Min-seok Yun

Investigation of PbI2 Film Fabricated by a New Sedimentation Method as an X-ray Conversion Material

Radiation detectors are currently fabricated by a screen-print method at room temperature. However, this method has many disadvantages in terms of thickness control and electron trapping. We fabricated polycrystalline PbI2 films by a new sedimentation method and compared the results with those obtained by the existing screen-print (SP) method. We investigated the electrical and structural properties of the films. We fabricated 2 ×2 cm2 sample films with a thickness of about 200 µm. A field emission scanning electron microscopy (FE-SEM) analysis showed that these films were of higher density than those fabricated by a conventional SP method. We also measured the photosensitivity and dark current of the films. The photosensitivity of the films fabricated by the new sedimentation method was 4.8 pC/(mR·mm2), and the dark current was 2.2 pA/mm2 at 1.0 V/µm. The linearity of the film ranged from 3 to 12 mAs, which is promising for diagnostic radiography.

Effect of Annealing on the X-ray Detection Properties of Nano-sized Polycrystalline Lead Oxide Films

Polycrystalline lead oxide (PbO) film is an excellent candidate material for a direct conversion X-ray detector. However, the thick-bulky film tends to significantly reduce the charge collection efficiency for recombination process, and the effective number of electron-hole pairs is lower than that of thin film, because it is difficult to fabricate high-dense and thick PbO films. In this paper, we first synthesized nano-sized PbO particles that could be used in a novel high-efficiency flat panel X-ray detector using a simple solution/combustion method. Energy dispersive X-ray spectrometry, X-ray diffraction, and field emission scanning electron microscopy were used to analyze the component ratio and morphology of the PbO particles as a function of annealing temperature. Then, 150-µm-thick PbO films were deposited on glass substrates using a particle-in-binder method at room temperature. The influences of annealing before deposition on the X-ray detection characteristics of the PbO films were investigated in detail. The key parameters–the dark current, X-ray sensitivity, signal-to-noise ratio, and signal decay–were measured. The annealing conditions strongly affected the electrical properties of the PbO films. The X-ray sensitivity of films annealed in oxygen gas increased dramatically with increasing annealing temperatures up to 500 °C.

Mercury iodide flat panel radiation detector for simultaneous acquisition of static and moving image

Mercuric iodide deposited on flat panel thin film transistor (TFT) array is one of the best alternate photoconductive materials for direct digital X-ray detectors for both static and moving image application in medical imaging. The mercuric iodide is coated onto the array by a Particle-In-Binder (PIB) process and scaled up to the 7inch 8.5inch size required in common medical imaging application. A TFT array with a pixel pitch of 139microns was used for detector. Mercuric iodide coating thickness around 200 microns was tested with beam energy between 40kVp and 100kVp utilizing exposure ranges typical for both static and dynamic imaging. Detector performances were evaluated by obtained images. Mercuric iodide deposited on flat panel thin film transistor (TFT) array is shown to exhibit high sensitivity to X-rays, excellent spatial resolution and high Detective Quantum Efficiency (DQE). Especially it is quite suitable for moving image because of low image lag. Resolution tests on resolution target phantoms showed that resolution is limited to the Nyquist frequency for the 139 microns (resolution ~3.6lp/mm) pixel detectors. The ability to operate at low voltages (~100V) gives adequate dark currents for most application and allows low voltage electronics designs. Also the detector can use exceptionally low dose-rate X-ray illumination because of the very high X-ray sensitivity, which exceeds any other known X-ray detector material. The fabricated detector represents the most advanced photoconductor material available today for flat panel, high resolution, x-ray, medical detector, which alternates conventional a-Se technology.

The optical characteristics of the sample based liquid crystal for using radiation detector

The digital radiation detectors are used clinically by diagnostic apparatus. However the digital radiation detector has some problem like high operating voltage, light blurring, low conversion efficiency and low fill factor. Thus we propose a new radiation detector that the photoconductor layer and liquid crystal layer are coupled in sandwich structure. X-ray absorption in the photoconductor layer controls the state of the liquid crystal via creation of charge carrier and the light modulation of liquid make image formation. The advantages of the new radiation detector are that high resolution image is acquired and the signal amplification is possible by external visible light source. In this study, we study the optical properties and electrical properties of the new radiation detector to irradiate X-ray. The HgI2 was used by photoconductor material, and the aluminum is used by reflective layer. The thickness of HgI2 is about 200um and the operating voltage of the liquid crystal is from 1.5V to 5V. The electrical properties of HgI2 were measured, and the transmission efficiency of liquid crystal was measured by modulation potential.

Characteristic study of multi-layer using Hybrid method for digital X-ray detector

Recently digital X-ray detector, often called Active-Matrix Flat Panel Detector (AMFPD), have been researched by using either the direct or the indirect conversion method in order to form the digital image. In this paper, we introduced the concept of Hybrid method using phosphor layer to complement a weak point of direct conversion method having low conversion efficiency about X-ray. The Hybrid receptor scheme consists of a photoconductor layer, phosphor layer and top/bottom electrodes to collect charges produced in the photoconductor layer. A light reflective layer was deposited under the phosphor as lower layer. Mercury Iodide (HgI2) is used as photoconductor layer to detect X-ray and light from the phosphor, and Cesium Iodide (CsI) is used as phosphor layer to convert X-ray to light. We fabricated a photoconductor (HgI2 200um) and a phosphor (CsI 50um) using PIB(Particle In Binder) method. As top and bottom electrodes, ITO (Indium Tin Oxide) layers were deposited to photoconductor layer and light reflective layer(Al 15 to 25um) in order to improve photon conversion efficiency. As results, higher X-ray sensitivity of Hybrid method exhibited comparing with that of direct conventional digital X-ray detector. The Hybrid method shows about 1.5 times higher sensitivity than the direct conversion method. This effect is caused by simultaneous detection of electric signal induced by the direct X-ray absorption in photoconductor layer and by the light absorption produced in phosphor layer. The Hybrid method can solve problems such as lower conversion efficiency of photoconductor, breakdown because of high voltage application in the direct conversion method as well as low fill fator of TFT, low electric image signals and low SNR in indirect conversion method.

HgI 2 Flat Panel Radiation Detectors for Medical Imaging Acquisition

Mercuric iodide deposited on flat panel thin film transistor (TFT) array is one of the best alternate photoconductive materials for direct digital X-ray detectors for both static and dynamic application in medical imaging. The mercuric iodide is coated onto the array by a Particle-In-Binder (PIB) process and scaled up to the 7inch × 8.5inch size required in common medical imaging application. A TFT array with a pixel pitch of 139microns was used for detector. Mercuric iodide coating thickness around 200 microns was tested with beam energy between 40kVp and 100kVp utilizing exposure ranges typical for both static and dynamic imaging. Detector performances were evaluated by obtained image. Mercuric iodide deposited on flat panel thin film transistor (TFT) array is shown to exhibit high sensitivity to X-rays, excellent spatial resolution and high Detective Quantum Efficiency (DQE). Resolution tests on resolution target phantoms showed that resolution is limited to the Nyquist frequency for the 139 microns (resolution 3.6lp/mm) pixel detectors. The ability to operate at low voltages (100V) gives adequate dark currents for most application and allows low voltage electronics designs. Also the detector can use exceptionally low dose-rate X-ray illumination because of the very high X-ray sensitivity, which exceeds any other known X-ray detector material. The fabricated detector represents the most advanced photoconductor material available today for flat panel, high resolution, x-ray, medical detector, which alternates conventional a-Se technology.