# Studies of GAGG:Ce scintillators for space missions dedicated to Terrestrial Gamma-Ray Flashes and Gamma-Ray Bursts observation

Melody Pallu, Damien Pailot, Eric Bréelle, Philippe Laurent, Jérôme Carron, François Lebrun, Charbel Koumeir, Claude Chapron, Kévin Biernacki

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Melody Pallu, Damien Pailot, Eric Bréelle, Philippe Laurent, Jérôme Carron, et al. Studies of GAGG:Ce scintillators for space missions dedicated to Terrestrial Gamma-Ray Flashes and Gamma-Ray Bursts observation. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2024, 1069, pp.169997. 10.1016/j.nima.2024.169997. hal-04759687

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Full Length Article

# Studies of GAGG:Ce scintillators for space missions dedicated to Terrestrial Gamma-Ray Flashes and Gamma-Ray Bursts observation

Melody Pallu<sup>a,b,●,*</sup>, Damien Pailot<sup>a</sup>, Eric Bréelle<sup>a</sup>, Philippe Laurent<sup>a,c</sup>, Jérôme Carron<sup>b</sup>, François Lebrun<sup>d,●</sup>, Charbel Koumeir<sup>e</sup>, Claude Chapron<sup>a</sup>, Kévin Biernacki<sup>a</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

**Keywords:**

Gamma ray spectrometer

TGF detection

Proton irradiation

GAGG crystal scintillator

GRB detection

## ABSTRACT

Fast Gamma ray Spectrometer (FGS) is an on-going detector development for the observation of transient gamma ray events such as Terrestrial Gamma ray Flashes (TGFs) produced in Earth's thunderstorms and Gamma Ray Bursts (GRBs) produced in the far Universe. In this work, we compare different Cerium-doped Gadolinium Aluminium Gallium Garnet (GAGG:Ce) scintillators, associated with Silicon Photo-Multipliers (SiPMs), quantifying their performances at atmospheric pressure and under vacuum, with different surface conditions. We also study their afterglow following proton irradiation, similar to what will be observed in space environment. Our measurements with GAGG:Ce scintillators after proton irradiation confirm that GAGG:Ce can be used in space.

## 1. Introduction

We present results about the Fast Gamma ray Spectrometer (FGS) (Fig. 1), already discussed in [1], which aims to be a modular spectrometer for space missions dedicated to the observation of Terrestrial Gamma ray Flashes (TGFs), Gamma Ray Bursts (GRBs), and solar flares. All these events are detectable from satellites placed on Low Earth Orbit (LEO). Therefore, we propose a new modular gamma-ray spectrometer, adaptable in crystal type, number and size to each scientific objective. Its particularity is to be fast and to cover a wide energy range. A detailed description of these objectives is given in Section 2.

The detection chain is composed of scintillators associated with silicon photomultipliers (SiPMs) (Hamamatsu S13360-6050CS), which convert the emitted light into an electrical signal, then analyzed with a spectroscopic chain. The optical interface between the SiPM and the crystals is made with optical glue EJ-500 from Eljen Technology. The readout chain is from Mirion technologies (formerly Canberra). The amplifier Modele 2024 Fast Spectroscopy Amplifier and the multichannel analyzer Multiport II MCA are used. A scintillator has been chosen for our spectrometer, the Cerium-doped Gadolinium Aluminium Gallium Garnet (GAGG:Ce), as it is non hygroscopic, non radioactive, bright, fast, dense and has got a high Z. This scintillator has already been tested in flight [2], and on the ground to study its behavior when

exposed to radiation [3,4]. A discussion to compare our results to theirs is made in Section 4.5. We have tested three GAGG:Ce types with different performances in light yield and fast fall time. The scintillators are wrapped in a reflective material in order to propagate the major part of the emitted light toward the SiPM window. Vikuiti™ has been chosen for the reflective envelope.

Tests have been performed in order to compare the different GAGG:Ce types performances. Measurements have been also performed under and after proton irradiations to qualify those crystals for space applications since they have to pass through high radiation regions such as the South Atlantic Anomaly (SAA) or the polar caps.

In Fig. 1, one can see the future configuration of FGS to be flown onboard balloon and/or satellites. The configuration used in this work is only one detector (instead of 16 pixels in the final configuration, shown here) with one SiPM.

