Marina Derdzyan

Consequences of Ca Codoping in YAlO3:Ce Single Crystals

The influence of Ca codoping on the optical absorption, photo-, radio-, and thermo-luminescence properties of YAlO3 :Ce (YAP:Ce) crystals has been studied for four different calcium concentrations ranging from 0 to 500 ppm. Ca codoping results in a partial oxidation of Ce3+ into Ce4+ , The luminescence time response under pulsed X-ray excitation of the Ce3+ /Ce4+ admixure clearly demonstrates the role of hole migration on both the rise time and the generally observed slow components. From an application point of view, Ca codoping significantly improves the timing performances, but the induced presence of Ce4+ ions is also the cause of a reduction in scintillation efficiency.

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.

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 were studied under excitation by gamma-rays from a 137Cs source. Both series 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 of LuAP:Ce crystals measured in 0.2 cm times 0.2 cm times 0.8 cm pixels increases linearly with the Ce concentration reaching at 0.58 at. % 6448 plusmn 322 ph/MeV and 9911 plusmn 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 plusmn 0.4% (I = 0.2 cm), 9.5 plusmn 0.5% (I = 0.8 cm). Intentional co-doping with Sc + ions was tested and resulted in increase of the Ce distribution coefficient to about 0.3. 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 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.

Dissimilar behavior of YAG:Ce and LuAG:Ce scintillator garnets regarding Li+ co-doping

Using a combination of experimental methods, the substitution tendencies of Li+ and involved charge compensation mechanisms are determined and compared in two important similar scintillators, LuAG:Ce and YAG:Ce. The studies were performed on polycrystalline samples prepared by solid phase reactions as well as in single crystals grown by the vertical Bridgman method. The results show that Li+ in LuAG:Ce, Li is localized mainly at Lu3+ sites with charge compensation brought about by Ce3+ → Ce4+ conversion and creation of anion vacancies. Surprisingly, unlike LuAG:Ce, Li, no evidence for Li+ substitution at Y3+ sites and conversion of Ce3+ to Ce4+ has been obtained in YAG:Ce, Li. It suggests that Li+ goes mainly to interstitial positions and does not interact with Ce3+. From an application point of view, Li+ co-doping of YAG:Ce is not efficient to improve the scintillationdecay parameters, however, it is efficient to reduce the amount of anion vacancies in both YAG and YAG:Ce.