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Once the explosive is laminated inside its (carefully cleaned) aluminum pouch, there's no way to detect the chemical signature of the PETN. The pouch seals that all in. The PETN and other components of the battery are too similar to one another to be detected with X-ray fluorescence, and the multi-layer construction of a battery also foils attempts to peer inside it with Spatially Offset Raman Spectroscopy.

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