The simple answer is: it won't kill immediately but depending on the length of the exposure before reaching a shielded area, it would definitely cause radiation sickness which would result in a whole bunch of symptoms similar to the symptoms recorded after the exposure to radioactive fallout during the Chernobyl reactor accident (like cancer, neural damage of some kinds, etc.). But all of those problems are long-term effects.
The armosphere and the suits the astronauts are wearing would already provide a relatively good shielding against a big part of the radiation. So I would assume, that exposure of several hours would definitely be not healthy, but managable.
An underground base would easily protect the astronauts from radiation, since the regolith is very good at blocking it, which is why some plans of mars bases include burrying modules under ground.