Sang Sik Kang

Synthesis and Characterization of Y2O3:Eu Phosphor Derived by Solution-Combustion Method

Y2O3:Eu powder was synthesized using a solution-combustion method by dissolving (CH3CO2)3Y and (CH3CO2)3Eu in a methyl-alcohol solution. The results of X-ray diffraction analysis (XRD) and thermogravimetry (TG)-differential thermal analysis (DTA) show that Y2O3:Eu crystallizes completely when the dry powder is sintered at 500°C. The optical properties were determined using the attained photoluminescence emission spectra, excitation spectra, and luminescence decay curves. No difference was observed in the optical spectra with respect to europium (Eu) concentration, which depends on emission intensity. The mean lifetime of the synthesized phosphors ranges from 2.3–2.6 ms.

Comparison of Amorphous Selenium and Mercuric Iodide Film for Diagnostic X-Ray Imaging

In this paper, we present the imaging parameters and compare both mercuric iodide (HgI2) and amorphous selenium (a-Se) films. Using MCNPX code, we designed the film structure and its thickness for the optimized detector in the diagnostic x-ray range. The mercuric iodide film was formed by a wet binder process, while the amorphous selenium film was deposited by physical vapor deposition (PVD). These deposition methods are capable of being scaled up to sizes required in diagnostic imaging applications. The electronic properties are investigated using dark current, x-ray sensitivity and signal to noise ratio (SNR). From our results, the developed HgI2 film as an alternative to a-Se photoconductor, which is in practical use in flat panel x-ray imaging detector, will prove its usefulness in the future design and the optimization for various diagnostic modalities.

Comparison of semiconductor radiation detectors for large area X-ray imaging

We have developed a large area, flat panel detector for general applications to digital radiology. This paper presents the x-ray detection characteristics with various semiconductor radiation detectors (HgI2, PbI2, PbO, CdTe) derived by a novel wet coating process for large area deposition. The wet coating process could easily be made from large area films with printing paste mixed with semiconductor and binder material at room temperature. X-ray performance data such as dark current, sensitivity and signal to noise ratio (SNR) were evaluated. The HgI2 semiconductor was shown in much lower dark current than the others, and also has the best sensitivity. In this paper, reactivity and combination characters of semiconductor and binder material that affect electrical and x-ray detection properties would be verified through our experimental results.

Characterization of a-Se(As:Cl) Film for Phosphor Coupled X-Ray Light Modulators

Amorphous selenium (a-Se) film has the potential to fulfill the requirements of a novel x-ray image detector because of its good photo-to-dark impedance ratio, large area coverage, and low temperature deposition. In this work were studied the structural, optical and electrical properties of thermally- evaporated a-Se film for the phosphor-light modulator (PLM). From the x-ray diffraction and electron microscopy experimental results, the deposited film had an amorphous phase without any re-crystallization or defects. Also, the light absorption in widely visible range of 400 ~ 630 nm was over 95 %. From the electrical measurements, the low dark current density of 2.8 nA/cm2 was obtained at 10 V/㎛. The x-ray sensitivity of the 270㎛-Gd2O3:Eu phosphor coupled 20㎛-Se film was 7.31 nC/cm2-mR. From such experimental results, the novel x-ray detector which incorporates phosphor coupled x-ray light modulator, makes an operation at low x-ray exposure possible, therefore, the applications as a medical imaging detector are shown below.

Implementation of Concurrently 6-Readout Integrated Circuit for High Speed Flat Panel X-ray Detector Systems

Flat panel x-ray detector systems researched not only static imaging but also dynamic imaging. The detector system required the high resolution and high frame rate. In this study, we researched the feasibility of concurrently readout integrated circuit for high speed readout time. We fabricated two type of direct method digital x-ray readout system based on mercury iodide, design of serial and concurrently readout system. The digital x-ray detector systems commonly consisted of 5 part, panel, readout module, gate module, control module, power module and interface module.

Monte Carlo simulation for the hybrid detector design

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.

New CsI:Na-selenium x-ray detector

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.

X-ray detection properties of polycrystalline Cd1-xZnxTe detectors for digital radiography

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.