A comparative study on the scintillation characteristics of Gd2SiO5:Ce and Lu1.2Gd0.8SiO5:Ce scintillators

In the present day, inorganic scintillators have been developed to detect high-energy photons such as x-rays and γ-rays. They have been applied in many works, such as astronomy, materials inspection, high energy physics, industrial application, and natural resources exploration. Especially in the medical field, they are used as radiation detectors in diagnostic equipment such as PET scanner, x-ray CT scanner, SPECT scanner, etc. This research studied the scintillation characteristics of Ce:GSO and Ce:LGSO single crystals. We found that at 662 keV gamma-ray energy, Ce:LGSO crystal has about 2.3 times more light yield than Ce:GSO crystal, but the energy resolution of both crystals is not different. For the degree of non-proportionality of both crystals, Ce:LGSO scintillator shows 0.08, which is better than the 0.19 obtained from Ce:GSO scintillator. This work also discusses the two scintillators’ peak-to-total ratio, loss parameter, and intrinsic light yield.


Introduction
The study and development of modern scintillators have gained interest due to the rapidly growing need for advanced medical technologies and high-energy physics research.Over the last 20 years, new scintillation materials have been seriously researched, especially inorganic scintillators doped with cerium.Some have been successfully developed for commercial manufacturing.There are many good reviews concerning the study of the characteristics of these materials, including from van Eijk [1], Moszynski [2], Nikl [3], and Lecoq et al [4].The desirable scintillators for various works should have the following attributes: high light-emitting, good light yield proportional response, excellent energy resolution, fast response time, non-hygroscopic, detect high-energy photons well, and almost no afterglow.The quick and efficient transition of (5d → 4f energy level) luminescence makes the Ce 3+ ion one of the most interesting activators.Especially in oxyorthosilicate groups such as Gd 2 SiO 5 :Ce (Ce:GSO) [5], Lu 2 SiO 5 :Ce (Ce:LSO) [6], and (Lu,Y) 2 SiO 5 :Ce (Ce:LYSO) [7,8] were intensively tested and found that they show very light-emitting, detects high-energy photons well and fast response time.The Ce:GSO scintillator is used in geographical investigations because of the scintillation stability at high temperatures [9].While the Ce:LSO and Ce:LYSO scintillators are utilized in PET scanners (positron emission tomography) because of their high light-producing and fast decay time.The Ce:GSO scintillator exhibits a light yield of roughly 10,000 photon/MeV [10], which is insufficient for a PET scan application.Replacing the Gd with an appropriate amount of Lu will make the Ce:GSO scintillator can emit more light, an alternative for PET scanners [11].The Ce:GSO scintillator is an inorganic non-hygroscopic scintillator.Its density is about 6.70 g cm −3 , and its effective atomic number is 57.Its emission wavelength is around 430 nm.Moszynski et al [10] observed the scintillation characteristics and found that at 662 keV γ-ray energy, the Ce:GSO scintillator showed an energy resolution of about 8.1% and a scintillation light yield of around 10,400 photon/MeV.The mixed oxyorthosilicate crystals (Lu 2x Gd 2−x SiO 5 :Ce or Ce:LGSO) at x = 0-1 were reported by Sidletskiy et al [12].They increased the light yield by around 70% compared to Ce:LSO crystals and achieved an energy resolution of about 7% at 662 keV ( 137 Cs) by selecting the appropriate Lu to Gd ratio and Ce 3+ concentration.The choice of host composition and activator concentration in Ce:LGSO crystals must be made with a minimum focus on Ce 3+ content in CeO 2 polyhedra and energy transfer between Ce1 and Ce2 luminescent centers.The 1-3 orders of magnitude also reduced the afterglow level compared to Ce:LSO.The difference in interatomic distances between the Ce 3+ luminescence centers (dopants) and the electron traps at the surrounding oxygen atoms that are not bonded to silicon can cause the Ce:LGSO's surprisingly low afterglow level when compared to Ce:LSO.The significant afterglow in LSO-based crystals is caused by the recombination of electrons released from traps with holes at luminescent centers, which the inclusion of Gd should inhibit.Sidletskiy et al [12,13] found that the Lu 2x Gd 2−x SiO 5 :Ce (x = 0.6) or Lu 1.2 Gd 0.8 SiO 5 :Ce scintillator showed an energy resolution of about 6.7% and a light yield of approximately 29,000 photon/MeV.The Ce:LGSO scintillator is a nonhygroscopic.Its density is 6.90 g cm −3 , and its effective atomic number is 64, greater than the Ce:GSO scintillator.The emission peak of the Ce:LGSO scintillator is about 405 nm.Wang et al [14] recently gave a review of the current status and future requirements for using scintillators in radiographic imaging and tomography (RadIT).At the SCINT22 conference, more than 160 different types of scintillators were presented, giving researchers.There are many options for implementing the new RadIT.The Ce:LGSO crystals are still attractive for future PET scanners due to their high density, low afterglow, good energy resolution, fast response time, etc.
In this work, we have chosen to study the scintillation characteristics of the Lu 1.2 Gd 0.8 SiO 5 :Ce (Ce:LGSO) and Gd 2 SiO 5 :Ce (Ce:GSO) scintillators.The photon yield, energy resolution, intrinsic resolution, light yield non-proportional response, intrinsic light yield, and loss parameter of both scintillators were investigated at room temperature.At 662 keV γ-ray energy, the peak-to-total ratio (PTR) value has been studied and compared to the cross-section ratio obtained from the NIST XCOM software [15].

