• sipm
• x-optogenetics
• shaper
• optogenetics
• x-ray
• radioactivity environmental monitoring
• gagg
• scintillator

Scintillators are materials that can convert high-energy radiation, such as X- rays or gamma rays, to near-visible or visible light. They have been playing a major role in many fields, such as in medical imaging, radiation detection, clinical diagnostics, geophysical exploration.
In this work, in order to understand the potential of these materials, two important applications will be analyzed and characterized: X-Optogenetics and Radioactivity Environmental Monitoring.
The scintillator used in the two applications is the GAGG:Ce, Gadolinium Aluminium Gallium Garnet (Gd3Al2Ga3O12) doped with Ce. Thanks to its excellent scintillation properties, chemical stability, biocompatibility and fast scintillation response, it is well suited for both X-optogenetics and radiation monitoring.
Traditional Optogenetics has been a transformative technology in the area of neuroscience, it is a biological method that aims to monitor and control the biological functions of neurons using visible light. Light is used to control photosensitive proteins, called opsins, that promote or inhibit the firing of action potentials. Although its great potential, this method is limited by the fact that visible light cannot penetrate brain tissue very deeply and therefore invasive techniques must be applied for light delivery, such as implantation of light sources. To overcome this limitation, in X-Optogenetics, scintillator microparticles exposed to x-rays produce light. The scintillator microparticles can be injected into the brain region of interest and externally stimulated by focused X-rays. The advantages of X-Optogenetics include deep penetration depth, no increase in tissue temperature, and no need for implantation of light sources. This unique and less invasive approach allows to study the roles of various neurons in disease states such as Parkinson’s and Alzheimer’s disease, and to understand the causality between a neuron’s firing and resulting action, such as movement and emotions.
With the recently developed X-ray Fluorescence Computed Tomography (XFCT) system, it has been proven that it is possible to map the GAGG:Ce microparticles for X- Optogenetics purposes. This feasibility has been demonstrated by the Radiation Detection and Imaging group at the University of Illinois at Urbana-Champaign, with whom I had the opportunity to collaborate for a few months participating at the initial bibliographical part of the project presented at the 2022 IEEE Nuclear Science Symposium, Medical Imaging Conference and Room Temperature Semiconductor Detector Conference [1]. In fact, the local activation of optogenetic effects can be revealed by the XFCT Image of the GAGG:Ce particles.
In the second part of this work, the scintillator-based method for radiation monitoring will be discussed. Radioactivity environmental monitoring is the collection and examination of certain environmental media, as in this case air, to identify the quantity of radioactivity present. The usage of radioactive materials and radiation sources is now widespread not only in the nuclear fuel cycle but also in civil and military installations and in nuclear medicine. Therefore, the ability to compare radioactivity levels and safety standards is crucial to ensure a safe environment and avoid radioactive pollution with serious consequences for living beings.
The cesium iodide-based gamma-ray detection and spectroscopy system developed by Chierici et at. [2] will be analyzed and scaled in order to use the GAGG:Ce as a scintillator in the detection system.
The system is composed by a GAGG:Ce crystal coupled with a Silicon Photomultiplier (SiPM). The output signal of the SiPM is passed through a readout circuit which includes a preamplifier, a shaping stage, a multichannel analyzer.
Finally, the pulse height analysis is performed, which consists in examining the amplitudes of signals to determine the energies of radiations striking the detector. The pulse-height spectrum shows the number of events detected (“counts”) versus the amplitude of those events.
In addition, a very useful tool has been implemented to analyze and characterize the analog front end of the readout circuit. The purpose of the waveform generator is to recreate a realistic signal that could be generated by the SiPM monitoring an environment with ionizing radiations. Such current signal represents the input of the analog front-end, and by processing and analyzing the front-end output it is possible to evaluate the behaviors of the pre-amplification and shaping circuitry. It is also a useful tool to calibrate the system in case this needs to be adapted for another scintillator. Therefore, such tool may allow to evaluate the system performance in advance before actually implementing the system.
A promising aspect of using GAGG:Ce as a scintillator in radiation detection is that, thanks to the presence of Gd isotopes with high thermal neutron capture cross sections, GAGG:Ce crystal can be a valid candidate to distinguish between gamma rays and neutron pulses by using pulse shape discrimination (PSD) technologies.
Thanks to the size and weight of the detection system implemented, this technology is available for unmanned aerial vehicles (UAVs) or more in general for remote and mobile radiation monitoring purposes. This makes remote access to radiation-contaminated areas possible, eliminating unnecessary exposure of civilians or military personnel, but also allowing to explore inaccessible locations. Moreover, this system has been designed to detect and identify radiations with a low-cost instrumentation.
All the advantageous properties of this novel scintillator have been analyzed further and in detail to explain how the GAGG:Ce is one of the most promising scintillators in these areas.

References
[1] S. Mandot, E. M. Zannoni, L. Cai and L.-J. Meng, X-ray Fluorescence Emission Tomography of Implantable Micro-Scintillators for X-ray-induced Optogenetic Applications, 2022.
[2] A. Chierici, A. Malizia, D. Di Giovanni, R. Ciolini and F. d’Errico, "A High-Performance Gamma Spectrometer for Unmanned Systems Based on Off-the-Shelf Components," Sensors, 2022.