In American-British drama miniseries Chernobyl, on the second episode, a scientist takes some dust from the outer surface of a window and places it in an instrument that tells her what radioactive isotope was in the sample.
She places the sample in the device in the middle and then fiddles with the electronics. The device on the right seems to include some form of oscilloscope. She then tells the colleague, upon handing him the printout,
"Iodine 131. It's not military, it's uranium decay, U-235."
What was the instrument that was used? And how was the scientist able to deduce that there was a nuclear explosion?