# Photo-Stimulated Luminescence Approach for Effective Identification of Irradiated Fruit
## 1 Introduction
Moreover, a few spices, the so-called \"clean spices\" such as white pepper or nutmeg, are so poor in silicates as provide weak PSL signals even after irradiation, which is not useful in detecting the treatment. The method is normally used for fast screening of samples through a first PSL measurement. However, when possible, a second measurement (PSL calibrated) after laboratory irradiation (1 kGy), would allow researchers to check the sample sensitivity (contamination), thus giving a more accurate response in those cases. The method has been validated for herbs and spices, but in principle, it can be applied to all vegetables sufficiently \"contaminated\". Regarding fruit, successful studies were carried out on several kinds of nuts [20] with the exception of chestnuts, which show a rather \"clean\" peel with mineral contamination and irradiation not detectable via the PSL technique [21; 22]. On the contrary, the PSL responses obtained from the irradiated samples are often non-positive for fresh fruit. This is true in the case of pomergnates irradiated with doses lower than 3 kGy [23], graprefruits and lemons irradiated with doses lower that 1 kGy and oranges irradiated with doses lower than 2 kGy [24; 25]. However, a study by [PERSON] et al. (2016) [26] reported positive PSL results for graprefruits and lemons irradiated with 1 kGy as well as for oranges, mandarins, limes and pineapples irradiated with the same dose. Positive results were also obtained by [PERSON] et al. (2006; 2008) [27; 28] when analyzing kiwi irradiated with doses of 1 and 2 kGy. Given the simplicity and rapidity of the PSL analysis, it seemed convenient to test further the method on this category of food, to verify the feasibility of its use in the official control, as an alternative to other methods, mentioned above, which require expensive equipment and/or long time of analysis. In particular, in this work, funded by the Italian Ministry of Health, 15 fruit types, not irradiated and irradiated (0.5 and 1 kGy), have been examined. The work included preliminary tests to select the matrices that responded to the PSL technique and an inter-laboratory blind test to validate the method. The preliminary tests focused mainly on fresh fruit that yielded inconsistent results in previous studies. In particular, the following products were tested:fresh kiwi, lemon, mango, orange, papaya, pineapple and tangerine, dried date, dried prune and raisin. The inter-comparative blind test involved four official control laboratories that analyzed eight matrices: four that gave good results in the preliminary tests (kiwi, orange, dried fig, raisin) and four types of nuts (hazelnut, peanut, pistachio and walnut), to confirm previous good PSL responses [20]. In total, 384 tests were carried out, 256 on aliquots of irradiated samples and 128 on aliquots of non-irradiated samples. Figure 1: (**a**) Stimulated luminescence according to the band model of insulating solids: on the left, an electron and a hole produced by ionizing radiation and trapped in the meta-stable levels due to the crystalline structure defects; on the right, the recombination of the charges with emission of luminescence after thermal or photo stimulation. (**b**) PSL measurement. The sample is exposed to pulsed infrared radiation (890 nm) that favors the de-trapping of the charges from the defects with subsequent emission of the luminescence (visible light) that is converted into electrical charge (counts) by the phototube.