## 2. Space application: Terrestrial gamma ray flashes and gamma ray burst detection

Terrestrial gamma ray flashes (TGFs) are bursts of high-energy photons produced in thunderstorms. They are short (<100 µs, [5]) and

<footnote><sup>a</sup> Université Paris Cite, CNRS, Astroparticule et Cosmologie, Paris, F-75013, France</footnote>

<footnote><sup>b</sup> CNES, Toulouse, 31401, France</footnote>

<footnote><sup>c</sup> Departement d'Astrophysique, Universite Paris-Saclay, Universite Paris Cite, CEA, CNRS, AIM, Gif sur Yvette, 91191, France</footnote>

<footnote><sup>d</sup> Gamma Detect, Champigny sur Marne, France</footnote>

<footnote><sup>e</sup> GIP ARRONAX, Saint-Herblain, France</footnote>

<footnote>* Corresponding author at: Universite Paris Cite, CNRS, Astroparticule et Cosmologie, Paris, F-75013, France.
E-mail address: pallu@apc.in2p3.fr (M. Pallu).</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** The FGS detector (left) is composed of four modules. One module (right) is composed of 4 GAGG:Ce crystals, each read by one SiPM. The size of the each GAGG:Ce crystal is 2 cm × 2 cm × 1 cm. This configuration is the future configuration of FGS to be flown onboard balloon and/or satellites.</figcaption></figure>

<table><caption>**Table 1**

Characteristics of the different types of GAGG:Ce given by the suppliers.</caption><thead><tr><th>GAGG:Ce type</th><th>Supplier</th><th>Particularity</th><th>Light yield</th><th>Resolution @662 keV</th><th>Decay time</th></tr></thead><tbody><tr><td>GaGG-T</td><td>EPIC crystal</td><td>None</td><td>42 ph/keV</td><td>6%</td><td>90 ns</td></tr><tr><td>GaGG</td><td>C&A Corp.</td><td>None</td><td>50 ph/keV</td><td>Not given</td><td>90 ns</td></tr><tr><td>GaGG-F</td><td>EPIC crystal</td><td>Fast</td><td>30 ph/keV</td><td>7%</td><td>50 ns</td></tr><tr><td>GFAG</td><td>C&A Corp.</td><td>Fast</td><td>30 ph/keV</td><td>Not given</td><td>~40 ns</td></tr><tr><td>GaGG-HR</td><td>EPIC crystal</td><td>High resolution</td><td>50 ph/keV</td><td>4.2%</td><td>150 ns</td></tr><tr><td>HR-GaGG</td><td>C&A Corp.</td><td>High resolution</td><td>45 ph/keV</td><td>Not given</td><td>400 ns</td></tr></tbody></table>

intense events, producing ~10¹⁸ gamma rays, with energies reaching more than 40 MeV [6,7]. Mainly detected by satellite up to now, most TGF spectra have been measured with gamma ray spectrometers designed for high-energy astrophysics. Those detections were limited by the fact that instruments were not designed to endure the very high photon flux characteristic of TGF. Since 2018, the first TGF detector, named ASIM [8], has been installed on the International Space Station (ISS) and allows for the detection of hundreds of TGFs every year, bringing new informations about their production mechanism. Further studies are still necessary to understand the whole process that leads to their production, their distribution on Earth in space and time and their impact on the Earth's Global Electrical Circuit (GEC). Therefore, FGS is designed to be one of the instruments able to perform their detection with a detection area adjusted to the mission requirements, using a new scintillator type, associated with fast electronics.

Able to detect TGFs, FGS will also be able to detect Gamma Ray Bursts (GRBs). GRBs are transient sources of gamma ray emissions associated with distant galaxies. They are among the most luminous events in the universe. Their duration is typically around a hundred milisecond (for the shortest) to hundreds of seconds (for the longest). As they have smaller peak fluxes, they impose less stringent requirements on the detector design than TGF.

In addition to TGFs and GRBs, FGS will be also able to detect solar flares, events which can last up to several hours. Its high energy threshold will enable to study in details gamma-ray lines, such as the neutron induced 2.2 MeV line, emitted during those flares. Due to its fast response, it will be also less saturated than other gamma-ray spectrometers by the flare very high fluxes.

TGFs, GRBs, and solar bursts being rare and fully independent events; observing all of them with a single instrument such as FGS is a way to reduce the cost of science.

### 3. Comparison of the performances of different GAGG:Ce type

In order to better choose the GAGG:Ce type as a function of the space mission main objectives, we bought three GAGG:Ce types from two different suppliers, EPIC crystal and C&A Corp. to compare them.

The size of the each GAGG:Ce crystal is 2 cm × 2 cm × 1 cm. Their characteristics given by their suppliers are shown in Table 1. Depending on the main scientific goal, we can design different FGS with different GAGG:Ce types, but using always the same electronics.

We performed tests on each type in order to characterize the differences in terms of gain (or light-yield), spectral performance and decay time. For that, we measured the energy spectra of radioactive sources, such as ²²Na, and ¹³⁷Cs, showing lines at known energies. Then, we calibrated the energy spectra using formula (2–14) given by Hamamatsu in their Technical note [9], to take into account the non-linearity of the SiPM response for high energies. Considering the fast GAGG:Ce, two different surface conditions have been tested: polished and unpolished.

#### 3.1. Measurement reproducibility

First we have estimated the reproducibility of the coating process, in order to estimate the associated uncertainties. One can note that Vikuiti sheet were cut out by a robot in a given pattern to reduce uncertainties that could occurred with a hand-made process.

