CdTe/CdZnTe pixellated radiation detector

The work in this thesis is focused on the study of CdTe/CdZnTe pixellated detectors. During this research, three main aspects have been covered in the development and application of these detectors. These broadly describe the fabrication process, characterisation of energy spectrum and imaging performance, and application of these detectors in industrial X-ray tomography. The first topic to be discussed in detail is the fabrication process of CZT pixellated detectors. This is a process that involves many steps, such as cutting the sample, cleaning the substrate, writing a masking pattern with e-beam lithography, photolithography, cleaning the surface using oxygen ash, metallisation and removing the resin in the development stage. However, these stages were grouped into three major steps to give a clearer image on what was going on throughout the fabrication process. They are grouped as sample preparation, pixellated structure deposition and sample passivation. The dry ash processing had been carried out in the James Watt Nano Fabrication Centre at Glasgow University. The motivation for the development of the dry ash process was to reduce the level of leakage current on the surface of the device so that a good energy spectrum resolution could be obtained. The research showed that a passivation technique using a layer of SiN as well as an oxygen plasma oxidation layer provided low leakage current in the pixel and lower interpixel leakage current. A good performance in energy spectrum was achieved after performing passivation. The FWHM of Co-57 is 10.78 keV where energy resolution is 8.8%. The second area covered in this thesis is the energy spectrum and imaging performance of pixellated CdTe Timepix detector. Two pitches of pixel have been studied namely, 110 µm pitch and 55 µm pitch. The work on characterisations has been performed with polychromatic and monoenergetic beams of X-rays. In both the Counting mode and ToT (Time over Threshold) mode it was discovered that there were large effects on performance due to charge sharing and fluorescence. In terms of image quality the result of MTF was found to be as expected, around half the contrast transfer (30%) recorded for 110 µm compared to 55 µm pixel detector at 4 lp/mm. Another field of interest explored was the defect factor. For this to be achieved studies had been performed in positive and negative bias of CdTe Timepix detector. The results showed that the presence of defects in the material affects the performance of the energy resolution of CdTe Timepix detector. The third area covered in this thesis was the study of CdTe Timepix detector for use in Industrial X-ray tomography. To give the sensor the best performance it could offer in capturing images for industrial use, work had been carried to optimise the technique for producing good quality images. The hardware setup involved the careful alignment all of the components of the system and the introduction of a collimator. In addition the sofware software it dealt with the flat field correction, noise filter and data projection algorithm. The results show that the CdTe Timepix detector can be used as a good detector for X-ray tomography. Here, the 3D of volume rendering studies had been performed to a pencil as a sample and voltage regulator. The result of the pencil image in volume rendering showed that CdTe Timepix detector had successfully provided good image for different density profile of material (paint coating, wooden and carbon). Not only that, another important finding besides the excellent image produced was that the effect of beam hardening, ring artifact and metal shrank artifact in tomogram of voltage regulator had been eliminated successfully.