Timing Resolution and Decay Time of LSO Crystals Co-Doped With Calcium

A comparative study of five 5 × 5 × 5 mm³ LSO:Ce samples with different co-dopings of calcium, from 0 to 0.4% is reported. All the results are compared to those obtained with selected 10 × 10 × 5 mm³ LSO crystal, tested in previous studies. The influence of Ca co-dopant on scintillator parameters was studied using a Photonis XP20D0 fast timing photomultiplier. The tests included measurements of light output in terms of number of photoelectrons, measurements of time resolution in coincidence experiments with annihilation quanta from a ²²Na gamma source, and decay time constant calculations on the basis of timing spectra obtained using the Bollinger-Thomas single photon method. Also energy resolution for 662 keV from ¹³⁷Cs gamma source is mentioned at the end of the paper. The results showed the significant influence of the Ca co-dopant on the properties of the LSO scintillators. For low Ca concentrations of 0.1%, photoelectron number and energy resolution were improved up to 7800 phe/MeV and 7.3 % respectively. On the other hand the shortest decay time constants were obtained for higher concentrations of 0.3% and 0.4% of Ca giving values equal to about 30 ns. The time resolution improvement was similar for all the samples and calculated values for a single detector were equal to about 140 ps.