The coating process (Vikuiti coating and optical grease deposit to make the optical interface with the SiPM) was performed 5 times on the same crystal/SiPM couple. We found out that the uncertainty led by the Vikuiti and optical grease deposit for our set-up is about 4.8% for the gain, and 10% for the 662-keV resolution.

Another uncertainty results from the absence of a precise characterization of each of the SiPM deposited on the crystals. A rough estimate of this uncertainty can be derived from measurements of 3 different SiPMs from the same model (Hamamatsu S13360-6050CS): ~4.1% on the gain and ~1.2% on the spectral resolution at 662 keV. Being independent, these two sources of uncertainty add quadratically. The global uncertainties are therefore of the order of 6% for the gain, and 10% for the spectral resolution at 662-keV. We will use these global uncertainties in the results.
<figure><img src="image_5.png" /><figcaption>Fig. 2. Gain of the different GAGG:Ce used as a function of their type and of their surface condition. When two points are represented per type, they correspond to two different samples tested.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Resolution at 662 keV of the different GAGG:Ce used as a function of their type and of their surface condition. When two points are represented per type, they correspond to two different samples tested.</figcaption></figure>

## 3.2. Performances depending on GAGG:Ce type

We compared 6 different types of GAGG:Ce. In this section, we show the comparison of GAGG:Ce crystals with polished and un-polished surfaces (respectively blue and red point in Figs. 2 and 3). We performed the same measurements on 2 copies of each GAGG:Ce types, to get an idea of the uncertainties.

Fig. 2 shows the gain calculated for each GAGG:Ce type, and Fig. 3 shows the spectral resolution at 662 keV. The uncertainties are the ones evaluated in Section 3.1. In view of the results, one can note that the gains and resolutions are quite similar for 5 of the GAGG:Ce. However, GFAG crystal seems to have poorer performances, with a smaller gain than the 5 other types, by almost a factor of 2, and a worst spectral resolution at 662 keV.

## 3.3. Surface condition of the crystal

We also performed the same measurements on fast GAGG:Ce with unpolished surface, in order to quantify the difference on the gain (or light-yield) and spectral resolution. For the crystals called unpolished, we mean that 5 of the sides are unpolished, but the side where the SiPM

is stuck is polished, to maximize the light transmission. The results are shown in red in Figs. 2 and 3. The uncertainties used are those determined in Section 3.1.

A significant difference in the gain can be noticed for polished and unpolished GAGG:Ce. Crystals with unpolished surface having a gain from 20% to 30% higher than the polished ones.

In order to try to explain this difference, we simulate the propagation of visible photons in GAGG:Ce crystals using a Monte Carlo toy model. This model assumes initial position and direction to be randomly distributed in the GAGG:Ce crystal of 2 cm × 2 cm × 1 cm. The Vikuiti is set to have a reflectivity of 98%. The absorption length in the crystal is chosen to be 600 mm. The reflection of photons on the Vikuiti is modeled as follows: the reflection direction is chosen randomly in a statistical distribution following a 2D-Gaussian around the “perfect” geometrical reflection on the Vikuiti. The standard deviation of the 2D-Gaussian distribution has been tested from $10^{-4}$ to $\pi$ rad. The upper face of the GAGG:Ce (2 cm × 2 cm face) is always polished, therefore the reflections on this face are assumed not to deviate from the geometrical reflection.

The results of the simulation are shown in Fig. 4. One can see that it seems possible to explain the 20% increase by a precise unpolished
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Results of the simulation concerning the impact of the surface state of GAGG:Ce light yields (or gains). The direction of the photon after reflection is normally distributed in a 2D Gaussian with a standard deviation equal to the angle of dispersion $\sigma$ (in radians), as illustrated on the four right figures. On the right, are represented angle distributions around the non deviated direction as a function of the $\sigma$ for 4 different $\sigma$ values.</figcaption></figure>

surface state, modifying reflection angles up to ~$\pi$/10 around the non deviated reflection.

### 3.4. Decay time

In this section, as in the other sections, we use a 2 × 2 × 1 cm GAGG:Ce and the SiPM described in Section 3.1. Using a $^{22}$Na radioactive source, we have recorded many pulses (20 pulses at 511 keV for each GAGG:Ce) using an oscilloscope, at the SiPM's output. Seeing the pulse shapes, the question of using a single or a multi exponential fit has been raised, thus we decided to evaluate the decay times of the pulses in calculating the time for the pulse to fall to 1/e of the peak amplitude. Thus, for each 511 keV pulse, we estimated its decay time calculating the time for the pulse to fall to 1/e of the peak amplitude, the mean of the 20 estimated decay times determined for each GAGG:Ce type is then used as the mean decay time. The error has been estimated as the standard deviation of the decay times found for the 20 pulses acquired for each GaGG:Ce type. We need the decay time to be as short as possible in order to minimize pile-ups when observing high flux TGF (~100 cts in <100 µs). The decay times measured for each GAGG:Ce type are shown in Table 2, and some 511 keV pulses are visible in Fig. 5.

