I'm quite new to scintillation detectors/radiation measurement, so pardon my ignorance if this has an obvious answer. One of my colleagues was recently conducting some tests using a CsI(TI) crystal wrapped in typical hardware store teflon tape. After being assembled for at least a couple weeks (I do not know the exact duration), he removed the crystal from the detector and unwound the teflon tape, only to find that the teflon tape had started turning slightly green (shown in the photo below, apologies for the size, this was all he sent me). As far as I am aware, he observed this green tint all the way along the length of the crystal, and the crystal itself did not exhibit any discolouration - only the teflon tape.
Just wondering if anyone here has observed this or has ideas of what may cause this? I'm aware that CsI(TI) is slightly hygroscopic and this wasn't kept in a sealed or dry-air environment. Is it possible this is an effect of moisture? There was also some optical grease applied between the crystal and SiPM. I have seen that optical grease will make the teflon tape almost completely translucent, but not turn green (though I did not conduct that experiment in proximity of a CsI(TI) crystal.
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-Luke