Experimental setup
Gd 2 SiO 5 :Ce (Ce:GSO) and Lu 1.2 Gd 0.8 SiO 5 :Ce (Ce:LGSO) single crystals were supplied by AMCRYS company, Ukraine.The Ce:GSO and Ce:LGSO crystals were grown with cerium concentrations of 0.5% and 0.3%, respectively, using the Czochralski technique.Both polished crystals on all six sides comprised the same dimensions (5 × 5 × 1 mm 3 ).The Archimedes method calculated the Ce:GSO and Ce:LGSO scintillators' density as 6.70 and 7.07 g cm −3 , respectively.Excitation and emission spectra of both scintillators were investigated using a Hitachi F-4600 fluorescence analyzer.The photoelectron yield (phel/MeV) calculation was carried out using optical grease, connecting the bare crystal to the Photonis XP5200B photomultiplier tube (PMT) window.Seven layers of Teflon and black tape coated the bare crystal and window of the PMT.Each radiation source is placed in front of the crystal.The radiation sources used include 133 Ba, 137 Cs, 241 Am, 152 Eu, and 22 Na.When gamma radiation hits the crystal, the gamma-ray photon will be converted into several light photons and enter the PMT.The electrical signal from an anode plate of the PMT was sent to the CANBERRA 2005 scintillation preamplifier, transferred to the CANBERRA 2022 spectroscopy amplifier, and entered a multichannel analyzer (MCA) [16] based on Tukan8k PC for analysis and display spectrum.The numbers of photoelectrons were obtained by the Bertolaccini method [17,18] by comparing the location of the photopeak of gamma radiation on the MCA display caught in scintillators with that of a peak of single photoelectron obtained from the cathode in the PMT.For intrinsic light yield and loss parameter testing, we took crystals cut from the same ingot in three sizes, 5 × 5 × 1 mm 3 , 5 × 5 × 3 mm 3 , and 5 × 5 × 6 mm 3 , and tested them with 662 keV energy gamma rays, and calculated photon yield for each of them.And we were using the two-ray model of Wojtowicz et al [19] to fit the curve to find the values in the equation.