We performed the same measurements using hundreds of strong muon-triggered pulses. Their summation allows for the study of the decay on much longer timescales. They have been obtained in adjusting the oscilloscope trigger very high, corresponding to energies >5 MeV, that could only be deposited by muons at ground level. The results are shown in Fig. 6, where one can clearly see different decay times between the different GAGG:Ce types. Clearly these decay times are longer than those measured with gamma-rays that is reminiscent of the distinction made by Yoneyama et al. [3] between photoluminescence and radio-luminescence. In this regard, the phosphorescence triggered by muons is probably closer to that induced by the SAA protons. Anyway, the ordering of the different crystals with regard to the decay time is the same with photons or muons (see Table 2); GFAG is the fastest, while C&A GAGG and HR-GAGG are the slowest. The C&A GAGG result is surprising since according to the manufacturers it should be as fast as the GAGG-T, a factor 4.5 faster.<fnref n="1" />

<caption>**Table 2**</caption>

Decay times ($\tau = 1/e$) of the 6 GAGG:Ce types, measured with a Hamamatsu S13360-6050CS SiPM. Pulses used are either 511 keV pulses, either muon pulses.

<table><thead><tr><th>GAGG:Ce type</th><th>Supplier</th><th>Decay time (1/e)<br>Supplier</th><th>Decay time (1/e)<br>511 keV</th><th>Decay time (1/e)<br>Muons</th></tr></thead><tbody><tr><td>GaGG-T</td><td>EPIC crystal</td><td>90 ns</td><td>230 ns ± 14 ns</td><td>280 ns</td></tr><tr><td>GaGG</td><td>C&A Corp.</td><td>90 ns</td><td>420 ns ± 40 ns</td><td>580 ns</td></tr><tr><td>GaGG-F</td><td>EPIC crystal</td><td>50 ns</td><td>170 ns ± 10 ns</td><td>280 ns</td></tr><tr><td>GFAG</td><td>C&A Corp.</td><td>~40 ns</td><td>230 ns ± 10 ns</td><td>200 ns</td></tr><tr><td>GaGG-HR</td><td>EPIC crystal</td><td>150 ns</td><td>270 ns ± 20 ns</td><td>360 ns</td></tr><tr><td>HR-GaGG</td><td>C&A Corp.</td><td>400 ns</td><td>470 ns ± 30 ns</td><td>620 ns</td></tr></tbody></table>

It is difficult to compare both our values with suppliers' values as we do not know about the suppliers' experimental set-up. Regarding the differences, we can imagine that they could have used photomultiplier tubes (PMTs) for their very fast response, whereas we used SiPMs with a longer time response. However, we remind that the aim of our study is mainly to characterize the behavior of GAGG:Ce/SiPM couples for future applications, and that the fast GAGG:Ce types show decay times in agreement with our needs.

### 3.5. Temperature dependency

We have also characterized the temperature dependency of GAGG:Ce/SiPM couples. The GAGG:Ce used in this section is a GaGG-F from EPIC crystal. A constant voltage has been applied to the SiPMs. No temperature-controlled feed-back was implemented in the detector. The gain has been measured using the 662 keV and 32 keV line of the $^{137}$Cs source.

The results are shown in Fig. 7. The SiPM (Hamamatsu S13360-6050CS) breakdown voltage has been characterized by the supplier, and is known to increase with temperature as 54 mV/°C. Therefore, it is expected that the gain increases (not necessarily linearly) for lower temperature, for a constant SiPM voltage, as the overvoltage applied is higher. Moreover, literature studies have shown that the intrinsic light yield of GAGG:Ce can increase/decrease with temperature depending on the doping [10] (that we do not know here). It is possible to decouple the SiPM contribution from the crystal contribution using for instance a constant light source. However, this point is out of the scope of this work: characterize the behavior of the GAGG:Ce/SiPM couples, to prepare future missions with this detector.

<footnote n="1">¹ The supplier has confirmed after the submission of the article that, given the decay time measured, the GAGG (C&A) crystal could have been mislabeled during the shipping process, and could in fact be a HR-GAGG.</footnote>
<figure><img src="image_8.png" /><figcaption>Fig. 5. Example of one pulse recorded by each crystal type, at 511 keV. The maximum pulse amplitudes have been normalized to 1.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. Sum of many muon pulses used to determine decay times for each crystal. The maximum pulse amplitudes have been normalized to 1.</figcaption></figure>

In order to reduce the gain change with temperature, it is possible to modify the SiPM voltage as a function of the temperature, thanks to a feedback system. Even though we do not have a clear explanation for the ~0 °C break, the feedback system should be fancy enough to follow the two behaviors visible in Fig. 7 for temperatures lower and higher than 0 °C.

