Simulation study of light transport in laser-processed LYSO:Ce detectors with single-side readout

Abstract

A tightly focused pulsed laser beam can locally modify the crystal structure inside the bulk of a scintillator. The result is incorporation of so-called optical barriers with a refractive index different from that of the crystal bulk, that can be used to redirect the scintillation light and control the light spread in the detector. We here systematically study the scintillation light transport in detectors fabricated using the Laser Induced Optical Barrier technique, and objectively compare their potential performance characteristics with those of the two mainstream detector types: monolithic and mechanically pixelated arrays. Among countless optical barrier patterns, we explore barriers arranged in a pixel-like pattern extending all-the-way or half-way through a 20 mm thick LYSO:Ce crystal. We analyze the performance of the detectors coupled to MPPC arrays, in terms of light response functions, flood maps, line profiles, and light collection efficiency. Our results show that laser-processed detectors with both barrier patterns constitute a new detector category with a behavior between that of the two standard detector types. Results show that when the barrier-crystal interface is smooth, no DOI information can be obtained regardless of barrier refractive index (RI). However, with a rough barrier-crystal interface we can extract multiple levels of DOI. Lower barrier RI results in larger light confinement, leading to better transverse resolution. Furthermore we see that the laser-processed crystals have the potential to increase the light collection efficiency, which could lead to improved energy resolution and potentially better timing resolution due to higher signals. For a laser-processed detector with smooth barrier-crystal interfaces the light collection efficiency is simulated to >42%, and for rough interfaces >73%. The corresponding numbers for a monolithic crystal is 39% with polished surfaces, and 71% with rough surfaces, and for a mechanically pixelated array 35% with polished pixel surfaces and 59% with rough surfaces.

Light Spread Manipulation in Scintillators Using Laser Induced Optical Barriers

“…Fig. 8 shows flood maps for this design compared to the half-way barrier pattern studied in [21]. These results showcase one way of using the LIOB technique to address the edge effect issues common to many detector designs.…”

“…The barrier/crystal interface roughness, as well as the outer pixel surface in the mechanical array, was described using the UNIFIED surface model in DETECT2000. The σα parameter was varied to account for different surface roughness, as explained in [21], [22]. Fig.…”

“…These simulations were also used to calculate the light collection efficiency as an average over all source locations, for each detector configuration. Further details regarding the simulation setup can be found in [21].…”

“…In previous work we performed simulations to compare the performance characteristics of laser-processed detectors with the two standard detector types being mechanically pixelated arrays and monolithic crystals [21]. For this baseline study we implemented laser-processed detectors with simple optical barrier patterns (1.05 mm pitch) extending all the way or half way through a 20 mm thick LYSO:Ce crystal, and the results of this study are summarized in Fig.…”

“…The values for the monolithic crystal, the mechanical array, as well as the all-way and half-way barrier patterns are taken from Ref. [21]. The uncertainties correspond to the one σ variations in light collection efficiency over the detector volume.…”