Scintillation light yield
The emission and excitation spectra of Ce:LGSO and Ce:GSO scintillators obtained with a fluorescence analyzer are illustrated in figure 1.The bands of excitation in the 230-400 nm region for both scintillators are caused by the allowed 4f → 5d 1,2,3 transitions of Ce 3+ center.The emission band resulting from the 5d 1 → 4f transitions of the Ce1 center in Ce:LGSO and Ce:GSO exhibited peak wavelengths of 410 nm and 430 nm, respectively.Typical Gd 3+ 8 S 7/2 → 6 I x absorption line around 275 nm can be seen in the excitation spectrum of Ce1 luminescence center in Ce:LGSO crystal.The energy transfer from Gd 3+ to Ce 3+ is possible because the Gd 3+ 6 P x → 8 S 7/2 emission line around 312 nm and the Ce 3+ 2 F 5/2 → 5d 2 absorption line overlap around 313 nm, as shown in figure 1.
Figure 2 presents the 662 keV energy spectra from the 137 Cs source of the XP5200B PMT-connected Ce: LGSO and Ce:GSO scintillators.In figure 2, the location of the photopeak of the Ce:LGSO scintillator is at a greater channel number than that of the Ce:GSO scintillator.This implies that the number of photoelectrons released from the Ce:LGSO scintillator's PMT photocathode is greater than that of the Ce:GSO scintillator.The photoelectron production obtained from the Ce:LGSO scintillator of around 7,710 ± 770 phel/MeV is greater than that obtained from the Ce:GSO scintillator (3,170 ± 320 phel/MeV).These photoelectrons can convert to the light photons emitted by the crystal using the quantum efficiency (Q.E.) value of the photocathode in XP5200B PMT.From figure 1, the Ce:LGSO and Ce:GSO crystals have maximum emission wavelengths of 410 nm and 430 nm, which are related to the quantum efficiency of the XP5200B PMT, which are 27.0%and 25.2%, respectively.The number of photons relative to the measured number of photoelectrons can be calculated from the equation: the number of photoelectrons the number of photons 100 1 = The scintillation light yield of Ce:LGSO and Ce:GSO scintillators will be 28,600 ± 2,900 photon/MeV and 12,600 ± 1,300 photon/MeV, respectively.The light yield of studied Ce:LGSO in this work and Ce:LGSO with the same size in [20] are comparable.Although the scintillation light yield of the Ce:LGSO scintillator is about 2.3 times more than that of the Ce:GSO scintillator, both scintillators' energy resolution was almost no different.For both scintillators, the energy resolution, the yield of photoelectron, and the yield of photon at 662 keV gamma radiation are shown in table 1.
It is notable that the photon yield of about 12,600 ± 1,300 photon/MeV obtained from the tested Ce:GSO sample is higher than that (10,400 photon/MeV) obtained from the bigger size (10 × 10 × 5 mm 3 ) Ce:GSO in [10].Its energy resolution is also superior (7.20% versus 8.10%).These results may be achieved by improving the performance of the new generation of Ce:GSO scintillators.For testing the Ce:LGSO scintillator, its energy resolution value and number of photons are slightly worse than the Ce:LGSO scintillator in [12] with the dimension of 10 × 10 × 2 mm 3 .This result could be due to the cross-sectional area being almost four times larger than the samples in [12], allowing crystals to emit more light photons and are more likely to enter the photomultiplier tube.In some studies, it was possible to increase the light yield of the Ce:LGSO crystal with 83% Lu in the host to 33,700 photon/MeV by using Ca 2+ co-doping, as in [13].However, the increased light yield did not improve the crystal's energy resolution (8.1% at 662 keV).

