Any organism will die if it absorbs to too much radiation.

For death to occur within hours of exposure to radiation, the dose would need to be 10 Gy (1000 rad) or higher.

For death to occur within 60 days, the dose would need to be 4-5 Gy (400-500 rad).

Doses less than 1.5-2 Gy (150-200 rad) would not be lethal but would incur a higher risk of cancers.