Hmmm, You could make a timer that starts when you get into the radiation zone and ends if you get out. But to knock the player out wire the counter to a respawn device. Target value reached (0) > respawn player. Make everything player scoped, and you can even display the amount of the time player has left using a property, block code, and an overlay.
(Guide for the timer:
How to make a colon-separated timer (Difficulty: 🟨))
ThatGim
Thanks!!!