Energy resolution
The energy resolution (ΔE/E) is the detector's efficiency in accurately resolving the incoming radiation's energy.It is known as the photopeak's full width at half maximum (FWHM) at a specific energy.This ΔE value is the FWHM value obtained from the radiation spectrum on the MCA display.As an equation (2) [21], the energy resolution of a full energy peak detected with a single crystal connected to a PMT can be written: where δ sc is the scintillator's intrinsic resolution, closely associated with its non-proportional response of light yield [21,22], inhomogeneity, and crystal defects in the scintillator, which can cause variations in light photon production.The δ st is the PMT resolution (the photomultiplier tube's statistical contribution to the resolution), and the δ p is the resolution of light transfer between scintillation crystal with PMT.The δ st is possible to characterize the statistical uncertainty of the photomultiplier tube signal as follows:  N and ε are the numbers of electrons released from the photocathode and the variance of increasing the number of electrons in the PMT.The ε value of the modern PMT is equal to 0.1.The calculated energy resolution versus gamma radiation energy for both studied scintillators is presented in figure 3.If δ p is negligible, δ sc can be calculated using equations (2) and (3) of a single crystal.A comparison of δ sc for both tested scintillators is shown in figure 4. Figure 3 shows that the Ce:LGSO scintillator's ΔE/E value was superior to the Ce:GSO scintillator's from across a radiation energy range of 31 to 245 keV, while the Ce:GSO scintillator exhibited better energy resolution in the energy range of 344 to 1,274.5 keV.It can be observed that the ΔE/E value will be better when the energy of the gamma radiation increases.It can be seen from figure 4 that the intrinsic resolution of the Ce:LGSO scintillator in the radiation energy range from 31 to 81 keV was superior to that of the Ce:GSO scintillator, while the radiation energy range from 245 to 1,274.5 keV showed a better intrinsic resolution of the Ce:GSO scintillator.
The light output ratio measured at any energy to light output at 662 keV energy is a non-proportional response of light yield or relative light yield.The non-proportionality of the oxide family crystals increases as the radiation energy increases, such as in the research of Wanarak et al [8] and Sreebunpeng et al [23].In contrast,  the halide family crystals decrease as the radiation energy increases, such as in the research of Chewpraditkul et al [24] and Valentine et al [25].Intrinsic resolution is majorly correlated with a non-proportional response of light yield [21,22].In addition, other parts are related to an intrinsic resolution, such as inhomogeneities in the materials, various atomic defects in crystals, and the ability to accumulate electrons at the photocathode.
Figure 5 presents the light yield non-proportional response of both studied single crystals versus gamma radiation energy.The non-proportionality of the Ce:LGSO scintillator at 31 keV decreased by 0.14 from 1.00 at 662 keV, while the non-proportionality of the Ce:GSO scintillator at 31 keV decreased by 0.30 from 1.00 at 662 keV.For easier comparison, the degree of non-proportionality for light yield (σ np ) was presented by Dorenbos [26].The σ np can be described as: N is the number of sources of gamma radiation.The light output at particular γ-ray energies and 662 keV γ-ray energy are Y(E i ) and Y(662).The lower this value is, the better.The σ np was 0.08 and 0.17 for the Ce:LGSO and Ce:GSO scintillator, respectively.These values are consistent with reports shown clearly in figure 5.
The contribution of different components to ΔE/E is displayed in table 2 to better understand the ΔE/E of both scintillators.It was possible to acquire N (number of photoelectrons emitted from the PMT photocathode plate) using the method of Bertolaccini [17,18].From the energy spectra, ΔE/E was defined, δ st was calculated by equation (3), and δ sc was determined using equations (2) and (3).It was possible to acquire the σ np using equation (4).As shown in table 2 in column 2, the research team found that the Ce:LGSO scintillator could emit 2.4 times quite so many photoelectrons as possible from the Ce:GSO scintillator (5,105 versus 2,098).This photoelectron number resulted in the Ce:LGSO scintillator's PMT resolution being superior (4.04% versus 6.30%) to the Ce:GSO scintillator.The Ce:LGSO scintillator, however, had a significantly poorer intrinsic resolution than the Ce:GSO scintillator (6.25% versus 3.49%), resulting in a comparable energy resolution of  LGSO scintillator showed a much better value for the σ np than the Ce:GSO scintillator (0.08 versus 0.17).It was shown that the Ce:LGSO scintillator's significantly poorer intrinsic resolution resulted from the crystal defects.Therefore, improving crystal defects during the growth of crystals will enable the crystal to enhance energy resolution.

