Flat-panel direct conversion detectors used in compound substance of semiconductor are being studied for digital x-ray imaging. Recently, such detectors are deposited by physical vapor deposition(PVD) generally. But, most of semiconductors (HgI2, PbI2, TlBr, PbO) deposited by PVD method have shown difficult fabrication process and instability for large area x-ray imaging. Consequently, in this paper, we propose applicable potentialities for screen printing method that is coated on a substrate easily. It is compared to electrical properties among semiconductors such as HgI2, PbI2, PbO, CdTe under investigation for direct conversion detectors. Each film detector was coated onto the substrate and Substrates of 2cmx5cm had been used to evaluate performance of semiconductor radiation detectors. Fabricated films consisted of ~25 to 35 μm thick layer of semiconductor. Dark current, sensitivity, linearity, lag, and morphologic property were measured for evaluation of films performance. Dark current of PbO was acquired the lowest, and dark current of HgI2 at the operation voltage of ~1V/μm was observed 8pA/mm2. Sensitivity is observed higher about ten times than the others. And then HgI2 is observed the best SNR in four materials. In four semiconductors, it is shown in good linearity. Such a value is not better than PVD process, but it is easy to be fabricated in high quality for large area X-ray Imaging. Our future efforts will concentrate on optimization of growth of film thickness that is coated onto a-Si TFT array.
We designed hybrid x-ray detector and simulated using Monte Carlo method. Hybrid x-ray detectors consist of scintillator coupled photoconductor structure. In the hybrid structure, x-ray photons are converted into the light photon in the scintillator layer and light photons are converted into the electric charge in the semiconductor layer. The electric charges can be generated from directly x-ray absorption in the semiconductor material. We design the columnar CsI:Na as scintillator layer and a-Se as photoconductor material. When x-ray photon incident the scintillator layer, the photons are distributed through the scintillator, and then generated light photon influence the semiconductor material. We study the light photon distribution according to the scintillator layer thickness and the detector pixel size which have influence on image resolution.
Digital x-ray imager known to flat-panel detector has been studied for the application of a various medical modalities. Currently, two types of detection methods have been realized in digital radiography. One is an indirect conversion method and the other is a direct conversion method. we have been developing a new x-ray detector that combines a columnar CsI:Na scintillation layer with a photosensitive a-Se with dielectric thin film. In this structure, an x-ray is converted to visible light in a CsI:Na scintillation layer and visible light is then converted to electric charges in a-Se layer. The electron-hole pairs can be also generated from x-ray interaction in the a-Se photoconductor, which can improve the detection efficiency of electric charge. We designed the thickness of CsI:Na scintillator by using computer simulation. MCNP is a general-purpose, continuous-energy, generalized-geometry, time-dependent, coupled neutron / photon / electron Monte Carlo transport code. The spectra of x-ray absorption was simulated by using MCNP 4C code. The morphology of the vacuum deposited CsI:Na scintillator and the parylene film were analyzed. Photoluminescence characterization of CsI:Na showed a light emission peak centered at 420nm as expected, which matched the absorption spectrum of amorphous selenium(a-Se). For an electric field of 10V/μm, the dark currents of our detector were below 370 pA/cm2 and the SNR of CsI:Na coupled a-Se detector with a dielectric layer was 1.8 times greater than that without CsI:Na layer.
Nowadays, large area, flat panel solid state detectors are being investigated for digital radiography. In this paper, development and evaluation of a selenium-based flat-panel digital x-ray detector are described. The prototype detector has a pixel pitch of 139μm and a total active imaging area of 7"× 8.5", giving a total of 1.9 million pixel.
This detector include a x-ray imaging layer of amorphous selenium as a photoconductor which is evaporated in vacuum state on a TFT flat panel, to make signals in proportion to incident x-ray. The film thickness was about 500μm. To evaluate the imaging performance of the digital radiography (DR) system developed in our group, sensitivity, linearity of the response of exposure, the modulation transfer function (MTF) and detective quantum efficiency (DQE) of detector was measured. The measured sensitivity was 4.16 x 106 ehp/pixel•mR at the bias field of 10 V/μm: The beam condition was 41.9 KeV. Measured MTF at 2.5 lp/mm was 52%, and the DQE at 1.5 lp/mm was 75%. And the excellent linearity was showed where the coefficient of determination (r2) is 0.9693.
There has been considerable recent progress in II-IV semiconductor materials and in methods for improving performance of the associated digital x-ray detectors. Cd1-xZnxTe is known as promising medical x-ray detector material. The CdTe and Cd1-xZnxTe (x=0.15,0.25,0.3) detectors were fabricated by vacuum thermal evaporator for the large area deposition. First, the stoichimetric ratio and the x-ray diffraction of the deposited (Cd,Zn)Te films were analyzed by EPMA and XRD. Secondly, leakage current, x-ray sensitivity, SNR, and linearity were measured to analyze the x-ray detection effect of Zn in (Cd,Zn)Te film. Experimental results showed that the increase of Zn concentration rates in Cd1-xZnxTe detectors reduced the leakage current and improved the x-ray detection performance.