### 3.6. Vacuum tests

GAGG:Ce/SiPM measurements have been also performed at ~0.1 mbar. During the measurements and the vacuum, the temperature was not stabilized and have shown some modifications. Gain modifications have been seen, due to this temperature change. Apart from that, the behavior of the instrument did not show any perturbation due to the vacuum. In particular the gains were identical to those measured before the vacuum tests and presented in Fig. 2.

## 4. SAA passage: Proton irradiation

### 4.1. Afterglow

When exposed to visible light (or to proton irradiation, see hereafter), GAGG:Ce crystals emit, for some time, additional light polluting

the gamma-ray spectra, called afterglow. It induces pile-up and exhibits a high count rate in the low energy channels polluting the measurements and increasing the low energy threshold. Afterglow has been observed on GAGG:Ce crystals during our tests. In this work, we highlight two main effects of this afterglow on the GAGG:Ce used. On the one hand, the noise at low energy (see Fig. 8) is likely due to the phenomenon of phosphorescence. This phenomenon has already been reported and quantified by Yoneyama et al. [3], for instance they showed that red light induces less phosphorescence than blue light. On the second hand, spectral continuum is higher (see Fig. 10), but we have only observed this effect when GAGG:Ce were exposed to a high energy proton beam.

In future space missions, the exposure to the high-energy protons of the SAA could trigger afterglow in the crystals installed on satellites. In that sense, an evaluation of the afterglow dependency on high energy particles present in orbit is necessary to qualify GAGG:Ce and FGS for our space applications.

Protons with energies <10 MeV would be stopped in the satellite without being able to interact in the scintillator. Between 10 MeV and 300 MeV, the proton flux in the SAA can reach 4000 pr/cm²/s [11]. An instrument on a LEO would be exposed to such a flux during <20 min for each orbit. A detailed plan has been defined to expose FGS under a
<figure><img src="image_10.png" /><figcaption>Fig. 7. Gain calculated as a function of the temperature for a constant voltage applied to the SiPM. The uncertainties are those determined in Section 3.1.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. Energy spectrum of GaGG-F before and after visible light exposure. Afterglow can be observed below channel 100. The measurements have been performed using a ²²Na source (511 keV and 1275 keV are visible around channels 2600 and 5900). At channel 290, the line is due to the lead fluorescence.</figcaption></figure>

similar proton flux with a proton accelerator to validate that FGS can be used on an LEO without being polluted by its afterglow.

## 4.2. Irradiations performed

We have performed measurements in the ARRONAX proton accelerator (cyclotron) [12] located in Nantes, France. ARRONAX is a high-energy and high-intensity cyclotron, mainly used for nuclear medicine. We used it to test the behavior of our detector exposed to proton fluxes similar to what will be encountered in space. The installation and experimental set up can be seen on Fig. 9, where the room was lighted in red light in order to reduce as much as possible the afterglow produced by visible light on the GAGG:Ce crystals (see [3]'s Figure 17).

The experiment aims to show how the different GAGG:Ce types behave after passing through the SAA. Therefore, the first part of the irradiation campaign was dedicated to the exposure of each of the 6

GAGG:Ce types to a proton flux of 10,000 pr/cm²/s with an energy of 70 MeV, that is a high flux compared to the proton flux in the SAA, in order to simulate a worst case. The results are shown in Section 4.4.1.

In the second part of the experiment, we focused on the fast GAGG:Ce types provided by our two suppliers because they are the best candidates for our main application. We performed a more detailed analysis using different energies and different fluxes to see their influence on the scintillator behavior. The proton beam has been used with fluxes ranging from 100 to 200,000 pr/cm²/s, and with energies of 30, 50 and 70 MeV. The results are shown in Section 4.4.2.

Each proton irradiation was performed during ~15 min. For each irradiation, consecutive 1-minute-gamma ray spectra were recorded during ~15 min after the end of the irradiation. Activated lines can thus be analyzed, and are presented in Section 4.4.3.

A precision must be given on the results shown in this section. As can be seen on Fig. 9, the paraffin wall (used to restrain the proton beam to the sensor) can be easily activated by the proton
<figure><img src="image_12.png" /><figcaption>Fig. 9. Experimental room of the Arronax proton accelerator installation. One can see the proton beam release and the Faraday cage (the white metallic box) with the GAGG:Ce scintillator and SiPM inside. The experiment was under red light to avoid afterglow induced by visible light.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 10. Spectrum acquired after a proton irradiation, immediately after, 3 min after and 15 min after. One can see the noise peak and the background level decreasing with time. The lines of the ²²Na source used during this measurement are visible.</figcaption></figure>

beam. Therefore, between two irradiations, the energy spectrum called “before irradiation” of a crystal is polluted by the emission of the activated paraffin. The activated paraffin emits $\beta^+$ from $^{11}$C with a decay time of ~20 min. Since ~30 min were needed to change crystal in the experiment room, only residuals from this activation should be present in spectra “before irradiation”.

