Most probably because in most commercial phosphors the trapping states are created by the doping impurities. If the phosphor is properly created, then the trapping states created by irradiation should have a negligible density compared to the traps created by the impurities. Thus, the irradiation will result mainly in populating the traps created by the impurities.

Thank you Dr. Radu...But is it sure that in most commercial phosphors the trapping states are created by the doping impurities? For example, are the main glow-peaks (2-4) in LiF:Mg,Ti are all due to the Mg (or Ti)?

How can we know if the trapping states are due to the doping or due to the radiation?

My reply is along the lines given by Dr. Radu. The main peaks in LiF:Mg,Ti are related to the Mg and Ti impurities or to other, unknown impurities and/or defects. In most cases, irradiation indeed contributes only to to the filling of existing traps, although the creation of new defects is possible, in particular if the sample is bombarded by high-energy heavy particles. A relatively easy way to tell whether traps are created or merely filled up by the irradiation and emptied during heating is to observe whether the effect is building up in a series of irradiations of the same sample. New created defects (if any) are not expected to disappear by heating. Thus, one may expect that the sensitivity of a sample will increase from one step to the next in a series of irradiations and heatings. If the sensitivity remains constant, most probably we are dealing only with filling and emptying of traps rather than creating new ones.