtained only over a range of typically 120°. By supplementing the missing projections iteratively an adequate reconstruction can still be achieved. A specimen of the unicellular organism thaumatomonas coloniensis [3] fixed on a 0.9 µm PE foil was investigated using 1.9 MeV He+-ions. The surface of the 7 µm wide cell including the 250 nm thick flagellum was reconstructed.
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Figure 3: Reconstruction of a thaumatomonas coloniensis cell
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A similar experiment was carried out with a 10 µm thick cell of the green algae species euglena gracilis [4]. By the combination of limited-angle STIM and PIXE (particle induced X-ray emission) tomography the phosphorus distribution (yellow) could be superimposed upon the density reconstruction (blue). The phosphorus-rich regions can be interpreted as the remains of the cells nucleus. The combination of limited-angle STIM and PIXE tomography is very fruitful since STIM has a better resolution and PIXE is more sensitive to intracellular structures [5].
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Figure 4: Reconstruction of euglena gracilis with phosphorus distribution in yellow
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Literature:
[1] M. Schwertner et al.: Ultramicroscopy 106 574 (2006)
[2] T. Andrea et al.: Nucl. Instr. and Meth. B 267 (2009) 2098-2102
[3] C. Wylezich et al.: J. Eukaryot. Microbiol. 54(4) 347 (2006)
[4] P. Guo et al.: Nucl. Instr. and Meth. B 161-163 (2000) 801-807
[5] T. Andrea: Diplomarbeit, Fakultät für Physik und Geowissenschaften, Universität Leipzig (2009)
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