1. Introduction
The family of lanthanide orthotantalates has been widely investigated due to its high chemical stability, proton conductivity [1–3], luminescence [4–12], and so on. One characteristic of these materials is the very high density ranging from7.8 g/cm3 of LaTaO4 to 9.75 g/cm3 of LuTaO4, which makes them very attractive as scintillator crystals [9] or X-ray phosphors [12]. Recently, crack- free GdTaO4 (hereafter abbreviated as GTO) crystal with large dimension was grown successfully by our group and it was found that GTO was a promising scintillator in terms of high density (8.94 g/cm3), and is the highest among current inorganic scintilla- tors, relatively fast decay and more favorable light yield compared to PbWO4 [13]. However, there exists a slow component in either photoluminescence (PL) decay or scintillation decay, which is detrimental to the timing resolution in application that requires fast response such as high energy calorimetry. It should also be noted that the light yield of GTO is still too low to be used in medical imaging, although around three times as that of PbWO4 [14]. Apparently, to find its way in practical scintillating applica- tions, the scintillation mechanism of GTO needs to be understood, thereby improving the scintillation efficiency and shortening the decay time.