### 4.3. Method to quantify the afterglow

Three spectra acquired after a proton irradiation (immediately after, 3 min after and 15 min after the irradiation stops) are shown in Fig. 10. One can see that the afterglow is characterized by a high number of counts in the low energy channels, considered as “noise”, and a higher continuous component, between 300 keV and 2 MeV, decreasing with time after the irradiation.

In Fig. 11, the afterglow spectrum (in magenta) was obtained by subtracting the energy spectrum before irradiation (in black), from the energy spectra after irradiation (in red). The afterglow spectrum shows a continuous spectrum with a maximum around 800 keV. The lines at low energy (up to 511 keV) are due to activation, that is to say nuclei that emit at those energies after being irradiated.

We decided to quantify the afterglow using two different methods: firstly, quantify the total number of counts in the energy spectrum, between 0 and 4 MeV. During the data analysis, we found out that this contribution was due to radioactivity (see hereafter Section 4.4.1 for the explanation). Therefore, this part is referred to as *radioactivity* in the next section. Secondly, we also used the peak at low energy (16–25 keV) quantifying the number of counts in this peak. This energy range has been chosen in order to remove the effect of a parameter, the threshold, that has been modified during the measurements. In fact, different thresholds induce different values for the minimum energy detected in the energy spectra. Thus, we fixed this low threshold at an energy detected in all the spectra. This low energy afterglow is referred to as *phosphorescence* in the next section.

### 4.4. Results

In this section, afterglow decreases are evaluated using exponential fits with one decay time. This method is sufficient for our measurements and study, that aims to show the future working of the detector for space measurements.
<figure><img src="image_14.png" /><figcaption>Fig. 11. (bottom panel) Afterglow spectrum of GaGG-F (EPIC) crystal, calculated by subtracting the energy spectrum before irradiation (top panel, black line) and the energy spectrum acquired immediately after a proton irradiation of 10,000 pr/cm²/s, at 70 MeV, during ~15 min (top panel, red line). The lines of the ²²Na source used during this measurement are visible in the top panel, they are removed in the bottom panel by subtraction.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 12. Number of counts in the energy spectrum between 0 and 4 MeV after proton irradiation (/min). The irradiations were performed for each crystal during ~15 min, with a flux of 10,000 pr/cm²/s, and an energy of 70 MeV. Dashed lines are exponential fits for each crystal.</figcaption></figure>

### 4.4.1. Afterglow depending on GAGG:Ce type

#### Radioactivity

Fig. 12 shows the number of counts in the energy spectrum, between 0 and 4 MeV, versus time, and as a function of the GAGG:Ce type. The number of counts decreases exponentially with a decay time of ~3 min (half-time of ~2 min), as can be seen in Fig. 13. Five different energy ranges have been used to detect a possible dependency with energy. According to this figure, the afterglow decay time does not depend on the energy and does not depend on the GAGG:Ce type either. The afterglow decay time is estimated to be ~3 min. This value compared to the duration between two SAA passages for a LEO orbit, that is ~1 h 30, is short enough to induce merely no impact on FGS operating in space.

We have also determined the energy spectrum as a function of the GAGG:Ce type. Clearly, all crystals present the same spectrum as Fig. 11. Such a continuous spectrum, extending up to approximately 2 MeV, independent of the crystal tested and decreasing exponentially

in 3 min (1/e) (half-time of ~2 min) is reminiscent of radioactivity. Several isotopes can be produced by such an irradiation in GAGG:Ce, for instance Gadolinium can generate $\beta^-$ radioactivity, Gallium and Aluminium can generate $\beta^+$ radioactivity with different decay times. In fact, the best known radioactive element that could be formed by spallation, and dominant in GAGG:Ce is ¹⁵O [13]. It is a $\beta^+$ emitter, which decays in ¹⁵N with a half-life of 2 min, i.e. a decay in 1/e after 2.94 min. The maximum kinetic energy of the positron produced is 1.7 MeV. With a total loss of 1 MeV/mm at this energy, only positrons emitted near the surface of the detector can exit, most deposits all their energy and annihilate by producing two photons of 511 keV. The maximum energy produced is reached when the two photons are completely absorbed, i.e. 2.7 MeV. The energy spectrum of the created positrons have a peak at 735 keV [14], which is in agreement with the energy spectrum of Fig. 11.

#### Phosphorescence

Fig. 14 shows the number of counts between 16 and 25 keV, considered as afterglow as well, but corresponding to phosphorescence,
<figure><img src="image_16.png" /><figcaption>Fig. 13. Decay times of the number of counts for different energy ranges, in shades of blue, and on the total energy range (0–4 MeV) in red. The error bars are indicated.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 14. Number of counts in the phosphorescence peak (between 16 keV and 25 keV) after proton irradiation. The irradiations were performed for each crystal during ~15 min, with a flux of 10,000 pr/cm²/s, and an energy of 70 MeV. An exponential fit has been done for GaGG-F and GAGG crystals, with a decay time estimated to be respectively 0.86 min and 0.68 min.</figcaption></figure>

as a function of time after irradiation. As for Fig. 12, the number of counts decreases with time, in decay times close to 0.8 min (for GaGG-F and GAGG in particular). This criteria shows a larger irradiation's impact at low energy for GaGG-F, and GAGG compared to the impact on the background level quantified in Fig. 12. However, the decay time being on the order of one minute, it will mostly not impact the measurements in space.

