HEAVY-ION AND PROTON-BEAMS IN HIGH-RESOLUTION IMAGING OF A FUNGI SPORE SPECIMEN USING STIM TOMOGRAPHY

Scanning transmission ion microscopy (STIM) tomography as a 3-D imagin g technique has been shown to have a range of applications. The energy of the transmitted ion is detected with nearly 100% efficiency as a f unction of position in the transverse plane. The parameters relating t o transmitted ion energy loss in the sample are imaged with statistics given by the energy loss process rather than Poisson counting statist ics. This enables very fast collection of a set of relatively noise-fr ee 2-D images. Each image is collected after a small rotation of the s ample, and a complete 3-D representation of the sample may be tomograp hically reconstructed. The small beam currents necessary mean that the technique is non-destructive. One of the fields where these non-destr uctive 3-D density structure maps are particularly useful is in the an alysis of biological tissue. The variation of energy loss with project ile atomic number may be exploited to tune the energy loss contrast to the size and density of the sample (heavy ion STIM). This work develo ps this point, and applies it to the imaging of the microscopic struct ure of a 90 mu m diameter mycorrhiza fungi spore. This specimen has be en imaged non-destructively in 3-D using both a 36 MeV C-12 beam and a 2.2 MeV proton beam, both with a spatial resolution of about 1 mu m. The gain in contrast in the carbon median energy loss maps was dramati c as expected. The corresponding improvement in the tomogram was found to be visible but less dramatic. The tomographic sections as well as the median energy loss maps of the vesicular-arbuscular mycorrhiza fun gi spore clearly show the internal structure. Wall morphology data has relevance to germination behaviour of the spores.