sterilization and mutation by irradiation

Radiation is used to keep food good condition. for example in order to kill kind of bacteria or germs on food, in order to prevent from potato's budding.

There is a interesting report on food irradiation, which says the result of experiment of ratio of sterilization and mutation by irradiation.
http://foodirra.jaea.go.jp/dbdocs/001005000010.html (japanese)

When almost all the little simple creature are killed by radiation, still some of them are survived, and some of surviver  have evolved to mutant. I am interested in that mutant ratio lines look linear.