Abstract
Scintillators play a key role in the detection chain of several applications which rely on the use of ionizing radiation, and it is often mandatory to extract and detect the generated scintillation light as efficiently as possible. Typical inorganic scintillators do however feature a high index of refraction, which impacts light extraction efficiency in a negative way. Furthermore, several applications such as preclinical Positron Emission Tomography (PET) rely on pixelated scintillators with small pitch. In this case, applying reflectors on the crystal pixel surface, as done conventionally, can have a dramatic impact of the packing fraction and thus the overall system sensitivity. This paper presents a study on light extraction techniques, as well as combinations thereof, for two of the most used inorganic scintillators (LYSO and BGO). Novel approaches, employing Distributed Bragg Reflectors (DBRs), metal coatings, and a modified Photonic Crystal (PhC) structure, are described in detail and compared with commonly used techniques. The nanostructure of the PhC is surrounded by a hybrid organic/inorganic silica sol-gel buffer layer which ensures robustness while maintaining its performance unchanged. We observed in particular a maximum light gain of about 41% on light extraction and 21% on energy resolution for BGO, a scintillator which has gained interest in the recent past due to its prompt Cherenkov component and lower cost.
Highlights
Introduction published maps and institutional affilScintillating materials are commonly used in high energy physics and medical applications because of their capability to downconvert high energy radiation into optical photons
The novel techniques, which we investigated include the use of Photonic Crystals (PhCs) nanoimprinted on the crystal output surface, as well as Distributed Bragg Reflectors (DBRs) and metal coatings applied to the other crysal surfaces
(2.15 for BGO vs. 1.8 for LYSO), and the fact that the simple use of a refractive index matching material is already providing good optical coupling for LYSO. This explanation is accredited by similar performance trends for both LYSO and BGO in experiments featuring PhCs deposited on crystals which are air-coupled to photodetectors
Summary
Scintillating materials are commonly used in high energy physics and medical applications because of their capability to downconvert high energy radiation into optical photons. Scintillators are usually instrumented with a photosensor and coupled to its sensitive surface. Photomultiplier Tubes (PMTs) were the most used photodetectors to read out the scintillation light, but nowadays silicon-based devices such as Silicon. Photomultipliers (SiPMs) have moved into a prominent position. This trend is due to their more compact structure, lower supply voltage, and robustness to magnetic fields. The light sensitivity of both PMTs and SiPMs do usually not exceed 30% at short wavelengths (e.g., 300 nm) in commercial devices [1], and peaks at around 50–60% in the visible for. The amount of light generated during the scintillation process in inorganic scintillators is in the range 8–60 k photons/MeV
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