PSI - Issue 47

Stavros Tseremoglou et al. / Procedia Structural Integrity 47 (2023) 119–124 Tseremoglou/ Structural Integrity Procedia 00 (2023) 000 – 000

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The AE values of LaCl 3 :Ce crystal scintillator are affected by the energy absorption efficiency (EAE), which describes the fraction of the incident X-ray absorbed in the crystal, and the Light Yield (LY) of the crystal. The total number of optical photons generated in the crystal can be calculated theoretically by the relation (Tseremoglou et al. 2022): = ∙ ̅ ∙ (1) where ̅ is the mean energy of the incident X-ray photons in MeV. At 90 kVp, the mean energy ̅ of the X-ray photons was calculated as 0,06048 MeV, by means of literature (Tseremoglou et al. 2022): ̅ = ∫ ( ) ∫ ( ) ⁄ (2) At room temperature, the LY value is 49.000 photons/MeV and the EAE value was calculated as 0,539, by an analytical formula described in literature (Tseremoglou et al. 2022). From relation (1), the number of optical photons generated per incident X-ray at room temperature was calculated as L=1597. The number of optical photons can be correlated with the experimental value of AE at room temperature through a transmission factor (F T ): ( )= ∙ (3) where F T denotes the fraction of the optical photons generated in the material that is actually escape to the output and AE(T) denoted the possible changes of AE with temperature. With exposure remaining constant, the F T value is mainly depended upon the optical properties of the crystal, which affect the propagation of photons within the crystal and their escape from it. Refractive index is a major factor for the efficient escape of the photons from the crystal, as it determines the critical angle for the total internal reflection (Salomoni et al. 2018, Cowles et al. 2022). The higher the value of the refractive index the lower the critical angle and therefore the probability of an incident photon at the exit surface of the crystal to escape from it. In general, with increasing temperature the value of refractive index decreases, due to density reduction (Wraxler et al. 1973). In the temperature range under investigation, due to the small dimensions of the crystal, we assume that such a change would not affect significantly the refractive index and thus the value of the transmission factor (Cowles et al. 2022). Other parameters that could affect the light collection efficiency of the crystal such as wrapping material, the edge quality and the shape of the crystal, are temperature independent (Salomoni et al. 2018, Lan et al. 2019, Roncali et al. 2017, Alenkov et al. 2013). For normal room temperature the transmission factor (F T ) was calculated from relation (3) as F T = 0,019. As the emission spectrum of the crystal is little affected by the temperature change (Guillot-Noel et al. 1999), the calculated number of optical photons L at different temperatures are shown in table 1.

Table 1. Calculated number of optical photons at different temperatures.

Temperature ( o C)

Absolute efficiency (AE)

Number of optical photons

29 45 55 69 84

33,14 20,80 19,43 18,26 17,96 17,88

1741 1093 1021

959 944 940

162

As it can be observed, with increasing temperature the number of optical photons produced is reduced, as a result of the LY reduction. This reduction, as mentioned above, is a result of de-excitation processes without the emission of radiation. In the matter of inorganic scintillators with Ce 3+ (or Eu 2+ ) activator, thermal quenching affects the 5d - 4f transition, which is responsible for the light emission. Due to the increase in temperature, the 5d excited electron receives the required energy to shift to the conduction band of La. The energy required for this transition is related to