Optimisation of fibre-optic readout of LSO scintillation crystals with acid etching

Optimising the collection of scintillation light is essential for good PET scanner performance, and even more so when the crystals are read out through optical fibres. Acid etching had been proposed as a cost-effective alternative to mechanical polishing, but there are discrepancies between the results published so far. The aim of this work is to gain a better understanding of acid etching and of its effects on the light yield and to assess its application to fibre-optic based configurations, We have examined the surface state of LSO crystals etched for various times and measured their light output in different configurations, with both direct and fibre-optic readout. Our results indicate that crystal etching is a complex process, where different crystal faces may be etched in different ways. Acid etching always resulted in an improvement the light yield and energy resolution of the crystals, and was as efficient as or in many cases superior to, mechanical polishing. While this improvement was somewhat limited for the configurations with direct readout of the crystals we tested, it was much larger for the fibre-optic based configuration.