New High Light Yield and Fast Ceramic Scintillator Y₃Al_2.5Ga_2.5O₁₂:Ce, Mg

The Reviewer 1 comment

Authors response

The creation of new scintillation materials is a pressing issue with a wide range of practical applications. In the peer-reviewed article, a new material Y₃Al_2.5Ga_2.5O₁₂:Ce,Mg was obtained and its properties were studied.

There are a number of questions and comments regarding the article under consideration:

- the peer-reviewed article does not cite related works:

Y. Yang et al., Influence of Mg²⁺/Ga³⁺ doping on luminescence of Y₃Al₅O₁₂:Ce³⁺ phosphors. Journal of Materials Science: Materials in Electronics 29, 17154 (2018).

W. Chewpraditkul et al., Optical and scintillation characteristics of Lu₂Y(Al_5-xGa_x)O₁₂: Ce, Mg multicomponent garnet crystals // Optical Materials 134, 113186 (2022).

K. Sreebunpeng et al., Luminescence and scintillation properties of fast Ce,Mg:Lu₂YGa_xAl_5-xO₁₂ ceramic scintillators fabricated from co-precipitated powders // Optical Materials 152, 115418 (2024).

K. Sreebunpeng et al., Temperature-dependent characteristics, light yield nonproportionality, and intrinsic energy resolution of Ce,Mg:Lu₂Y(Al, Ga)₅O₁₂ garnet ceramics // Radiation Physics and Chemistry 236, 112886 (2025).

Thank you very much for the articles. We investigated the data published and found some of them in line with our publication. They are included in the references. The others will be quoted in coming publications on the topic.

-from Fig. 3, one can note a fairly high similarity of the luminescence decay kinetics of YG25-1.1 and YG25-2.1 crystals upon excitation with radiation with wavelengths of 340 and 450 nm. In this case, only the constant tau1 is indicated for the excitation line in Table 2. What is the reason for this?

The table is corrected. We agree with the reviewer; kinetics at excitation 450 nm are pretty good coincide. Both are well fitted with a single exponential function. However, at excitation 340 nm, we see that the sample solely doped with Ce has a bit longer tail. It was confirmed by approximation with two exponents. 

- it is not clear from the text in Section 2 what is the purity of the components used for synthesis;

Now it is indicated in the corrected manuscript.

- there is extra text "Y₃" in line 71;

Corrected

the subscripts in line 78 need to be corrected;

Corrected

-the density dimension in Table 1 is not in English;

Corrected

-the text uses different spellings of the "±" sign;

Corrected

in the second column of the top line of Table 3, the wavelength is in brackets instead of energy;

Corrected

- in line 256, perhaps there should be "introduction of aluminum into the lattice"? Judging by the chemical formula, gallium is already present in the composition.

It is corrected as follows.

An increase in the scintillation yield of the compound is explained by the role of the Ga ions in the DOS forming. When gallium is introduced into the lattice, a branch is split off from the major group of conduction band branches in DOS.

1. The rationale for selecting the specific Al/Ga ratio (2.5/2.5) and Mg doping concentration (50 ppm) should be elaborated. Are these values empirically optimized or theoretically predicted?

The corresponding results of the theoretical consideration are given in the section “Discussion.” Two findings were used when choosing the Ga content. First, as small as possible deep in the DOS of the conduction band, and an optimal red shift of the STE luminescence. Our consideration demonstrated that the Al/Ga ratio of 2.5/2.5 looks to be close to the optimal.

In our previous work [  now ref. 20 ] we demonstrated that to compensate for the influence of different traps in the aluminum-gadolinium garnets, a small concentration of Mg2+ codoping, at the level of 20 ppm, is required. In our study, we follow this line.

1. “The residual porosity was defined to be 0.08 and 0.1%”, how is the residual porosity are identified?

The surface residual porosity of the ceramic material was estimated by analyzing a series of SEM images, in which voids in the microstructure were contrasted using a software filter (open-access “ImageJ” software), followed by summing up the area of pores and assigning them to the total area of the ceramics image.

3.       The reviewer recommends that the authors should present the in-line transmittance of the prepared ceramics.

Just to avoid overloading the article with the figures, we did not include the transmittance or absorption curves. Typical absorption spectrum of the ceramic samples of YAGG: Ce,Mg is bellow.

1. The PL/PLE spectra (Figure 2) and decay kinetics (Figure 3) demonstrate the improvement in material performance. However, the origin of the slow components in PL kinetics (Table 2) should befurther investigated. Are these related to defect states or energy transfer processes?

An excitation with 450 and 340 nm corresponds to excitation of 4f¹→4f⁰5d₁ and 4f¹→4f⁰5d₂ transitions of Ce³⁺ ions. Typically in aluminum-gallium garnets, the radiation state 4f⁰5d₁ is below the conduction band bottom, whereas the next one, 4f⁰5d₂, is already located in the condition band. When the latter level is excited, the electron can be delocalized into the conduction band, providing the creation of metastable Ce⁴⁺ ions. Therefore, diffusion of the nonequilibrium electrons in the conduction zone and their recapturing by either Ce⁴⁺ or traps give rise to the slower components in the scintillation kinetics.

