E′1 (bridging oxygen vacancy) defect densities have been measured in thermal SiO2 films implanted with ions having various energies and masses. From a plot of the defect production per unit volume versus energy deposited into elastic processes per unit volume we observe a linear slope yielding one E1 defect per 2000 eV deposited in displacement energy. Comparison with the measured average displacement energy per ion of 4 eV suggests that up to 500 defects of other forms might be created for every E′1. The predictions of a simple model for E1 creation/annihilation are further tested and the magnitude of the microscopic creation and annihilation cross sections found to be of the order of 10-13 cm2 per ion in both cases. The ratio of the creation and annihilation cross sections is found to decrease monotonically with decreasing implant ion mass.