Comparison of spectral and scintillation properties of LuAP:Ce and LuAP:Ce, Sc single crystals

Scintillation properties of LuAP:Ce and LuAP:Ce,Sc crystal series grown by the Bridgman method were studied under excitation by γ-rays from a ¹³⁷Cs source. Both series were prepared using the same quality of starting oxides and demonstrated comparable optical quality in terms of underlying absorption at 260 nm, slope of the optical edge and transmission in the range of emission. The light yield in the present series of LuAP:Ce crystals measured in 0.2 cm × 0.2 cm × 0.8 cm pixels increases linearly with the Ce concentration reaching at 0.58 at.% 6448±322 ph/MeV and 9911±496 ph/MeV in the long and in the short directions respectively (the light yield ratio is 65%) and shows no sign of light saturation. The energy resolution is found to depend, among other factors, on the uniformity of Ce concentration within the pixels and is improved to 7.1±0.4% (l=0.2 cm), 9.5±0.5% (l=0.8 cm). Intentional co-doping with Sc³⁺ ions was tested and resulted in increase of the Ce distribution coefficient from 0.17 in LuAP:Ce to about 0.3 in LuAP:Ce,Sc. This enabled to increase the concentration of Ce in LuAP:Ce,Sc crystals up to 0.7 at.%, while conserving high optical quality. In contrast to LuAP:Ce, the light yield in LuAP:Ce,Sc crystals does not increase with Ce concentration, the photo peak being gradually suppressed. The involved mechanisms are discussed basing on the results of measurements of the unit cell volumes, Ce concentration uniformity, x-ray rocking spectra, absorption spectra of pure and variously doped LuAP crystals, and emission spectra under different excitations.