Intrinsic light yield and loss parameter
For accuracy compared to the light yield of the scintillator, Dujardin et al [27] measured the scintillation light yield (LY) versus the height of the cuboidal sample, which changed in steps by cutting off slices one after the other.One very interesting thing they notice is that the light yield doesn't change much with width, but it does change a lot as the height of the cuboidal crystal on top of the PMT window goes down.Because of this, the most interesting conclusion and most important for the model that will be given in the article is that the LY changes with the size of the crystal because of a single main parameter (crystal height).As a result, Wojtowicz et al [19] present a one-dimensional model wherein they additionally assume that the gamma rays, which are weakly absorbed, uniformly excite the entirety of the crystal.Hence, the involvement stemming from the slender section (dx) of the crystal positioned at a distance x from the PMT window will encompass two components representing two scintillation light rays (2R-model).One ray progresses directly towards the PMT window, while the other ray moves in the opposite way.Following reflection without any loss, the second beam will be sent onto the window of the PMT.It is anticipated that they will have: d LY e e LY dx 1 2 0 5 The determination and computation of the overall LY necessitates the process of integrating with respect to the variable x.Therefore, they obtain: In the given context, the symbol 't' denotes the vertical dimension of the crystal, while 'μ' represents the loss parameter encompassing both absorption and scattering effects.Additionally, 'LY(0) or LY 0 ' signifies the assumed constant intrinsic light yield throughout the crystal.
Tables 3 and 4 show the photon yield (light yield) results for Ce:LGSO and Ce:GSO scintillators of different sizes, respectively.Both tables show that as the thickness of the crystals increases, the amount of light emitted decreases.Using the two-ray model, the curve of the light yield depends on the thickness of the two crystals obtained, as in figure 6.
Figure 6 is summarized in table 5.This table shows that the intrinsic light yield and the loss parameter of the Ce:LGSO crystal are 31,200 ± 3,100 photon/MeV and 0.65 cm −1 , respectively.While the Ce:GSO crystal shows the 13,900 ± 1,400 photon/MeV intrinsic light yield and 1.20 cm −1 loss parameter.The intrinsic light yield and loss parameter of the Ce:LGSO (Lu 1.8 Gd 0.2 SiO 5 :Ce) in the research of Yawai et al [20] are about 29,900 ± 3,000 photon/MeV and 0.40 cm −1 , respectively.Although the intrinsic light yield of the studied Ce:LGSO (Lu 1.2 Gd 0.8 SiO 5 :Ce) crystal is slightly better than that of the Ce:LGSO (Lu 1.8 Gd 0.2 SiO 5 :Ce) crystal in [20], but the loss parameter value of the studied Ce:LGSO crystal is worse.These loss parameter values of both Ce:LGSO crystals show that the luminous efficiency of the studied Ce:LGSO crystal is worse than that of the crystal in [20] if the crystal thickness is increased.

Peak-to-total ratio
The peak-to-total ratio (PTR) value [28] shows the scintillator's ability to detect high-energy photons (high stopping power property), which is important for medical radiography.As measured by a scintillator, the peakto-total ratio value is defined as the ratio between the full energy peak area (the photoelectric absorption peak area or the photopeak area) and the whole spectrum area (the sum of total attenuation).Table 6 presents the PTR of both studied single crystals.The cross-section ratio (σ-ratio) between the photoelectric absorption and the overall one (the photoelectric and Compton scattering absorption sum) evaluated using the NIST XCOM program [15] is shown for comparison.The data shows that, in a similar trend to the cross-section ratio calculated by the NIST XCOM software, the PTR of the Ce:LGSO scintillator was slightly greater than the Ce:GSO scintillator.Usually, the PTR would be proportional to the product of Z eff 5 and ρ [29].The cause is the slightly greater density (7.07 g cm −3 versus 6.70 g cm −3 ) and an effective atomic number (59 versus 57) of the Ce:LGSO scintillator.

Conclusions
For γ-ray detection, the scintillation characteristics of small Ce:LGSO and Ce:GSO scintillators were tested.At 662 keV γ-ray energy, both single crystals showed comparable energy resolution (∼7%).Although the Ce:LGSO scintillator emits approximately 2.4 times more light than the Ce:GSO scintillator, its energy resolution does not  differ.The reason is that the Ce:LGSO scintillator has more crystal defects than the Ce:GSO scintillator.If this aspect improved, its energy resolution value would be better, making it more suitable for medical radiography (PET scan and CT scan applications) and high-energy physics research in the future.

Figure 1 .
Figure 1.Excitation and emission spectra for both tested scintillators.

Figure 2 .
Figure 2. Pulse height spectra of γ-rays for both tested scintillators.

Figure 3 .
Figure 3. Energy resolution for both tested scintillators.

Figure 5 .
Figure 5. Relative light yields as a function of γ-ray energy for both tested scintillators.

Figure 6 .
Figure 6.Light yield as a function of thickness for both tested scintillators.

Table 2 .
The comparative analysis of the energy resolution components at 662 keV γ-ray energy for both tested scintillators.

Table 4 .
The photoelectron, photoelectron yield, photon yield at 662 keV γ-ray energy for 3 tested Ce:GSO scintillators of different sizes.

Table 6 .
Peak-to-total ratio at 662 keV γ-ray energy for both tested scintillators.