>how would I get caught

lets hope that in post 9/11 world dosimeters are deployed at various important (i hope it includes "crowded with normal people") places.

>what would I be prosecuted with?

>walk around terrorizing people...

seems like you answered your own question.

>feel like it's not too difficult for someone today to build something like that

your old CRT ("cathode ray tube") TV/monitor :)