Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
Space applications of GAGG:Ce scintillators: a study of afterglow emission by proton irradiation
Introduction
The need for this work arose in the context of HERMES-TP/SP1 mission concept during 2017. HERMES aims to localize and study bright high-energy transients – such as Gamma-Ray Bursts – by hosting miniaturized X/soft- detectors on board nano-satellites in low-Earth orbit (LEO). HERMES detectors are designed around the so-called “siswich” concept [1] in which silicon detectors play the double role of sensor for the scintillation light emitted by suitable scintillator crystals and of independent detector for low energy X-rays. The scintillator selected for use on HERMES units is GAGG:Ce (GdAlGaO12:Ce, Cerium-doped Gadolinium Aluminum Gallium Garnet). It is a promising scintillation crystal displaying a wide array of appealing features for space applications: very high light-yield, fast-decay times, very low intrinsic background and mechanical robustness.
However, GAGG:Ce is still a recently developed scintillator [2] and, as a consequence, literature is lacking on points crucial to its applicability in space. For example, GAGG:Ce is characterized by unusually intense and long-lasting afterglow emission [3], a slow phosphorescence component in scintillation light. Afterglow emission is a source of background noise and is induced by the exposure of GAGG:Ce crystals to electromagnetic and particle radiation. Hence, in space applications, an effective degradation of the detector energy resolution should be expected as a result of the phosphorescence induced by the interaction of the energetic particles in the near-Earth radiation environment with the GAGG:Ce scintillators; the extent and dynamics of such phenomena depending on both the host spacecraft orbit and the crystal intrinsic properties. Besides degradation of the energy resolution, the current induced in the SDD sensors by the background light due to the afterglow emission may become too large, impairing the functionality of the Front-End Electronics (FEE). To tackle this last concern we conducted an irradiation campaign at Trento Proton Therapy Center (TPTC) in which a GAGG:Ce sample was irradiated with 70 MeV protons. The choice of particle specie, energy and fluences was driven by the need to simulate the nature of the radiation environment of near-equatorial LEO orbits and the constraints of the cyclotron particle accelerator available at TPTC and of our equipment.
The corpus of this paper is arranged in three parts. In the first part we describe the experiment set-up and timeline. In the second part we discuss GAGG:Ce phosphorescence, introduce our model of the afterglow emission, discuss the impact of activation on our observations and outline the fit procedure and results. The afterglow model development is discussed in detail in Appendix. Finally we make use of the afterglow model, supported by the AE9/AP9 trapped radiation belt models [4], to estimate the impact of the afterglow emission resulting from the orbital radiation environment on the performance of the instrument.
Section snippets
Experiment outline
A GAGG:Ce scintillation crystal – dimensions 3 × 1 × 1 cm – was housed in a lightproof metal case hosting two PMT detectors. The large faces of the crystal were wrapped in thin white teflon to minimize scintillation light dispersion, while both the unobstructed small faces were coupled to cm long quartz light guides by means of optical grade silicone grease. The same coupling technique was used to interface the light guides to the photocathodes of the Hamamatsu R4125 PMTs. The anode signal
Afterglow emission models
Long-lived afterglow emission in scintillators is attributed to the existence of intrinsic or impurity defects within the crystal lattice. Some of the charge carriers (electrons or holes) liberated by the ionizing radiation can be trapped at defect sites into metastable states. At later times, charge carriers escape these sites by different processes (e.g. to the conduction band by thermal energy absorption [6] or to nearby recombination centers by direct or thermally assisted tunneling [7]).
Crystal activation
The energies of the protons, both in the near-Earth radiation environment and in the irradiation campaign, are high enough to induce a level of activation in GAGG:Ce crystals. In these settings, activation poses different challenges. In orbit, crystal activation is expected to result in a decay spectrum from the unstable nuclides that will interfere with the observation of astronomical gamma-ray sources. From the point of view of the irradiation tests, the scintillation accompanying the decays
Fitting data to the afterglow emission model
Due to a significant presence of scintillation light in the measurements the application of the afterglow models we developed is not justified because it entails precise meaning to the fit parameters. Nonetheless, being interested in a conservative estimation of the afterglow emission during the mission, the additional component due to crystal activation can be considered as yet another source of overestimation and we can still apply our modeling in a purely empirical way to reproduce the light
In-orbit impact of GAGG:ce afterglow on silicon drift detectors
On average the GAGG:Ce afterglow emission manifests itself as a continuous stream of optical photons with monotonically decreasing flux after stimulation. The randomly arriving photons are able to induce detectable anode current pulses on PMTs but not on the highly-efficient yet unamplifying SDDs. Hence scintillator afterglow will not result in triggering HERMES SDD front-end electronics. Instead it will behave as an equivalent leakage current component adding to the true device current.
In
Conclusions
In this paper we discussed our investigation of GAGG:Ce for applications in the context of the HERMES-TP/SP nanosatellite mission. The goal was to determine whether the delayed luminescence caused by the interaction between the scintillators and the energetic particles of the near-Earth radiation environment could pose a threat to the well-functioning of the detector. For this reason, we conducted an irradiation campaign in which a GAGG:Ce sample was irradiated with protons at dose levels
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 project has received funding from the European Union Horizon 2020 Research and Innovation Framework Programme under grant agreement HERMES-Scientific Pathfinder n. 821896 and from ASI-INAF Accordo Attuativo HERMES Technologic Pathfinder, Italy n. 2018-10-HH.0. We kindly thank the ReDSoX collaboration, the TIFPA staff and the anonymous reviewers.
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