Thats not how you would do it, the way you would do it is to place it in a shielded briefcase. There would be a mechanism on the briefcase that would allow you

Thats not how you would do it, the way you would do it is to place it in a shielded briefcase. There would be a mechanism on the briefcase that would allow you to open up an exposure focal point effectively exactly like a radiotherapy device. Taking this into a subway, coffee shop etc. opening up the aperture and aiming it at a person quietly for a hour would give them a potentially lethal dose of radiation. A serial killer, assuming they could construct and acquire the necessary radioactive materials could probably perform this many many times before being caught.