A cylindrical proportional chamber has a central anode wire of radius 0.02 mm and an outer cathode of radius 10mm with a voltage of 500 volts applied between them. What is the electric field at the surface of the anode? If the threshold for ionisation by collision is 750 kV/m and the mean free path of the particles being detected is 4 × 10⁻⁶ m, estimate the number of ion pairs produced per primary particle.