Onet unearthed an interesting story from 1980 #Poland when the communist intelligence services used highly radioactive isotopes to scan through safes in Western embassies to covertly copy documents.

The agents used iridium-192 and cobalt-60 isotopes, which are normally used for medical or industrial purposes, seeking for hidden defects in steel and other dense materials. High energy gamma rays emitted by the isotopes allow photography through steel, as if it was paper. For the same reason they are also dangerous for humans, as the high energy radiation damages cells and DNA, causing radiation disease and cancer.

As many things in the communist Poland, the state did not care for its servants and the intelligence operated the isotopes without any protection. They did deliver intelligence, as the gamma ray photography allowed them to see the safe lock mechanism and recover the opening combinations, but some of them became ill and died as matter of months after participating in the program.

Source: https://www.onet.pl/informacje/lowcy-szpiegow/elita-elit-to-dzieki-nim-kontrwywiad-prl-u-byl-jedna-z-najlepiej-poinformowanych/zcxrt3k,0666d3f1 (in Polish)