SOS INDUCTION BY LOW ACTIVITY CR SOURCES - MICRODOSIMETRIC CONSIDERATIONS

The SOS response to alpha-radiation was measured using two Escherichia coli strains with differential repair capabilities. Monolayers of exp onentially growing cells, placed on cellulose acetate filters, were ex posed to an electrodeposited 252 Cf alpha source to total doses from 4 6 to 165 cGy at a dose-rate of 0.8 cGy/min. A mean alpha energy of 2.9 MeV, with a corresponding LET of 133 keV/mu m (for water) was attaine d by means of a filter placed between the bacterial culture and the ra dioactive source. Owing to the low fluences ranging from 0.013 to 0.03 7 alpha-particles/cell and short irradiation times, calculations of th e estimated number of irradiated cells were done to obtain the total a bsorbed doses by the culture. The results show that in spite of the lo w doses, the damage was sufficient to induce the SOS function in one o f the strains used (PQ37). Nevertheless in another strain IN88, lackin g double strand break repair and which should be more sensitive, SOS i nduction could not be observed, probably due to the killing effect of the high LET radiation.