Folks, it's not supposed to be a passive transmitter of radiation, like a chemical element.
It'd rather be made somewhat like modern day X-ray devices, with a power supply, radiation generator, and ... well, some simple computer logic to determine when to fire it, and computer being in a protected shell. Also, explosive charge, just enough to blow up the mine when it's PSU is dried.
Actually, it wouldn't really be too high tech device even by today's standards.
Exactly as 11 said, it wouldn't be releasing chemicals all the time I mean there are things like this in use today, obviously once the mine was released it would begin emitting the radiation either through some sort of chemical combination or other techno-babble reason.