Megavoltage bremsstrahlung end point voltage diagnostic

In a material, a beam of x rays is accompanied by various kinds of secondary radiation, including Compton electrons from collisions between the x rays and the material's electrons. For megavoltage bremsstrahlung in air, many of these Compton electrons are forward-directed and fast enough to be deflected outside the beam's edge by a magnetic field perpendicular to the beam. At the beam's edge, the dose from the deflected Compton electrons has a pattern that depends on the radiation's end point energy. Dose patterns measured with radiochromic film on a nominally 1 and 2 MV linear accelerator agree reasonably well with the corresponding Monte Carlo computations. With further development, the dose pattern produced outside the beam by such a sweeper magnet could become a noninvasive way to monitor megavoltage bremsstrahlung, when the end point energies are difficult to determine with other methods.