Cell death after irradiation occurs by apoptosis in certain cell popul
ations in tissues. The phenomenon also occurs after high linear energy
transfer (LET) irradiation, and the relative biological effectiveness
(RBE) is 3 to 4 (with respect to low-LET radiation and apoptosis in i
ntestinal crypts) for neutrons with energies of 14 MeV and up to 600 M
eV. It is thought that p53 plays a role in the phenomenon, as radiatio
n-induced apoptosis is not observed in p53-null animals.