Design of Free-Air Ionization Chambers for the Soft X-Ray Region (20–100 kv)

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Abstract
The roentgen has been recommended (1) by the International Commission on Radiological Units and Measurements as the unit of exposure dose. A measurement with a free-air ionization chamber is the most convenient way of accurately determining exposure dose in the soft x-ray region. Design criteria for standard free-air chambers have been summarized by Wyckoff and Attix (2) in National Bureau of Standards Handbook 64for moderately and heavily filtered x-rays generated at potentials from 50 to 500 kvcp. Agreement to about 0.5 per cent has been reached in international intercomparisons of the roentgen in this energy region. In intercomparisons involving lightly filtered low-energy x-rays, however, differences of 1 per cent or more have been observed (3, 4). The present experiment was undertaken to provide design criteria for 20 to 100-kv radiation with filtration ranging from 2 mm. of beryllium to 2 mm. of beryllium plus 4 mm. of aluminum. A typical free-air ionization chamber is shown schematically in Figure 1. The roentgen is defined in terms of the ionization produced by the interaction of the x-ray beam with a specified mass of air (1). In practice, however, one defines a volume represented in Figure 1 by the cross hatched region. The cross-sectional area of the volume is determined by the diaphragm, and the length of the collecting region by the length of the collector and by the electric field between the high-voltage and collecting plates. The presence of the grounded lead box tends to distort the electric field in the collecting region, but guard plates and guard strips can be used to eliminate such distortion. The guard strips are uniformly spaced between the ground and high-voltage plates and are parallel to them. The potentials of the strips are fixed by a resistor network to give a linear potential gradient between the plates. Distortion can also be caused by the guard strips themselves. This effect becomes important when the strips are close to the edge of the collector or when the centerline-to-centerline spacing between adjacent strips is large. The x-ray beam interacts with the air to produce electrons like e1, which produce ionization in the collecting region. The precise determination of the roentgen requires that such electrons expend their energy in the air before striking the high-voltage or collecting plates. Thus the proper design of a free-air chamber requires a knowledge of the range of these primary electrons. Electrons like e2 cause ionization outside the collecting region, but under conditions of electronic equilibrium this loss is compensated by electrons like e3. Conditions may also exist in which a photon is scattered out of the primary beam and interacts to produce an electron like e4. This represents a contribution that is not included in the definition of the roentgen. This scattered photon contribution must be determined and subtracted from the total ionization measured by the chamber.

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