RBE and OER values of negative pion beams from growth inhibition of Vicia faba roots.

Two pion beams of different momentum width have been used to expose meristems of Vicia Faba roots under aerobic and hypoxic conditions. The measurements of the resulting 10 days growth inhibition after exposures at various locations on the pion beam axes have been made and RBE and OER values evaluated for 50% effects compared to 60Co gamma-rays. The results have been related to the fractional doses from star products defined by telescope measurements of stopped pions along the same beams. It has been found that the RBE value increases with the fractional "star dose" up to a maximum after which the RBE decreases. The OER values, however, were found to decrease with increasing "star dose" fraction rather rapidly after which it was found to be independent of the "star dose" contribution.