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Cerium-doped YAlO3:Ce (YAP:Ce) is an interesting oxide scintillator that exhibits a wider range of light yield nonproportionality on a sample-to-sample basis than most other well-known oxide scintillators. In general, most oxide materials, such as Bi4Ge3O12 and Lu2SiO5:Ce, are thought to have an intrinsic proportional response that is nearly constant between samples and independent of growth conditions. Since light yield nonproportionality is responsible for degrading the achievable energy resolution of all known scintillators, it is important to understand what contributes to the behavior. In an attempt to understand if the phenomenon can be affected by growth parameters or by other means, seven samples of YAP:Ce were collected from various sources, and eight samples were grown in-house using the Czochralski method. Based on optical and scintillation measurement as well as direct measurement of the cerium concentration, it was determined that the light yield proportionality in YAP:Ce is strongly related to the cerium concentration. Samples that were found to have higher relative cerium concentration displayed a more proportional light yield response. In addition, it was determined that samples with higher cerium concentration also exhibit a faster decay time and an enhanced energy resolution when compared to samples with less cerium. It was also determined that growth in a reducing atmosphere can effectively suppress a parasitic optical absorption band. [ABSTRACT FROM PUBLISHER]