I would assume that the base would have been properly built to withstand radiation or the government would never have signed off on a manned operation, and radiation affects various individuals at different rates, and gradually over time, but in the movie it was like clockwork. For these two reasons I think it was engineered. Also, they couldn't risk a clone somehow not dying on time when another clone is about to be brought on line, or wondering where the ship is, or other stuff. Definitely would need the timing to be spot on.