Issue 50

N. Martini et alii, Frattura ed Integrità Strutturale, 50 (2019) 471-480; DOI: 10.3221/IGF-ESIS.50.39 472 the blue region of the spectrum which provides excellent compatibility with photocathodes, incorporated in various types of photomultipliers (PMTs), charge-coupled devices (CCD), as well as, with non-passivated amorphous hydrogenated silicon photodiode (a-Si:H), employed in thin film transistors of active matrix flat panel detectors [5]. When rare earth phosphors came into the spot-light, CaWO 4 was dis-continued for medical imaging applications. Thereafter, terbium-doped gadolinium oxysulfide (Gd 2 O 2 S:Tb) rare earth phosphor, and later cesium iodide (CsI) were established for digital imaging applications (medical, industrial radiography, etc.) including CCDs and complementary metal oxide semiconductors (CMOS) [3,4,6- 11]. In industrial radiography, non-destructive testing (NDT) is used; it consists of a variety of non-invasive inspection techniques that is used to evaluate material properties, components, or entire process units. Radiographic testing is one of the most frequently used NDT techniques that involve the use of X-rays and digital detector systems, such as amorphous silicon, CCDs and CMOS sensors [12-14]. However, beyond the dominance of terbium-doped gadolinium oxysulfide, the interest for CaWO 4 has been renewed in applications such as, particle astrophysics in the quest for dark matter in the uni- verse [15-17], for WIMP-nucleon elastic scattering interactions [18,19], as well as, for customs and border control [15,20]. The resolution properties of this material [21,22], along with the adequate luminescence emission efficiency, at specific X- ray energies [5] could be also considered for dual energy applications [23-28]. All the aforementioned applications require efficient detectors, of high resolution, with good spectral matching between the phosphor’s emission light and the sensitivity of the sensor, thus the aim of this study is to investigate further the properties of a thin layered calcium tungstate screen, coupled to a state-of-the-art NDT active pixel sensor (APS) CMOS sensor, in order to enhance the imaging capabilities of the integrated detector. Measurements were conducted, following standardized methodologies for medical imaging configurations (sensors and scin- tillator material combinations) [11]. Standardized protocols were used for both resolution and efficiency measurements [5, 29-31]. The latest IEC 62220-1-1:2015 protocol from the International Electrotechnical Commission (IEC) 62220 series was used [32,33]. M ATERIALS AND METHODS Phosphor screens amples of CaWO 4 were extracted from an Agfa Curix universal screen. For the resolution measurements samples with dimensions 2.7x3.6 cm 2 were used. The phosphor is used in the intensifying screens employed in X-ray imaging [31,34,35]. The internal properties of the samples were examined via scanning electron microscopy (SEM) [36]. X-ray absorption (@50keV) 35% Light conversion efficiency 4-5% Melting point 1570-1670 o C Molar mass 287.9156 g/mol Atomic number 74 Density 6.06-6.1 g/cm 3 Afterglow From 5x10 -6 sec up to a few sec Refractive index 1.94 K-edge 69.5 keV Decay time 6-8x10 3 ns Table 1 : CaWO 4 properties [3-5,15,17,37,38]. Scanning electron microscopy (SEM) Parameters such as particle size and thickness of the CaWO 4 compound were verified via SEM micrographs using the Jeol JSM 5310 scanning electron microscope and the INCA software. Within this system, gold can be used to image a site of interest of the sample. For the elementary particle analysis, a carbon thread evaporation process was used. Carbon was flash evaporated under vacuum conditions to produce a film suited for the CaWO 4 SEM specimen in a BAL-TEC CED 030 carbon evaporator (~10 -2 mbar) [36]. S