Which of these factors has no effect on response of cells to irradiation?

The dose required to produce a certain biologic effect is reduced as the linear energy transfer (LET) of the radiation is increased. Higher LET radiations (e.g., α particles) are more efficient in damaging biologic systems, because their high ionization density is more likely than X rays to induce double-strand breakage in DNA. The greater cell damage sustained in the presence of oxygen is related to the increased amounts of hydrogen peroxide and hydroperoxyl free radicals formed. The term 'dose rate' indicates the rate of exposure. Exposure of biologic systems to a given dose at a high dose rate causes more damage than exposure to the same total dose given at a lower dose rate. White and Pharoah's Oral  Radiology Principles and Interpretation 8th edition