If you are doing 4th ed Gamma World, prep multiple characters in advance. It's deadly, and with little healing. Some creatures are extremely deadly for their level.
I'd almost recommend each player playing two characters at once if they can handle it.
I ran the starter adventure for a group of 5 over the course of several weeks ( we game once a week) and I think we lost at least two people a night, who then made up new characters.
It's fun, but deadly.