Novel multi-energy X-ray imaging methods: Experimental results of new image processing techniques to improve material separation in computed tomography and direct radiography.

We present novel multi-energy X-ray imaging methods for direct radiography and computed tomography. The goal is to determine the contribution of thickness, mass density and atomic composition to the measured X-ray absorption in the sample. Algorithms have been developed by our own to calculate new X-ray images using data from an unlimited amount of scans/images of different tube voltages by pixelwise fitting of the detected gray levels. The resulting images then show a contrast that is influenced either by the atomic number of the elements in the sample (photoelectric interactions) or by the mass density (Compton scattering). For better visualization, those images can be combined to a color image where different materials can easily be distinguished. In the case of computed tomography no established true multi-energy methodology that does not require an energy sensitive detector is known to the authors. The existing dual-energy methods often yield noisy results that need spatial averaging for clear interpretation. The goal of the method presented here is to qualitatively calculate atomic number and mass density images without loosing resolution while reducing the noise by the use of more than two X-ray energies. The resulting images are generated without the need of calibration stan-dards in an automatic and fast data processing routine. They provide additional information that might be of special interest in cases like archaeology where the destruction of a sample to determine its composition is no option, but a increase in measurement time is of little concern.

A carbon nanotube based x-ray detector.

X-ray detectors based on metal-oxide semiconductor field effect transistors couple instantaneous measurement with high accuracy. However, they only have a limited measurement lifetime because they undergo permanent degradation due to x-ray beam exposure. A field effect transistor based on carbon nanotubes (CNTs), however, overcomes this drawback of permanent degradation, because it can be reset into its starting state after being exposed to the x-ray beam. In this work the CNTs were deposited using a dielectrophoresis method on SiO2 coated p-type (boron-doped) Si substrates. For the prepared devices a best gate voltage shift of 244 V Gy-1 and a source-drain current sensitivity of 382 nA Gy-1 were achieved. These values are larger than those reached by the currently used MOSFET based devices.

Printing nearly-discrete magnetic patterns using chemical disorder induced ferromagnetism.

Ferromagnetism in certain alloys consisting of magnetic and nonmagnetic species can be activated by the presence of chemical disorder. This phenomenon is linked to an increase in the number of nearest-neighbor magnetic atoms and local variations in the electronic band structure due to the existence of disorder sites. An approach to induce disorder is through exposure of the chemically ordered alloy to energetic ions; collision cascades formed by the ions knock atoms from their ordered sites and the concomitant vacancies are filled randomly via thermal diffusion of atoms at room temperature. The ordered structure thereby undergoes a transition into a metastable solid solution. Here we demonstrate the patterning of highly resolved magnetic structures by taking advantage of the large increase in the saturation magnetization of Fe60Al40 alloy triggered by subtle atomic displacements. The sigmoidal characteristic and sensitive dependence of the induced magnetization on the atomic displacements manifests a sub-50 nm patterning resolution. Patterning of magnetic regions in the form of stripes separated by ∼ 40 nm wide spacers was performed, wherein the magnet/spacer/magnet structure exhibits reprogrammable parallel (↑/spacer/↑) and antiparallel (↑/spacer/↓) magnetization configurations in zero field. Materials in which the magnetic behavior can be tuned via ion-induced phase transitions may allow the fabrication of novel spin-transport and memory devices using existing lateral patterning tools.