Assessment and development of novel transition metal oxide radiation detectors.
Doctoral thesis, UCL (University College London).
New radiation detector materials based on transition metal oxides [TMOs] are being developed for applications in radiation protection. The TMOs are cheap, simple to manufacture and have semiconducting properties, indicating either p-type or n-type characteristics. Thus, p-n hetrojunction diodes may be formed by combining p-type and n-type specific TMOs to produce radiation sensitive detectors. This is done by layering the materials in a flame-spraying process. Work by Mott on amorphous semiconductors has inspired this work to addressing the theoretical analysis of the disordered TMOs. It is found that charge carriers are polarons due to the presence of localised states in the TMOs. Moreover, the charge transport mechanism is ―Mott Hopping‖, whereby conductivity obeys the relationship of log σ ~ -[1/T]1/4. Practically, early studies have shown that TMO detectors perform successfully at room temperature as ionising radiation detectors. However, limitations existed in their very early design, which used Cu tape electrodes and, though demonstrated acceptable levels of response. This response did not meet the full expectation of the TMO detectors. New electrode deposition and materials, therefore, are sought to form ohmic and well-deposited electrodes. In this work, electrodes of Ag and Al were physically vapour deposited on either side of the detector. The electron microscope was used to fully evaluate the structure of the TMO materials. It is found that the flame-spray deposition is uniform with 3.56% of surface variation. Bulk and surface were scanned and were found to vary by 3.18% with variation in oxygen level of 0.55%. Mechanical polishing has shown a high level of validity to improve the surface by 94%. TMOs have demonstrated superior absorption efficiency of 19%, compared to Si [5%] of 200 μm thickness with 33 keV of incident photon energy. In contrast, CZT showed a superior intrinsic efficiency of 0.47%, compared to TMOs [0.02%] at 33 keV. In terms of x-ray and UV radiation tests, excellent correlations between TMO detectors, and IC and Si photodiode were observed. Also, detection was found to be uniform as expected across the p-n hetrojunction with a variation of 6% as an average. Finally, the TMO detectors were claimed to be feasible for detection of photons in the energy range of UV up to diagnostic x-ray.
|Title:||Assessment and development of novel transition metal oxide radiation detectors|
|Open access status:||An open access version is available from UCL Discovery|
|UCL classification:||UCL > School of BEAMS > Faculty of Engineering Science > Medical Physics and Bioengineering|
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