A new type of standard free-air chamber for high quantum energies

The conventional free-air chamber is employed for the exposure standardization of X-rays generated at potentials up to a few hundred kilovolts, i.e. for secondary electron ranges below 1 m. Two methods of attaining higher energies have been published: suppressing electron ranges by operating the chamber under high pressure, and forcing electron trajectories into spirals by a magnetic field parallel to the photon beam. The first method leads to increased ion recombination, causing additional inaccuracy, the second leaves the required chamber length unchanged. The secondary electron trajectories can also be confined by a magnetic field orthogonal to the photon beam. This is achieved by means of a toroidal winding, which is intersected by the beam parallel to the toroid symmetry axis, z. The free-air chamber inside the torus consists of two cylinders (a high voltage electrode and a collector-guard electrode assembly) and two end plates (guard strip assemblies). The magnetic flux density is inversely proportional to the distance r from the axis, the lines of induction being around it. The required chamber cross-section is defined by the secondary electron trajectories in the rz-plane. These are periodic loops with a net drift along z, and can be expressed in terms of Bessel functions. The product, mass of the winding multiplied by power dissipation in it depends on the maximum energy T of the secondary electrons but not significantly on other design parameters. For copper, a value below 107T4kg.W can be achieved, where T is measured in MeV. Thus this method is feasible, for example, for the 60Co quantum energies (T=1.1 MeV). An experimental chamber for X-ray measurements is described. The chamber length is 250 mm and the electrode radii are 85 and 195 mm. The design allows of selection among eight collector electrodes and an adjustable beam-to-axis distance. The winding consists of 36 turns of water cooled copper tubing, carrying a current of 833 A maximum. Measurements of ionization as a function of the magnetic induction are reported. (author)