### 4.4.2. Afterglow depending on energy and flux

We quantified the number of counts between 1 and 3 MeV in the energy spectra, acquired by the fast GAGG:Ce types (GaGG-F and GFAG), as a function of the proton energy and flux. The results can be seen in Fig. 15.

One can clearly see that the strength of the afterglow triggered by the irradiation, depends on both the energy and the flux for the fast GAGG:Ce types. The quantity of afterglow increases with the proton energy, and increases with the proton beam flux.

We also measured the decay time of the afterglow depending on the energy and flux, however, it does not show any modification with energy and flux of the proton beam for GaGG-F and GFAG.

We finally show in Fig. 16 the energy spectrum of the afterglow as a function of the energy and flux of the proton beam.

### 4.4.3. Activated lines

Immediately after a proton irradiation, we have observed several activated lines produced by the irradiated crystal itself. Five activation lines are visible (see Fig. 17): the 511 keV line, that is the highest, and 4 others that quickly fade away. According to the measurements, we estimate the decay time of each line:

* 48 keV line: ~24 min ± 35 min

* 85 keV line: 37 s ± 64 s

* 160 keV line: 8 min ± 2 min

* 320 keV line: 7 min ± 6 min

Those decay times are of the order of several minutes to tens of minutes. In ~30 min, no more of the four lines are visible on the measurements. The 511 keV line, shown in Fig. 17 is mixed up with the 511 keV from $^{22}$Na source. In order to study better its decay, we performed measurements without the $^{22}$Na source (energy spectra not shown here, but used for Fig. 18's measurements).
<figure><img src="image_18.png" /><figcaption>Fig. 15. Number of counts detected after proton irradiation. The irradiation were performed for each crystal during ~15 min, with a flux between 100 and 100,000 pr/cm²/s, and an energy between 30 and 70 MeV.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Fig. 16. Energy spectra of the afterglow depending on energy and flux for GaGG-F crystal, after an irradiation with protons at an energy E equal to 30, 50 or 70 MeV.</figcaption></figure>

In addition to GAGG:Ce measurements itself, we have performed
longer measurements with a HPGe detector, to detect GAGG:Ce emis-
sions, to identify lines and to quantify the decay of the activated lines
on a longer period (up to ~1 h after the end of the irradiation). The 511
keV decay is visible on Fig. 18 for GaGG-F crystal, combining data from
GAGG:Ce measurements itself and HPGe measurements. Using a Monte
Carlo algorithm simulating the geometry used, we have estimated a
factor of ~1.44 between HPGe data and GAGG:Ce data, taking into
account the solid angle intercepted by HPGe compare to the GAGG:Ce
measurements. In other words, we estimate that HPGe will detect only
1/1.44 ≈ 0.69 of GAGG:Ce detected photons. This geometrical factor
has been used to scale HPGe data on GAGG:Ce data.

Using GAGG:Ce and HPGe data, we noticed that there are possibly
two different decay time for the 511 keV line. In fact, it can be seen on
Fig. 18 that HPGe and GAGG:Ce data cannot be fitted with an exponen-
tial with a unique decay time. Yoneyama et al. [3] have also reported
the presence of 2 decay times in their 511 keV line measurements,
of respectively 14.0 ± 3.0 min and 82.0 ± 13.1 min. Unfortunately,

our HPGe measurements are not sufficient to get a proper estimation
of the slow component decay time. If we anyway try to determine
the two components of the decay time with our measurements, we
find them to be of the order of ~7 min and ~54 min respectively,
with large uncertainties. The main question raised by these similar
results is: could the long component be a problem in space for scientific
measurements? Indeed, if the 511 keV line does not completely decay
before two consecutive SAA passages, the line flux can pile-up at every
passage and become an important background source for the scientific
measurements. In order to estimate if stacking could be produced, we
used OMERE Software [11] to estimate the number of SAA passage in a
day. SAA passages occur in groups of 5 to 6 passages by half-day, with
~1h30 between each passage in a half-day, and ~5 h between each
group. We created a simple toy model to simulate the background level
with time. The background level is enhanced every ~1h30, by adding
to the background signal an exponential signal, the afterglow, with a
decay time of 82 min, for a duration of 5 months. The simulation shows
that the signal is not stacked and therefore the long component of the
<figure><img src="image_20.png" /><figcaption>**Fig. 17.** Energy spectrum acquired with GaGG-F (EPIC) after a proton irradiation of ~15 min with an energy of 70 MeV and a flux of 10,000 pr/cm²/s. 4 activated lines are visible (48, 85, 160, and 320 keV). The 511 keV-line is also an activated line, but mixed with the ²²Na 511 keV line here. 1275 keV mentioned in black is the ²²Na line.</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**Fig. 18.** GAGG:Ce and HPGe measurements of the 511 keV emitted by GFAG crystal, after an irradiation of ~15 min at 70 MeV and 100,000 pr/cm²/s. A geometrical factor of 1.44 is used to scale HPGe data on GAGG:Ce data (explanation given in the text). Two decay times are estimated with an exponential fit: 6.6 min and 54 min. A ¹³⁷Cs source has been used during this measurement, but should not impact the decay of the 511 keV line.</figcaption></figure>

511 keV will not pollute the measurements in space even after several years of exposure time.

