Secondary-electron distribution for heavy ions.

ions in passing through matter. The transverse distribution results from absorption of energy by the medium due primarily to the slowing down of the secondary electrons which have been ejected from the primary particle's track. Delta (5) rays are generally those secondary electrons that have an initial energy above some arbitrary threshold value so that there is a high probability that most of their energy will not be deposited in the vicinity of the ion track. An alternative definition would be that a rays are all secondary electrons which are ejected further than a given arbitrary distance from the ion track. In radiation biology, many radiation effects of heavy ions have been interpreted in terms of an inactivation cross section. Part of this total cross section is due to the ion track, and another part is due to 6 rays. The estimated cross section due to the 5 rays is subtracted from the total cross section to obtain the cross section due to the ion track. This procedure is called the 5-ray correction. Although it is well known that the contribution of 5 rays to the radiation effect of heavy ions is quite significant, our knowledge of 5 rays is very poor experimentally as well as theoretically. Thus far, almost all the 5-ray corrections have been made with several simplified assumptions about the processes of 5-ray production as well as on their slowing down, which have not yet been verified by experiment. Although it is most desirable to obtain complete experimental knowledge of the physical properties of the 8 rays, some specific information on 5 rays may suffice (depending on the methods of interpretation of the radiation effects, such as the target-theoretical analysis) for the study of the biological effect. Therefore, first let us discuss two alternative possible approaches to the 5-ray correction in connection with the present status of experimental information available for the low-energy secondary electrons. Then we shall present results of the measurements carried out on the secondary electrons from heavy ions from the Lawrence Radiation Laboratory's Hilac.