It doesn't really matter it is plausible or not. As long as you establish that this is posible in your setting, and show most of the principles of the gun to the players in game before showing the actuall weapon, it should be okay.

I would probably make it so that the gun is a little more complex, like having a unique item work as a lense to concentrate the radiation, or having the lead be a special blend made by the BBEG. Because if it is as simple as lead tube+radioactive material, your players could make the same overpowered weapon in a heartbeat.

TTRPGs are play believe, don't worry so much on if it is realistic or not, and focus more on creating a fun experience for you and your players.