RBE as a function of depth in a 650 MeV He++ beam determined for growth delay in Vicia faba and for induction of chromosome aberrations in Allium cepa

Variation of RBE as a function of depth in the irradiated medium was determined for a 650 MeV He++ beam. Two biological systems were investigated: growth delay in Vicia faba bean roots and induction of chromosome aberrations in Allium cepa onion roots. Irradiations were performed at four depths: in the initial plateau region (taken as a reference radiation quality); at the beginning: at the middle, and at the end of the spread-out Bragg peak (2 cm, 13 cm, 15 cm and 17 cm in depth respectively). The spread-out Bragg peak was about 5 cm broad. For both vegetal systems investigated, RBE increases with depth, but RBE values observed for chromosome aberrations are systematically higher than for growth delay. For an absorbed dose of 0.5 Gy of the test radiation quality, RBE values for growth delay are 1.45, 1.5 and 1.95 at 13 cm, 15 cm and 17 cm in depth respectively. For chromosome aberrations, RBE values are 1.4, 2.0 and 2.5 respectively. Present results are compared with those previously obtained for d(50)-Be neutrons. Biological results are correlated with microdosimetric data obtained for the same radiation qualities