PSI - Issue 10

N. Martini et al. / Procedia Structural Integrity 10 (2018) 326–332

327

N. Martini et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

Phosphor materials made of calcium tungstate (CaWO 4 ) (Kaugars and Fatouros (1982); Derenzo et al. (2003)) and zinc sulfide (ZnS) were used as radiation detectors, in the form of thin screens, for almost a century (Brixner (1987); Nikl (2006)). CaWO 4 properties were questioned for medical imaging applications, since the appearance of rare earth phosphors. Thereafter, terbium-doped gadolinium oxysulfide (Gd 2 O 2 S:Tb) rare earth phosphor dominated digital imaging applications (medical, industrial radiography, etc.) including charged-coupled devices (CCD) and complementary metal oxide semiconductors (CMOS) (Brixner (1987); Cavouras (1998a); (1998b); Nikl (2006); Michail et al. (2011); (2014); (2015a); (2016)). In particular, non-destructive testing (NDT) that is used in industrial radiography consists of a variety of non-invasive inspection techniques 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, charge coupled devices, and complementary metal oxide semiconductors (Souza et al. (2008); IAEA (2011); Kim et al. (2018)). 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 universe (Moszyski et al. (2005); Munster et al. (2017); Zdesenko et al. (2005)), for WIMP-nucleon elastic scattering interactions (Mikhailik and Kraus (2010); Mikhailik et al. (2015)), as well as, for customs and border control (Moszyski et al. (2005); Shahabinejad et al. (2014)). The resolution properties of this material (Koukou et al. (2017a)), along with the adequate luminescence emission efficiency, at specific X-ray energies (Michail et al. (2018)) could be also considered for dual energy applications (Sotiropoulou et al. (2015); Koukou et al. (2015); Sotiropoulou et al. (2016); Koukou et al. (2017b); Koukou et al. (2017c); Martini et al. (2017)). All the aforementioned applications require efficient detectors, of high resolution, thus the aim of this study is to investigate further the resolution 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 scintillator material combinations) (Michail et al. (2016)). Two protocols from the International Electrotechnical Commission (IEC) 62220 series were used for comparison purposes, i.e. the 2003 IEC62220-1 (IEC (2003)) and the 2015 modified IEC 62220-1-1:2015 standards (IEC (2015)).

2. Materials and methods

2.1. Phosphor screens

CaWO 4 samples, with dimensions of 2.7x3.6 cm 2 , were extracted from an Agfa curix universal screen. The phosphor is used in the intensifying screens employed in X-ray imaging (Kandarakis et al. (1997); (1998); Michail et al. (2007)). The internal properties of the samples were examined via scanning electron microscopy (SEM).

2.2. 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).

2.3. CMOS sensor

The CaWO 4 scintillating screen was manually coupled to an optical readout device including a CMOS Remote RadEye HR photodiode pixel array (Michail (2015a)). The CMOS photodiode array consists of 1200x1600 pixels with 22.5 μ m pixel spacing. This sensor was initially manufactured for non-destructive testing (NDT)/industrial inspection applications and especially for tight or difficult to reach spaces. However, due it its unique resolution