### 4.5. Discussion about the proton irradiation

Studying GAGG:Ce crystals, we have observed a phenomenon called *afterglow*. We found out that this phenomenon was composed of two components: phosphorescence and radioactivity. The phosphorescence contribution is both triggered by exposure to visible light and high-energy protons, contrary to radioactivity contribution that has been revealed by our measurement campaign and is only triggered by high-energy protons. This afterglow could be a problem for space applications, if the data were polluted during a large part of the orbit. The irradiations performed in this study simulate what will be encountered by GAGG:Ce crystals when used for space applications in the SAA. We quantified that it is present only for ~3 min after the end of the exposure, that is very short compared to an orbit duration of ~1h30. All the GAGG:Ce types used in this study are then usable for space

applications. Moreover, the activated lines due to high energy proton irradiation, can be used in flight in order to perform/verify the energy calibration during few minutes after the SAA passage.

For comparison, we can talk about the rate and energy spectrum of the events we aim to detect in future missions. We expect about hundreds TGFs per year to be detected (for our spectrometer of 64 cm² detection area). Their energy spectrum is a continuous spectrum, with an exponential cut-off at 7 MeV. Concerning GRBs, we could expect to detect tens of GRBs per year and their energy spectrum is ∝ $E^{-2}$, with a maximum around some MeV. The 3 min-afterglow after a SAA passage of 15 min will not impact their detection.

We could try to make comparisons with works of Yoneyama et al. [3] and Dilillo et al. [4] for instance, which also reported results about GaGG:Ce proton irradiations. However, both these studies and ours are different in several points, which limits the comparison of the results. We have irradiated GaGG:Ce crystals with doses corresponding to ~1 SAA passage, and a total dose received corresponding to ~1 month for GaGG-F crystal, which was the most exposed one. Our aim was
to see the impact of the irradiation on our measurements, thus we quantified the afterglow using the GaGG:Ce itself. Yoneyama et al. [3] have irradiated their GaGG:Ce sample at least ~100 times more, with 10 krad using 70-MeV protons, equivalent to ~10 years' exposure in LEO. They have measured radioactive afterglow emission emitted by GaGG:Ce using a HPGe detector, thus the afterglow that is seen "outside" the GaGG:Ce. Dilillo et al. [4] have irradiated their GaGG:Ce samples with doses corresponding to 6 month to 10 years in orbit. They have quantified the afterglow in terms of current. Moreover, we have no clues on the precise chemical composition or on the doping (e.g. magnesium) of each GaGG:Ce used in each study, thus it is difficult to explain the differences noticed in the results. Finally, Yoneyama et al. [3] have quantified the afterglow between 30 keV and 600 keV using a HPGe detector, whereas we did it between 1 MeV and 3 MeV, using the GaGG:Ce itself. In conclusion, we cannot compare precisely the results of our 3 studies, but one can easily see that the presented observations show that afterglow is present, but should not hamper measurements in space for our application, namely the detection of TGFs and GRBs.

## 5. Conclusions

The characterization of the different GAGG:Ce performances we performed in this work is important to adapt the sensor to the scientific goals, such as detecting fast gamma-ray events like TGFs. The afterglow after proton irradiations, simulating SAA passages, has been found to be small enough not to pollute TGF or GRB detections. It should not impact much the spectrometer measurements along the orbits in view of the decay times involved. The 511 keV and 160 keV lines in particular could even be an advantage to establish the energy calibration during flight.

We conclude that the different GAGG:Ce scintillators tested are well suited for space applications, but the fast EPIC GAGG:Ce, called GaGG-F, seems the best candidate for our TGF dedicated mission.

## CRediT authorship contribution statement

**Melody Pallu:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis. **Damien Pailot:** Writing – review & editing, Project administration, Methodology, Investigation. **Eric Bréelle:** Writing – review & editing, Methodology, Investigation, Formal analysis. **Philippe Laurent:** Writing – review & editing, Validation, Investigation, Formal analysis. **Jérôme Carron:** Project administration, Funding acquisition. **François Lebrun:** Writing – review & editing, Validation, Investigation, Formal analysis. **Charbel Koumeir:** Writing – review & editing, Resources, Formal analysis. **Claude Chapron:** Resources. **Kévin Biernacki:** Resources.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

This work is supported by the French space agency (CNES).

## Data availability

Data will be made available on request.

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