PSI - Issue 66

Martha Dima et al. / Procedia Structural Integrity 66 (2024) 153–160 Author name / Structural Integrity Procedia 00 (2025) 000–000

154

2

1. Introduction Scintillators are used as ionizing radiation sensors in a variety of applications from X-ray medical imaging to application in extreme environmental conditions [1–8]. Cerium-doped Gadolinium Aluminum Gallium Garnet (Gd ₃ Al ₂ Ga ₃ O ₁₂ -Ce or GAGG:Ce) is a material with attractive properties for the aforementioned applications, which has been studied for applications in X-ray biomedical imaging, γ -spectroscopy, astrophysics, and in environmental applications [9–15]. Some of the advantages that make it suitable for biomedical imaging and therefore worthy of study are its high density of 6.63 g/cm 3 , high light yield (LY) up to 56000 photons/MeV, fast scintillation time (~100ns), ability to absorb radiation due to high density [16–18]. Medical imaging is a clinical tool and consistent scintillator performance needs further investigation as it is crucial to the process. GAGG:Ce has stable physical and chemical properties as well as high thermal conductivity (ranging from 510 W/mK@12K to 7.7 W/mK@ room temperature) with an atomic number of 54.4 and the maximum wavelength of the emission spectrum at 520nm which matches the range of photomultipliers and silicon photodiodes [19–23]. Moreover, the material is non-hygroscopic compared to other high light yield materials which however are fragile and hygroscopic, thus require encapsulation for environmental protection, such as the cerium bromide (CeBr 3 ). CeBr 3 has high light yield (6x10 4 photons/MeV) , 45.9 effective atomic number, 19 ns decay time, afterglow of 0.1@3 ms and emission maximum at 380nm [24–26]. Indicative properties of the examined inorganic scintillators are summarized in Table 1 [27–31]. This study aimed to measure the luminescence performance of two Gd ₃ Al ₂ Ga ₃ O ₁₂ (GAGG-45 and GAGG-49) single crystals within a temperature range between 23 and 157 Celsius, under X-ray irradiation. The results were compared with data for CeBr 3 crystals [32].

Table 1. Gd ₃ Al ₂ Ga ₃ O ₁₂ and CeBr 3 scintillator properties. Properties Units

Gd ₃ Al ₂ Ga ₃ O ₁₂

CeBr 3

Density

g/cm³

6.63 54.4 1850

5.1

Atomic number (effective)

45.9 722

Melting point

°C

-

17.7 x 10 -6

Coefficient of thermal expansion

°C -1 Mho

Mineral hardness

8

-

Maximum of emission

nm

520

380 Yes

Hygroscopic

No

2. Materials and Methods 2.1. Experimental set-up

Figure 1 shows the two examined (10X10X10mm) Gd ₃ Al ₂ Ga ₃ O ₁₂ crystals: (i) GAGG-45 with fast decay (<50 ns) and LY of 45000 photons/MeV whereas the second ii) GAGG-49 with a balanced decay (<90 ns) and LY of 49000 photons/MeV) single crystals, both obtained from Advatech [33]. For the irradiation of the crystal samples, a typical radiological X-ray tube (CPI model CMP 200 DR high frequency X-ray unit and IAE SpA-RTM90HS X-ray tube) was used, with voltage settings 90 kVp @ 63mAs, and large focus (1.2 mm). The source to detector distance (SSD) was set to 0.725m with additional external aluminum (Al) filter of 20 mm [34]. Samples were inserted in the input port of an integrating sphere (Oriel model 70451). The light that was produced by the samples, after X-ray excitation, was collected by a photomultiplier tube (PMT, EMI model 9798). The PMT’s photocathode was the extended sensitivity (S20). The dose was monitored using an RTI Piranha P100B dosimeter. In our experiments, the PMT works with a 22 volts difference between the initial dynode (all dynodes shorted) and the photocathode. The crystal’s temperature within the examined range (23°C to 157°C) was monitored with a thermocouple that was attached on samples surface with Teflon tape covering all sides except from the side looking within the integrating