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Radiation Hardness Performance of Amorphous Silicon Flat Panels

As the digital Radiography (DR) amorphous silicon flat panel (a-Si FP) continues to advance, new applications are being considered on a regular basis. Some of these applications require high doses in excess of 10 kGy (1 MRad) to be delivered cumulatively. However, very little performance data on the aSi-FP is documented for these high dose applications. Thus, when specifying a FP for an application, in addition to the typical performance characteristics such as contrast to noise ratio (CNR) and basic spatial resolution (SRb) as defined by ASTM E2597 and ASTM E2737, there is a question to performance as function of cumulative dose. A-Si FP’s have several components that include the scintillator, a-Si converter (consisting of the glass, switchingtransistors and embedded photo-diodes), and (crystalline) silicon-based electronics for digitization. Since each ofthese components may receive various levels of dose, the performance of a-Si FP over time is dependent upon howeach of these components responds to dose it receives. While there is published documentation that the non-radiation hard silicon-based electronics begin to exhibit significant failures in performance at 100 Gy (10 kRad),other radiation hard components such as the a-Si converter have been less well studied. Moreover, the performanceof the scintillator also often degrades as a function of dose.This paper will evaluate the performance of a 2530HE a-Si FP with a GOS scintillator as a function of cumulative dose up to at least 40 kGy (4 MRad). In order to perform this evaluation, a portion of the active area of a 2530HE was shielded and the remaining area was irradiated. Regions of interest (ROIs) for comparison were set up in the irradiated and shielded areas for comparison. Measurements of the mean offset (dark field) values and standard deviations of the flood fields were made at least every 10kGy (1.0 MRad) delivered over a period of about 60 days up to an exposure of 40 kGy (4 MRad) after which a full ASTM E2597 evaluation was run for comparison to the initial FP performance. Results showed that the offsets remained relatively constant up to 2 MRad and gradually increased by about 100 counts per pixel in some of the irradiated ROIs at 40 kGy (4 MRad). The standard deviations in the ROIs remained statistically constant to 40 kGy (4 MRad). This implies that the panels should calibrate and remain functional up to 4 MRad. Sample images and contrast to noise measurements will also be presented at various dose intervals. A separate set of electronics was irradiated and confirmed that the typical silicon-based electronics do fail at 100 Gy (10 kRad) and thus should be shielded from the direct radiation source. Careful characterization of a-Si FP as a function of dose will allow the imagers to be used in variety of applications with a confidence in the performance level of imager. Lastly, another panel was irradiated at 9 and 15 MV for a total dose of 5 MRad over 3 days; preliminary results and sample images will also be shown from this experiment.


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