1. The short-wavelength bands are missing in Table 3.

Corrected

1. The DFT calculations (Figure 6) provide insights into the electronic structureof Y3Al5Ga2.5O12:Ce, Mg. However, the correlation between the calculated DOS and the observed scintillation properties (e.g., light yield enhancement) needs to be more explicitly linked. For instance, how does the split-off conduction band branch affect the STE-Ce³⁺ resonance?

We agree  with the Reviewer, this  is  interesting and may be the matter of separate article. Itr will require much more undoped and Ce doped samples.

Moreover, the problem of correlation between electronic structure and efficiency
of STE creation and their energy transfer to Ce is complex due to
multiplicity of different factors which influence on the different
channels, it demands additional experimental and theoretical studies
which we are caring out now. We are preparing now separate publication
concerning the results of these investigations.

7.      Comparisons with YAG:Ce are presented, but the manuscript would be much more interesting if there were a table summarizing the key parameters (light output, decay time, density) of competing materials, including LuAG:Ce and other scintillators

In this article we focused on the search for the lightweight scintillator, which can be an alternative to YAG. Light, high-light-yield scintillators are highly demanded for X-ray imaging. LuAG:Ce is perfect heavy scintillation material, which, however, has another mission: the detection of gamma rays above 100 keV.

1. The role of Mg²⁺ in alleviating quenching is interesting, but more mechanistic details are needed. Does Mg2+alter defect chemistry or energy transfer pathways?

The role of the Mg²⁺ codoping in crystalline scintillators on a base of oxide compounds is described in  [https://doi.org/10.1002/pssa.201700798]. It creates a point structure defect with extremely fast nonradiative relaxation. It prevents the capturing of the nonequilibrium electrons by other traps.

1. The format and language of the manuscript need to be carefully double checked.

Text is checked by the language carrier.

1. In Table 1, the authors should check the unit of density, r/cm3?

Corrected.

1. In the abstract, line 17, a scintillation kinetic decay constant of 47 ns was mentioned. However, it was mentioned in the manuscript that the decay contains both fast and slow components, which is 39 (85%) and 97 (15%) ns respectively. Is 47 ns the effective decay time?

Yes, it is effective decay time. Abstract is corrected correspondingly.

1. In the part of materials and methods, the authors did not mention the sintering atmosphere, such as vacuum sintering, reductive sintering or in a muffle furnace, which is important to the densification of pores free ceramic. And if the ceramic sample was sintered in a muffle furnace, whether the presence of oxygen atmosphere affect the valence state of Ce-ions, thereby affecting its scintillation performance used in the experiment.

The type of atmosphere is indicated. In addition, we would like to point out that an influence of the sintering atmosphere depends on the rote of the raw material preparation. In our case we use coprecipitated raw material having the garnet structure in which Ce³⁺ is already incorporated.

1. Continuing with the previous question, should the effect of valence state of Ce³⁺ or Ce⁴⁺ be considered in the explanation of LY-mechanism, when Ce, Mg was co-doped in the YGAG transparent ceramic?

We investigated the appearance of Ce⁴⁺ in the ceramics of the garnet of a very close composition. Results are described elsewhere [https://doi.org/10.1016/j.jlumin.2021.118140]. We found a small quantity of Ce⁴⁺ ions in the ceramics we made. Worth noting, an appearance of Ce⁴⁺ in the process of the crystal growth is quite a different process in comparison with ceramics. Crystal is grown from the melt, whereas no melting of the green body occurs at the ceramics preparation.

The Reviewer comment

Authors response

1. It appears that no photoluminescence decay time of LuAlO₃:Ce is reported in Ref. 30. Could the authors explain the anomaly where the decay constant of the scintillation kinetics is smaller than that observed in photoluminescence?

Yes, Ref [30] reports scintillation decay time. More information on the scintillation decay time of Lu-containing perovskites is in [Lecoq, P., Gektin, A, Korzhik, M, 2016. Inorganic scintillators for detector systems: physical principles and crystal engineering. Springer Berlin Heidelberg, New York, NY. ISBN 978-3-319-45521-1, Ch 9.2.3.3-9.2.3.4]. We did not include the latter article in the literature list to avoid self-citing.

2.     Figure 2 includes three legends, but only two graph lines are visible. Is there a missing graph line? Please verify and clarify.

Two graphs of the samples YG25-1.1 and YG25-2.1 perfectly coincide. This is now mentioned in the corrected manuscript.

3.     The legend 'YG-1.1' in Figures 4 and 5 is mislabeled as 'YG-1.2'. Please correct the labeling.

Corrected.

4.     It is recommended to relocate Table 1, Figure 1, and their related descriptions to the '3.

The data presented in Fig.1 and Table 1   confirm that the materials studied are typical garnet structure materials. Therefore, they characterize samples as the specific objects for future investigation. Due to this reason we prefer to keep them in paragraph 2 “Materials and methods”.