All members of the CEI are dedicated to creating a centre for excellence in semiconductor imaging technology. The facilities we have aid us in carrying out our research.
In order to measure the increased high energy X-ray QE sensitivity of deep depletion CCDs, such as the CCD247-FI, a reference detector with a high QE was required. The reference detector chosen was a lithium drifted silicon crystal (SiLi) with a collecting area of 30 mm2 that was depleted to ~4.5 mm by applying a 500 V bias across the crystal, allowing X-ray photons in the energy range 5 keV to 20 keV to be sampled with a QE of approximately 100%. The QE of the SiLi detector is modelled taking into account X-ray photon absorption in the beryllium entrance window and nickel electrode contact. The beryllium entrance window is used to filter out the optical wavelengths and has a thickness of between 8 µm and 12 µm. The uncertainty of the Be window thickness has negligible impact on the QE measurements taken for photon energies greater than 5 keV. The nickel electrode covering the entire crystal surface is used to apply the high bias potential to drive the depletion and has a thickness of ~8 nm, although the exact thickness is unknown. Even though the Ni-K absorption edge is within the energy range of interest, the error imposed by the uncertainty of the thickness is negligible due to its relatively small size. The modelled QE for the SiLi detector with both 8 µm and 12 µm beryllium entrance windows. The true QE of the SiLi detector therefore lies somewhere between these two curves. The SiLi crystal must be cooled during operation to suppress dark current; this is achieved by thermal conduction with liquid nitrogen that is stored in the insulated white tank. The liquid nitrogen tank prohibits the SiLi detector from being translatable and was therefore permanently fixed onto a custom made vacuum flange with the CCD assembly mounted next to it.
It has been demonstrated that p-channel charge coupled devices (CCDs) are more radiation hard than conventional n-channel devices as they are not affected by the dominant electron trapping caused by the displacement damage defect the E-centre (phosphorus-vacancy). The dopant used in the creation of p-channel is boron, which possesses no known mid-gap traps. This test equipment is being used to perform a comparative study of two types of n-channel and p-channel CCDs each type operated under the same conditions. The first type of CCD tested is the e2v technologies CCD47-20, a 1024 × 1024 frame transfer device with a split output register, fabricated using the same mask to form n-channel and p-channel devices. The second type is the high resistivity p-channel CCD227 (1st generation High-Rho) and the n-channel CCD247 (2nd generation High-Rho), both 2008 × 512 pixel devices based on the popular astronomy format of the CCD42. The n-channel devices were irradiated to a 10 MeV equivalent proton fluence of 1.68×109 protons.cm-2, with the p-channel devices irradiated to 10 MeV equivalent proton fluence of 5.03×109 protons.cm-2 and 15.09×109 protons.cm-2. The charge transfer efficiency, leakage current, leakage current non-uniformity, and energy resolution at Mn-Kα is being measured as a function of radiation dose.
David Hall's equipment allows him to use a scintillator coupled to an EMCCD in order to resolve individual interactions inside the scintillator. Multiple frames can be taken in quick succession with hundreds of interactions per frame. These interactions can be analysed individually using sub-pixel centroiding and the data compiled to create an image of a much higher resolution than that achieved with one integrated frame. Analysis of individual events opens up the possibility of energy discrimination through the profiling of each interaction. The equipment allows him to look into the possibilities of using such a system as a high-resolution multi-label imager for medical applications, such as Single Photon Emission Computed Tomography (SPECT).
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