The sensitivity and spatial resolution dependence on the microstructures of CsI:Tl scintillation layer for X-ray imaging detectors Bo Kyung Cha, Jun Hyung Bae, Chae-hun Lee, Sungho Chang, Gyuseong Cho n Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejoen 305-701, Repulic of Korea article info Available online 20 June 2010 Keywords: Scintillation properties CsI:Tl scintillator X-ray imaging abstract Thallium doped cesium iodide (CsI:Tl) scintillator films for the use as a converter for X-ray imaging detectors were fabricated by the thermal deposition method. The microstructures of these scintillating layers were affected by various deposition conditions such as vapor pressure, substrate temperature and post-heat treatment or rapid thermal annealing (RTA). CsI(Tl) scintillator films with various polycrystalline structures were manufactured under different process conditions and prepared for experiments. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate the crystal structure and morphology properties. Light output and spatial resolution of the samples were strongly affected by the microstructures, which are determined by the deposition conditions and post-heat treatment. Imaging characteristics of the various CsI:Tl films were also measured under X-ray exposure conditions by coupling them to a CCD image sensor. & 2010 Elsevier B.V. All rights reserved. 1. Introduction In the last decade, thallium doped cesium iodide (CsI:Tl) scintillators have been widely used in X-ray imaging detectors for medical and industrial applications because of their high light output, proper emission wavelength (550 nm) and high spatial resolution due to light-guiding function within needle structures [1]. In this work, CsI:Tl scintillator thin films were fabricated by thermal evaporation method on glass substrates. The micro- structures of these scintillating layers were determined for various deposition parameters such as vapor pressure, substrate temperature and post-heat treatment or rapid thermal annealing. CsI:Tl scintillator films with various polycrystalline structures were prepared under different process conditions and rapid thermal annealing. Their X-ray imaging characteristics, such as light output, linearity and spatial resolution were measured by X-ray imaging systems [2]. 2. Materials and methods CsI:Tl scintillating layers were fabricated by the thermal evaporation method in our laboratory of Korea Advanced Institute of Science and Technology (KAIST) on 2 Â 2 cm 2 area glass substrates at various deposition conditions using a mixture of CsI and 1.0 mol% Tl powder. A variety of samples were prepared for several different deposition parameters. The evaporation pressure was either 10 À2 or 10 À5 Torr and substrate temperature was either 30 or 200 1C. Rapid thermal annealing in a quartz tube at 250 1C for 30 min was carried out as a post-heat treatment of the CsI:Tl layers in order to diffuse the Tl dopant into the thin CsI scintillating layers [3,4]. The morphology of CsI:Tl scintillating layers deposited at different evaporation conditions is shown in Fig. 1. The measurements were done using field emission SEM (JEM-2100F HR, Jeol Ltd., Japan). The thickness of CsI:Tl scintillating layers manufactured at 10 À2 and 10 À5 Torr pressure was 90 and 50 mm, respectively. Polycrystalline layers with dense, needle (or columnar) shape structures are seen at 10 À2 and 10 À5 Torr evaporation pressure. As substrate temperature increases during the evaporation process, some cracks and voids in the surface section of the CsI:Tl layers disappeared. Crystal structures of the CsI:Tl layers were observed by high resolution X-ray diffraction (RIGAKU Ultima IV) with an analysis range 2y of 20–901. The XRD patterns according to evaporation pressure, substrate temperature and rapid thermal annealing are shown in Fig. 2. CsI:Tl layers deposited at 10 À2 Torr pressure with 30 and 200 1C substrates temperature without rapid thermal annealing have X-ray peaks at (1 1 0), (2 0 0), (2 1 1), (2 2 0), (3 1 0), (2 2 2), (3 2 1) and (4 0 0), which are almost equal to those of cubic phase of pure CsI material. X-ray peaks at (2 0 0) and (2 2 0) were sparingly observed in case of samples deposited at 10 À5 Torr with 30 and 200 1C substrate temperature after post-heat treatment. 3. Results and discussion The light output was measured by combining the CsI:Tl scintillating layers with a lens-coupled CCD imaging system Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.06.194 n Corresponding author. Tel.: + 82 42 350 3821; fax: + 82 42 350 5861. E-mail address: gscho@kaist.ac.kr (G. Cho). Nuclear Instruments and Methods in Physics Research A 633 (2011) S297–S299