Spatial and temporal image characteristics of a real-time large area a-Se x-ray detector

Tousignant, O.; Demers, Y.; Laperrière, Luc; Mani, H.; Gauthier, Philippe; Leboeuf, Jonathan

doi:10.1117/12.602183

Cited by 17 publications

(15 citation statements)

References 8 publications

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“…The ghosting measurement is carried out as described in Ref. 23. The dashed line represents numerical results considering only recombination (f = 1.0), the dash-dotted line represents numerical results considering recombination (f = 1.0) and field dependent charge carrier generation, the dotted line represents numerical results considering effective recombination (f = 0.3) and field dependent charge carrier generation, and the solid line represents numerical results considering effective recombination (f = 0.3) and field dependent charge carrier generation and x-ray induced new trap center generation.…”

Section: Resultsmentioning

confidence: 99%

Ghosting mechanisms in a-Se based direct conversion x-ray image sensors

Kabir

Yunus

Kasap

et al. 2005

Medical Imaging 2005: Physics of Medical Imaging

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The dependence of the x-ray sensitivity of a-Se based x-ray image detectors on repeated x-ray exposures is studied by considering deep trapping of charge carriers, trapped charges due to previous exposures, trap filling effects, recombination between trapped and drifting carriers, x-ray induced new deep trap center generation, space charge effects, and electric field dependent electron-hole pair creation energy. We simultaneously solve the continuity equations for both holes and electrons, trapping rate equations, and the Poisson's equation across the photoconductor for a pulse x-ray exposure by the finite difference method. We also perform Monte Carlo Simulations of carrier transports and obtain almost identical results. The change in relative sensitivity (ghosting) as a function of cumulative x-ray exposures for different levels of trapping and different detector operating conditions are examined. The relative sensitivity decreases with increasing cumulated x-ray exposure. The amount of ghosting in a-Se detectors increases with decreasing applied electric field. The sensitivity reduction at negative bias is greater than at positive bias. The theoretical model shows a very good agreement with the experimental relative sensitivity vs. cumulative x-ray exposure characteristics. The comparison of the model with the experimental data reveals that the recombination between trapped and the oppositely charged drifting carriers and x-ray induced new deep trap centers are mainly responsible for the sensitivity reduction in biased a-Se-based x-ray detectors.

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Section: Resultsmentioning

confidence: 99%

Ghosting mechanisms in a-Se based direct conversion x-ray image sensors

Kabir

Yunus

Kasap

et al. 2005

Medical Imaging 2005: Physics of Medical Imaging

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“…Although the pulse shaping time with SWAD can be greatly reduced by unipolar charge sensing, it needs to be optimized when reabsorption of k-fluorescence photons create a separate charge cloud that arrives with a time delay. A more realistic model of the multi-well structure will be implemented to incorporate the electric field distribution in both the bulk and well, and any position dependent charge collection efficiency can be taken into account through charge transport simulation 11 . Additionally, the effects of energy loss and charge sharing will be investigated for polychromatic spectra used in breast imaging.…”

Section: Discussionmentioning

confidence: 99%

“…Direct conversion active matrix flat-panel imagers (AMFPI) using a-Se with thicknesses from 200 to 1000µm, have had great success in commercial systems for mammography and radiography 10,11 . While a-Se can attribute this success to its high detection efficiency and low cost for large area deposition, current AMFPI detectors have limited performance in low dose applications due to electronic noise in the TFT readout electronics.…”

Section: Existing A-se Detector Technologymentioning

confidence: 99%

SWAD: inherent photon counting performance of amorphous selenium multi-well avalanche detector

2016

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Photon counting detectors (PCDs) have the potential to improve x-ray imaging, however they are still hindered by several performance limitations and high production cost. By using amorphous Selenium (a-Se) the cost of PCDs can be significantly reduced compared to crystalline materials and enable large area detector fabrication. To overcome the problem of low carrier mobility and low charge conversion gain in a-Se, we are developing a novel direct conversion aSe field-Shaping multi-Well Avalanche Detector (SWAD). SWAD circumvents the charge transport limitation by using a Frisch grid built within the readout circuit, reducing charge collection time to ~200 ns. Field shaping permits depth independent avalanche gain in wells, resulting in total conversion gain that is comparable to Si and CdTe. In the present work we investigate the effects of charge sharing and energy loss to understand the inherent photon counting performance for SWAD at x-ray energies used in breast imaging applications (20-50keV). The energy deposition profile for each interacting x-ray was determined with Monte Carlo simulation. For the energy ranges we are interested in, photoelectric interaction dominates, with a k-fluorescence yield of approximately 60%. Using a monoenergetic 45 keV beam incident on a target pixel in 400um of a-Se, our results show that only 20.42 % and 22.4 % of primary interacting photons have kfluorescence emissions which escape the target pixel for 100um and 85um pixel sizes respectively, demonstrating SWAD's potential for high spatial resolution applications.

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“…Figure 4-4 shows an X-ray image obtained with a PbO detection layer 13 . Until now, only amorphous selenium is used in commercial X-ray detectors 7,9 , the other materials still being in their research phases. A common problem of the amorphous or polycrystalline direct conversion materials is their temporal behavior.…”

Section: Direct Conversion Materialsmentioning

confidence: 99%

“…Usually these materials have a high resistivity (10 10 to 10 16 Ωcm) and are operated with relatively strong electric fields (between 0.1 and 20 V/µm). For applications in X-ray imaging, several classical photoconducting materials have been studied, 6 such as amorphous selenium [7][8][9] (a-Se) and the polycrystalline materials lead iodide 10, 11 (PbI 2 ), lead oxide [12][13][14] (PbO) and mercury iodide 11,15,16 (HgI 2 ). Some properties of these materials are listed in Table 4-1.…”

Section: Direct Conversion Materialsmentioning

confidence: 99%

Detectors for X-ray Imaging and Computed Tomography

Overdick

Philips Research

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Spatial and temporal image characteristics of a real-time large area a-Se x-ray detector

Cited by 17 publications

References 8 publications

Ghosting mechanisms in a-Se based direct conversion x-ray image sensors

Ghosting mechanisms in a-Se based direct conversion x-ray image sensors

SWAD: inherent photon counting performance of amorphous selenium multi-well avalanche detector

Detectors for X-ray Imaging and Computed Tomography

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