Permanent radiation effects are those changes in materials which persist after the material is removed from the radiation field. In most cases these effects are due to lattice displacements. In many materials annealing processes take place, the rate being a function of the temperature, which tend to return the material to a condition similar to that prior to irradiation. It is possible with some materials for this back-reaction to proceed at such a rate that evidence of radiation damage cannot be observed. The term “relaxation effects” is sometimes applied to changes which anneal out at room temperature in a matter of minutes or hours.