Sorry for bumping an old thread.
But its true that various ratios can tell what the source are. Also, depending on the "fingerprint" (what radionuclides are seen) it can tell what temperatures might be relevant. Especially in nuclear power plant accidents, some elements are released relatively easy, such as noble gases, krypton, xenon and whatnot, but also Iodine. But some elements require relatively high temperatures to be released, and of course, if you see like plutonium or uranium in the measurement, then you have fuel fragments, and that is pretty much an indication of an explosion that just blasted away all kinds of stuff.
As for the source, well, I cant provide one. This stuff is not really mainstream science, and perhaps there are either no desire to spread info on this further (to avoid teaching how things can be detected) or no one is interested and have asked the questions
As for the measurement of these radionuclides, you measure them by gamma spectrometry using a high purity germanium detector (HPGe). All countries should have some air monitoring system, they take the filter, and then measure it (137,134Cs and I-isotopes are gamma emitters) and can be seen in the spectrum.
As for a plug-in detector to phones, could work maybe. There are some semi conductor detector system (including electronics) that can be powered from an usb port. But the resolution on these semi conductors operating in room temperature (in contrast to liquid nitrogen for HPGe) is really bad, so might be difficult to see all 4 radionuclides clearly.